1. An intra prediction method of an encoder, the method comprising: generating reference pixels for intra prediction with respect to an input prediction unit; determining an intra mode for the prediction unit5 ; generating a prediction block based on the reference pixels and the intra mode; and generating a residual block for the prediction unit and the prediction block, wherein at least one of the reference pixels and pixels of the prediction block is predicted based on a base pixel, and a pixel value of the predicted pixel is a sum of a pixel 10 value of the base pixel and a variation in pixel value from the base pixel to the generated pixel.

2. The intra prediction method of claim 1, wherein a reference pixel of a neighboring block disposed on a top left corner of the prediction block is set as a first base 15 pixel, a value obtained by applying a variation in pixel value from the first base pixel to a lowest pixel among reference pixels of a neighboring block disposed on a left boundary of the prediction block and a variation in pixel value from the first base pixel to a rightmost pixel among reference pixels of a neighboring block disposed on an upper boundary of the prediction block to the base pixel is set as a pixel value of a second base pixel as a 20 diagonal pixel in a bottom right corner of the prediction block, and pixel values of diagonal pixels of the prediction block are predicted from the first base pixel and the second base pixel.

3. The intra prediction method of claim 2, wherein non-diagonal pixels of 25 the prediction block are predicted by interpolation or extrapolation using the diagonal 40 pixels and the pixels of the neighboring blocks on the upper and/or left boundaries of the prediction block.

4. The intra prediction method of claim 1, wherein a reference pixel of a neighboring block disposed on a top left corner of the prediction block is set as the bas5 e pixel, and a value obtained by applying a variation in pixel value from the base pixel to a neighboring pixel disposed in the same row as a prediction target pixel among reference pixels of a neighboring block disposed on a left boundary of the prediction block and a variation in pixel value from the base pixel to a neighboring pixel disposed in the same 10 column as the prediction target pixel among reference pixels of a neighboring block disposed on an upper boundary of the prediction block to the base pixel is predicted as a pixel value of the prediction target pixel.

5. The intra prediction method of claim 1, wherein a pixel disposed in the 15 same row or column as a prediction target pixel among pixels of neighboring blocks disposed on left or upper boundary of the prediction block is set as the base pixel, and a value obtained by applying a variation in pixel value from the base pixel to the prediction pixel to the base pixel is predicted as a pixel value of the prediction target pixel. 20 6. The intra prediction method of claim 5, wherein the prediction target pixel is a diagonal pixel of the prediction block, and a non-diagonal pixel of the prediction block is predicted by interpolation using the diagonal pixel and the pixels of the neighboring blocks. 25 7. The intra prediction method of claim 1, further comprising generating a 41 reference pixel disposed on a boundary between an inter-mode block and the prediction unit when a block neighboring to the prediction unit is the inter-mode block, wherein a pixel disposed on a boundary of the prediction unit among pixels of an intra-mode block disposed on a left side or lower side of the reference pixel is set as a first base pixel, a pixel disposed on the boundary of the prediction unit among pixels of an intra-mode bloc5 k disposed on a right side or upper side of the reference pixel is set as a second base pixel, and the reference pixel is generated based on a distance from the first base pixel to the reference pixel and a distance from the second base pixel to the reference pixel. 10 8. The intra prediction method of claim 7, wherein a pixel value of the first base pixel is an average pixel value of pixels disposed on the boundary of the prediction unit among the pixels of the intra-mode block that the first base pixel belongs to, and a pixel value of the second base pixel is an average pixel value of pixels disposed on the boundary of the prediction unit among the pixels of the intra-mode block that the second 15 base reference belongs to.

9. The intra prediction method of claim 7, wherein a pixel value of the first base pixel is a pixel value of the reference pixel when an intra-mode block is disposed only on the left side or lower side of the reference pixel, and a pixel value of the second 20 base pixel is a pixel value of the reference pixel when an intra-mode block is disposed only on the right side or upper side of the reference pixel.

10. An intra prediction method of a decoder, the method comprising: entropy-decoding a received bitstream; 25 generating a reference pixel used for intra prediction of a prediction unit; 42 generating a prediction block from the reference pixel based on a prediction mode for the prediction unit; and reconstructing a picture from a residual block obtained by the entropy-decoding and the prediction block, wherein at least one of the reference pixels and pixels of the prediction block i5 s predicted based on a base pixel, and a pixel value of the predicted pixel is a sum of a pixel value of the base pixel and a variation in pixel value from the base pixel to the generated pixel. 10 11. The intra prediction method of claim 10, wherein a reference pixel of a neighboring block disposed on a top left corner of the prediction block is set as a first base pixel, a value obtained by applying a variation in pixel value from the first base pixel to a lowest pixel among reference pixels of a neighboring block disposed on a left boundary of the prediction block and a variation in pixel value from the first base pixel to a rightmost 15 pixel among reference pixels of a neighboring block disposed on an upper boundary of the prediction block to the base pixel is set as a pixel value of a second base pixel as a diagonal pixel in a bottom right corner of the prediction block, and pixel values of diagonal pixels of the prediction block are predicted from the first base pixel and the second base pixel. 20

12. The intra prediction method of claim 11, wherein non-diagonal pixels of the prediction block are predicted by interpolation or extrapolation using the diagonal pixels and the pixels of the neighboring blocks on the upper and/or left boundaries of the prediction block. 25 43

13. The intra prediction method of claim 10, wherein a reference pixel of a neighboring block disposed on a top left corner of the prediction block is set as the base pixel, and a value obtained by applying a variation in pixel value from the base pixel to a neighboring pixel disposed in the same row as a prediction target pixel among reference pixels of a neighboring block disposed on a left boundary of the prediction block and 5 a variation in pixel value from the base pixel to a neighboring pixel disposed in the same column as the prediction target pixel among reference pixels of a neighboring block disposed on an upper boundary of the prediction block to the base pixel is predicted as a pixel value of the prediction target pixel. 10

14. The intra prediction method of claim 10, wherein a pixel disposed in the same row or column as a prediction target pixel among pixels of neighboring blocks disposed on a left or upper boundary of the prediction block is set as the base pixel, and a value obtained by applying a variation in pixel value from the base pixel to the prediction 15 pixel to the base pixel is predicted as a pixel value of the prediction target pixel.

15. The intra prediction method of claim 14, wherein the prediction target pixel is a diagonal pixel of the prediction block, and a non-diagonal pixel of the prediction block is predicted by interpolation using the diagonal pixel and the pixels of the 20 neighboring blocks.

16. The intra prediction method of claim 10, further comprising generating a reference pixel disposed on a boundary between an inter-mode block and the prediction unit when a block neighboring to the prediction unit is the inter-mode block, wherein a 25 pixel disposed on a boundary of the prediction unit among pixels of an intra-mode block 44 disposed on a left side or lower side of the reference pixel is set as a first base pixel, a pixel disposed on the boundary of the prediction unit among pixels of an intra-mode block disposed on a right side or upper side of the reference pixel is set as a second base pixel, and the reference pixel is generated based on a distance from the first base pixel to the reference pixel and a distance from the second base pixel to the reference pixel5 .

17. The intra prediction method of claim 16, wherein a pixel value of the first base pixel is an average pixel value of pixels disposed on the boundary of the prediction unit among the pixels of the intra-mode block that the first base pixel belongs to, and a 10 pixel value of the second base pixel is an average pixel value of pixels disposed on the boundary of the prediction unit among the pixels of the intra-mode block that the second base reference belongs to.

18. The intra prediction method of claim 16, wherein a pixel value of the first 15 base pixel is a pixel value of the reference pixel when an intra-mode block is disposed only on the left side or lower side of the reference pixel, and a pixel value of the second base pixel is a pixel value of the reference pixel when an intra-mode block is disposed only on the right side or upper side of the reference pixel. 20 19. The intra prediction method of claim 10, comprising acquiring an instruction to generate the pixels of the prediction block based on the base pixel through the entropy-decoding. 25 45

20. The intra prediction method of claim 10, comprising acquiring an instruction to generate the reference pixels based on the base pixel through the entropy-decoding.

【Technical Field5 】
The present invention relates to a video processing technique, and more
particularly, to an intra prediction method in encoding/decoding video information.
【Background Art】
Recently, demands for high-resolution and high-quality images have increased in
10 various fields of applications. As images have higher resolution and higher quality,
amount of information on the images also increases. Accordingly, when video data is
transferred using media such as existing wired and wireless broad band lines or is stored in
conventional storage media, transfer and storage costs of the video data rise.
Thus, high-efficiency video compression techniques may be utilized to effectively
15 transmit, store or reproduce images with superior resolution and superior quality.
【Disclosure】
【Technical Problem】
An aspect of the present invention is to provide a method of performing effective
intra prediction on a texture with directionality in consideration of variations of reference
20 pixels of neighboring blocks.
Another aspect of the present invention is to provide a method of performing
planar prediction considering variations in pixel values of adjacent blocks to a prediction
block in conducting intra prediction.
Still another aspect of the present invention is to provide a method of generating a
25 reference pixel based on an intra-mode neighboring block in a position of an inter
3
prediction mode neighboring pixel and using the reference pixel for intra prediction when
constrained intra prediction (CIP) is employed.
Yet another aspect of the present invention is to provide a method of generating a
reference pixel considering variations in pixel value when the reference pixel is generated
based on an intra-mode neighboring block in a position of an inter prediction mod5 e
neighboring pixel.
【Technical Solution】
An embodiment of the present invention provides an intra prediction method of an
encoder, the method including generating reference pixels for intra prediction with respect
10 to an input prediction unit, determining an intra mode for the prediction unit, generating a
prediction block based on the reference pixels and the intra mode, and generating a
residual block for the prediction unit and the prediction block, wherein at least one of the
reference pixels and pixels of the prediction block is predicted based on a base pixel, and a
pixel value of the predicted pixel is a sum of a pixel value of the base pixel and a variation
15 in pixel value from the base pixel to the generated pixel.
A reference pixel of a neighboring block disposed on a top left corner of the
prediction block may be set as a first base pixel, a value obtained by applying a variation
in pixel value from the first base pixel to a lowest pixel among reference pixels of a
neighboring block disposed on a left boundary of the prediction block and a variation in
20 pixel value from the first base pixel to a rightmost pixel among reference pixels of a
neighboring block disposed on an upper boundary of the prediction block to the base pixel
may be set as a pixel value of a second base pixel as a diagonal pixel in a bottom right
corner of the prediction block, and pixel values of diagonal pixels of the prediction block
may be predicted from the first base pixel and the second base pixel.
25 Here, non-diagonal pixels of the prediction block are predicted by interpolation or
4
extrapolation using the diagonal pixels and the pixels of the neighboring blocks on the
upper and/or left boundaries of the prediction block.
Further, a reference pixel of a neighboring block disposed on a top left corner of
the prediction block may be set as the base pixel, and a value obtained by applying a
variation in pixel value from the base pixel to a neighboring pixel disposed in the sam5 e
row as a prediction target pixel among reference pixels of a neighboring block disposed on
a left boundary of the prediction block and a variation in pixel value from the base pixel to
a neighboring pixel disposed in the same column as the prediction target pixel among
reference pixels of a neighboring block disposed on an upper boundary of the prediction
10 block to the base pixel may be predicted as a pixel value of the prediction target pixel.
Also, a pixel disposed in the same row or column as a prediction target pixel
among pixels of neighboring blocks disposed on left or upper boundary of the prediction
block may be set as the base pixel, and a value obtained by applying a variation in pixel
value from the base pixel to the prediction pixel to the base pixel may be predicted as a
15 pixel value of the prediction target pixel.
Here, the prediction target pixel may be a diagonal pixel of the prediction block,
and a non-diagonal pixel of the prediction block may be predicted by interpolation using
the diagonal pixel and the pixels of the neighboring blocks.
The intra prediction method may further include generating a reference pixel
20 disposed on a boundary between an inter-mode block and the prediction unit when a block
neighboring to the prediction unit is the inter-mode block, wherein a pixel disposed on a
boundary of the prediction unit among pixels of an intra-mode block disposed on a left
side or lower side of the reference pixel may be set as a first base pixel, a pixel disposed
on the boundary of the prediction unit among pixels of an intra-mode block disposed on a
25 right side or upper side of the reference pixel may be set as a second base pixel, and the
5
reference pixel may be generated based on a distance from the first base pixel to the
reference pixel and a distance from the second base pixel to the reference pixel.
Here, a pixel value of the first base pixel may be an average pixel value of pixels
disposed on the boundary of the prediction unit among the pixels of the intra-mode block
that the first base pixel belongs to, and a pixel value of the second base pixel may be a5 n
average pixel value of pixels disposed on the boundary of the prediction unit among the
pixels of the intra-mode block that the second base reference belongs to. Further, a pixel
value of the first base pixel may be a pixel value of the reference pixel when an intramode
block is disposed only on the left side or lower side of the reference pixel, and a
10 pixel value of the second base pixel may be a pixel value of the reference pixel when an
intra-mode block is disposed only on the right side or upper side of the reference pixel.
Another embodiment of the present invention provides an intra prediction method
of a decoder, the method including entropy-decoding a received bitstream, generating a
reference pixel used for intra prediction of a prediction unit, generating a prediction block
15 from the reference pixel based on a prediction mode for the prediction unit, and
reconstructing a picture from a residual block obtained by the entropy-decoding and the
prediction block, wherein at least one of the reference pixels and pixels of the prediction
block is predicted based on a base pixel, and a pixel value of the predicted pixel is a sum
of a pixel value of the base pixel and a variation in pixel value from the base pixel to the
20 generated pixel.
A reference pixel of a neighboring block disposed on a top left corner of the
prediction block may be set as a first base pixel, a value obtained by applying a variation
in pixel value from the first base pixel to a lowest pixel among reference pixels of a
neighboring block disposed on a left boundary of the prediction block and a variation in
25 pixel value from the first base pixel to a rightmost pixel among reference pixels of a
6
neighboring block disposed on an upper boundary of the prediction block to the base pixel
may be set as a pixel value of a second base pixel as a diagonal pixel in a bottom right
corner of the prediction block, and pixel values of diagonal pixels of the prediction block
may be predicted from the first base pixel and the second base pixel.
Here, non-diagonal pixels of the prediction block may be predicted b5 y
interpolation or extrapolation using the diagonal pixels and the pixels of the neighboring
blocks on the upper and/or left boundaries of the prediction block.
A reference pixel of a neighboring block disposed on a top left corner of the
prediction block may be set as the base pixel, and a value obtained by applying a variation
10 in pixel value from the base pixel to a neighboring pixel disposed in the same row as a
prediction target pixel among reference pixels of a neighboring block disposed on a left
boundary of the prediction block and a variation in pixel value from the base pixel to a
neighboring pixel disposed in the same column as the prediction target pixel among
reference pixels of a neighboring block disposed on an upper boundary of the prediction
15 block to the base pixel may be predicted as a pixel value of the prediction target pixel.
Further, a pixel disposed in the same row or column as a prediction target pixel
among pixels of neighboring blocks disposed on a left or upper boundary of the prediction
block may be set as the base pixel, and a value obtained by applying a variation in pixel
value from the base pixel to the prediction pixel to the base pixel may be predicted as a
20 pixel value of the prediction target pixel.
Here, the prediction target pixel may be a diagonal pixel of the prediction block,
and a non-diagonal pixel of the prediction block may be predicted by interpolation using
the diagonal pixel and the pixels of the neighboring blocks.
The intra prediction may further include generating a reference pixel disposed on a
25 boundary between an inter-mode block and the prediction unit when a block neighboring
7
to the prediction unit is the inter-mode block, wherein a pixel disposed on a boundary of
the prediction unit among pixels of an intra-mode block disposed on a left side or lower
side of the reference pixel may be set as a first base pixel, a pixel disposed on the
boundary of the prediction unit among pixels of an intra-mode block disposed on a right
side or upper side of the reference pixel may be set as a second base pixel, and th5 e
reference pixel may be generated based on a distance from the first base pixel to the
reference pixel and a distance from the second base pixel to the reference pixel.
Here, a pixel value of the first base pixel may be an average pixel value of pixels
disposed on the boundary of the prediction unit among the pixels of the intra-mode block
10 that the first base pixel belongs to, and a pixel value of the second base pixel may be an
average pixel value of pixels disposed on the boundary of the prediction unit among the
pixels of the intra-mode block that the second base reference belongs to. Further, a pixel
value of the first base pixel may be a pixel value of the reference pixel when an intramode
block is disposed only on the left side or lower side of the reference pixel, and a
15 pixel value of the second base pixel may be a pixel value of the reference pixel when an
intra-mode block is disposed only on the right side or upper side of the reference pixel.
The decoder may acquire an instruction to generate the pixels of the prediction
block based on the base pixel through the entropy-decoding. Also, the decoder may
acquire an instruction to generate the reference pixels based on the base pixel through the
20 entropy-decoding.
【Advantageous Effects】
As described above, according to the present invention, intra prediction on a
texture with directionality may be achieved effectively in consideration of variations of
reference pixels of neighboring blocks.
25 Further, planar prediction may be performed considering variations in pixel values
8
of neighboring blocks to a prediction block, thereby enhancing prediction efficiency.
In addition, when constrained intra prediction (CIP) is employed, a reference pixel
is generated based on an intra-mode neighboring block in a position of an inter-mode
neighboring pixel and used for intra prediction, considering variations in pixel value,
thereby enhancing prediction efficiency5 .
【Description of Drawings】
FIG. 1 is a block diagram illustrating a configuration of a video encoder according
to an exemplary embodiment of the present invention.
FIG. 2 is a block diagram schematically illustrating a configuration of an intra
10 prediction module according to an exemplary embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration of a video decoder according
to an exemplary embodiment of the present invention.
FIG. 4 schematically illustrates a planar prediction method.
FIG. 5 schematically illustrates an alternative planar prediction method.
15 FIG. 6 schematically illustrates that a diagonal pixel of a current prediction block
is predicted first.
FIG. 7 schematically illustrates a method of deriving other pixel values in the
prediction block based on the diagonal pixel.
FIG. 8 schematically illustrates a method of predicting a pixel value considering a
20 reference pixel value and a variation from a reference pixel.
FIG. 9 schematically illustrates a method of deriving diagonal pixels of a
prediction block first and then pixel values of remaining pixels.
FIG. 10 schematically illustrates that diagonal pixels are derived first and other
pixels than the diagonal pixels are derived in the same method as used for the diagonal
25 pixels.
9
FIG. 11 schematically illustrates an CIP method.
FIG. 12 schematically illustrates an alternative CIP method.
FIG. 13 schematically illustrates that a system according to the present invention
conducts CIP in consideration of variations in pixel value.
FIG. 14 is a flowchart schematically illustrating an operation of the encoder in th5 e
system according to the present invention.
FIG. 15 illustrates a prediction direction of an intra prediction mode.
FIG. 16 is a flowchart schematically illustrating an operation of the decoder in the
system according to the present invention.
10 【Mode for Invention】
Although elements shown in the drawings are independently shown so as to
describe different features and functions of a video encoder/decoder, such a configuration
does not indicate that each element is constructed by a separate hardware constituent or
software constituent. That is, the elements are independently arranged and at least two
15 elements may be combined into a single element, or a single element may be divided into
a plurality of elements to perform functions. It is to be noted that embodiments in which
some elements are integrated into one combined element and/or an element is divided into
multiple separate elements are included in the scope of the present invention without
departing from the essence of the present invention.
20 Hereinafter, exemplary embodiments of the present invention will be described in
detail with reference to the accompanying drawings. Like reference numerals in the
drawings refer to like elements throughout, and redundant descriptions of like elements
will be omitted herein.
FIG. 1 is a block diagram illustrating a configuration of a video encoder according
25 to an exemplary embodiment of the present invention. Referring to FIG. 1, the video
10
encoder includes a picture partition module 110, an inter prediction module 120, an intra
prediction module 125, a transform module 130, a quantization module 135, a
dequantization module 140, an inverse transform module 145, a deblocking filter 150, a
memory 160, a rearrangement module 165 and an entropy encoding module 170.
The picture partition module 110 may receive input of a current picture and divid5 e
the picture into at least one coding unit. A coding unit is a unit of encoding conducted by
the video encoder and may be also referred to as a CU. A coding unit may be recursively
subdivided with a depth based on a quadtree structure. A coding unit having a maximum
size is referred to as a largest coding unit (LCU), and a coding unit with a minimum size
10 as a smallest coding unit (SCU). A coding unit may have a size of 8 x 8, 16 x 16, 32 x
32 or 64 x 64. The picture partition module 110 may partition or devide the coding unit
to generate a prediction unit and a transform unit. The prediction unit may be also
referred to as a PU, and the transform unit may be also referred to as a TU.
In an inter prediction mode, the inter prediction module 120 may perform motion
15 estimation (ME) and motion compensation (MC). The inter prediction module 120
generates a prediction block based on information on at least one of previous and
subsequent pictures of the current picture, which may be referred to as inter-frame
prediction.
The inter prediction module 120 is provided with a partitioned prediction target
20 block and at least one reference block stored in the memory 160. The inter prediction
module 120 performs motion estimation using the prediction target block and the
reference block. The inter prediction module 120 generates motion information
including a motion vector (MV), a reference block index and a prediction mode as a result
of motion estimation.
25 Further, the inter prediction module 120 performs motion compensation using the
11
motion information and the reference block. Here, the inter prediction module 120
generates and outputs a prediction block corresponding to an input block from the
reference block.
The motion information is entropy-encoded to form a compressed bitstream,
which is transmitted from the video encoder to a video decoder5 .
In an intra prediction mode, the intra prediction module 125 may generate a
prediction block based on information on a pixel in the current picture. Intra prediction
is also referred to as intra-frame prediction. In the intra prediction mode, a prediction
target block and a reconstructed block reconstructed through encoding and decoding are
10 input to the intra prediction module 125. Here, the reconstructed block is a picture that
has not been subjected to the deblocking filter. The reconstructed block may be a
previous prediction block.
FIG. 2 is a block diagram schematically illustrating a configuration of the intra
prediction module according to an exemplary embodiment of the present invention.
15 Referring to FIG. 2, the intra prediction module includes a reference pixel generation
module 210, an intra prediction mode determination module 220, and a prediction block
generation module 230.
The reference pixel generation module 210 generates a reference pixel needed for
intra prediction. Pixels in a rightmost vertical line of a left block neighboring to a
20 prediction target block and pixels in a lowest horizontal line of an upper block
neighboring to a prediction target block are used for generating the reference pixel. For
example, when the prediction target block has a size of N, 2N pixels in each of left and
upper directions are used as reference pixels. The reference pixel may be used as it is or
via adaptive intra smoothing (AIS) filtering. When the reference pixel is subjected to
25 AIS filtering, information about AIS filtering is signaled.
12
The intra prediction mode determination module 220 receives input of the
prediction target block and the reconstructed block. The intra prediction mode
determination module 220 selects a mode minimizing quantity of information to be
encoded among prediction modes using the input picture and outputs information on the
prediction mode. Here, a preset cost function or Hadamard transform may be used5 .
The prediction block generation module 230 receives input of the information on
the prediction mode and the reference pixel. The prediction block generation module
230 spatially predicts and compensates a pixel value of the prediction target block using
the information on the prediction mode and a pixel value of the reference pixel, thereby
10 generating a prediction block.
The information on the prediction mode is entropy-encoded to form a compressed
bitstream along with video data and transmitted from the video encoder to the video
decoder. The video decoder uses the information on the prediction mode when
generating an intra prediction block.
15 Referring back to FIG. 1, a differential block is generated by difference between
the prediction target block and the prediction block generated in the inter or intra
prediction mode and is input to the transform module 130. The transform module 130
transforms the differential block in a transform unit to generate a transform coefficient.
A transform block with a transform unit has a quadtree structure within maximum
20 and minimum sizes and thus is not limited to a predetermined size. Each transform block
has a flag indicating whether the current block is partitioned into sub-blocks, wherein
when the flag is 1, the current transform block may be split into four sub-blocks.
Discrete cosine transform (DCT) may be used for transformation.
The quantization module 135 may quantize the values transformed by the
25 transform module 130. A quantization coefficient may change based on a block or
13
importance of a picture. The quantized transform coefficient may be provided to the
rearrangement module 165 and the dequantization module 140.
The rearrangement module 165 may change a two-dimensional (2D) block of the
transform coefficients into a one-dimensional (1D) vector of transform coefficients by
scanning so as to enhance efficiency in entropy encoding. The rearrangement modul5 e
165 may change scanning order based on stochastic statistics to enhance entropy encoding
efficiency.
The entropy encoding module 170 entropy-encodes the values obtained by the
rearrangement module 165, and the encoded values are formed into a compressed
10 bitstream, which is stored or transmitted through a network abstraction layer (NAL).
The dequantization module 140 receives and dequantizes the transform
coefficients quantized by the quantization module 135, and the inverse transform module
145 inverse-transforms the transform coefficients, thereby generating a reconstructed
differential block. The reconstructed differential block is merged with the prediction
15 block generated by the inter prediction module 120 or the intra prediction module 125 to
generate a reconstructed block. The reconstructed block is provided to the intra
prediction module 125 and the deblocking filter 150.
The deblocking filter 150 filters the reconstructed block to remove a distortion on
a boundary between blocks that occurs in encoding and decoding processes and provides a
20 filtered result to an adaptive loop filter (ALF) 155.
The ALF 155 performs filtering to minimize an error between the prediction
target block and the final reconstructed block. The AFL 155 performs filtering based on
a value resulting from comparison of the reconstructed block filtered by the deblocking
filter 150 and the current prediction target block, and a filter coefficient information on the
25 ALF 155 is loaded into a slice header and transmitted from the encoder to the decoder.
14
The memory 160 may store the final reconstructed block obtained through the
ALF 155, and the (final) stored reconstructed block may be provided to the inter
prediction module 120 to perform inter prediction.
FIG. 3 is a block diagram illustrating a configuration of a video decoder according
to an exemplary embodiment of the present invention. Referring to FIG. 3, the vide5 o
decoder includes an entropy decoding module 310, a rearrangement module 315, a
dequantization module 320, an inverse transform module 325, an inter prediction module
330, an intra prediction module 335, a deblocking filter 340, an ALF 345, and a memory
350.
10 The entropy decoding module 310 receives a compressed bitstream from an NAL.
The entropy decoding module 310 entropy-decodes the received bitstream, and also
entropy-decodes a prediction mode and motion vector information if the bitstream
includes the prediction mode and the motion vector information. An entropy-decoded
transform coefficient or differential signal is provided to the rearrangement module 315.
15 The rearrangement module 315 inverse-scans the transform coefficient or differential
signal to generate a 2D block of transform coefficients.
The dequantization module 320 receives and dequantizes the entropy-decoded and
rearranged transform coefficients. The inverse transform module 325 inverse-transforms
the dequantized transform coefficients to generate a differential block.
20 The differential block may merge with a prediction block generated by the inter
prediction module 330 or intra prediction module 335 to generate a reconstructed block.
The reconstructed block is provided to the intra prediction module 335 and the deblocking
filter 340. The inter prediction module 330 and the intra prediction module 335 may
perform the same operations as the inter prediction module 120 and the intra prediction
25 module 125 of the video encoder.
15
The deblocking filter 340 filters the reconstructed block to remove a distortion on
a boundary between blocks that occurs in encoding and decoding processes and provides a
filtered result to the ALF 345. The ALF 345 performs filtering to minimize an error
between the prediction target block and the finally reconstructed block. The memory
160 may store the final reconstructed block obtained through the ALF 345, and the store5 d
(final) reconstructed block may be provided to the inter prediction module 330 to perform
inter prediction.
Meanwhile, in an area with insignificant changes in texture, for example, a
monotonous background of the sky or sea, planar intra prediction is used to further
10 enhance encoding efficiency.
Intra prediction is classified into directional prediction, DC prediction and planar
prediction, wherein planar prediction may be an extended concept of DC prediction.
Although planar prediction may be broadly included in DC prediction, planar prediction
may cover a prediction method that DC prediction does not deal with. For example, DC
15 prediction is preferable for a uniform texture, while planar prediction is effective for block
prediction in a pixel values having directionality.
The present specification illustrates a method of improving planar prediction
efficiency with respect to a texture with directiionality using variations in pixel values of
reference pixels of neighboring blocks.
20 FIG. 4 schematically illustrates a planar prediction method.
Referring to FIG. 4(A), a pixel value 425 of a pixel in a bottom right corner of a
current block is predicted. The pixel value 425 of the pixel on the bottom right corner of
the current block may be predicted as a DC value.
Referring to FIG. 4(B), pixel values of pixels located on a right boundary of the
25 current block and pixel values of pixels located on a bottom boundary of the current block
16
are predicted. For example, a pixel value 445 located on the right boundary of the
current block may be predicted by linear interpolation of a pixel value 450 of an upper
block and the DC value 425. Further, a pixel value 435 located on the bottom boundary
of the current block may be predicted by linear interpolation of a pixel value 430 of a left
block and the DC value 4255 .
Referring to FIG. 4(C), pixel values of remaining pixels, other than the pixel on
the bottom right corner, the pixels on the right boundary and the pixels on the bottom
boundary in the current block, may be predicted by bilinear interpolation using the pixel
values of the upper and left blocks and the already predicted pixel values in the current
10 block. For example, a pixel value 475 in the current block may be predicted by
interpolation using a pixel value 460 of the upper block, a pixel value 455 of the left block,
the already predicted pixel value 445 located on the right boundary of the current block
and the already predicted pixel value 435 located on the bottom boundary of the current
block.
15 Referring to FIG. 4(D), the prediction samples (predicted samples) obtained via
the foregoing process may be refined. For example, a pixel value X 485 in the current
block may be refined using an upper sample value T 480 and a left sample value L 490.
Specifically, X' refined from X may be obtained by X'={(X<<1)+L+T+1}>>2. Here,
"x<>y" indicates that the two's complement integer
expression of x is arithmetically shifted to the right by the binary unit y.
FIG. 5 schematically illustrates an alternative planar prediction method.
In the method of FIG. 5, pixel values of pixels diagonally located in a current
pixel are predicted first, and pixel values of remaining pixels in the current block are
25 predicted using the predicted pixel values. For convenience of description, the pixels
17
located diagonally from the top left to the bottom right among the pixels constructing the
block are referred to as diagonal pixels below.
Referring to FIG. 5(A), pixel values of diagonal pixels 540 of a current block 510
are predicted using a pixel value 510 of an upper reference block and a pixel value 530 of
a left reference block. For instance, a pixel value of a diagonal pixel P in the curren5 t
block may be obtained using a pixel value of a pixel AboveRef located on a boundary
between the current block and the upper block among pixels of the upper block and a pixel
value of a pixel LeftRef located on a boundary between the current block and the left
block among pixels of the left block by P=(LeftRef+AboveRef+1)>>1.
10 Referring to FIG. 5(B), pixel values of pixels in the current block 510 other than
the diagonal pixels 540 may be obtained by linear interpolation using the pixel value
obtained in FIG. 5(A) and pixel values of the pixels of the upper and left blocks on the
boundaries. For example, P1 may be obtained using the pixel AboveRef of the upper
block and the obtained diagonal pixel P by P1 = (AboveRef*d2 + P*d1)/(d1+d2).
15 Further, P2 may be obtained by P2 = (LeftRef*d3 + P*d4)(d3+d4).
Meanwhile, the planar prediction methods illustrated in FIGS. 4 and 5 are
effective for a uniform texture without directionality, whereas these methods may have
reduced efficiency in prediction in a case of a texture with directionality, such as luma
pixels in which luminance changes substantially in one direction, for example, a
20 horizontal direction, but hardly changes in another direction, for example, a vertical
direction.
Thus, planar intra prediction considering variations in pixel value may be needed.
Planar intra prediction according to the present invention selects or predict a base pixel
value and applies variations in pixel values between a base pixel and a target pixel to the
25 base pixel value, thereby predicting a pixel value of the target pixel.
18
Hereinafter, examples of the present invention will be described with reference to
the drawings.
Example 1
FIG. 6 schematically illustrates that a diagonal pixel Pii of a current prediction
block is predicted first. Although FIG. 6 illustrates an 8 x 8 prediction block fo5 r
convenience of description, the present invention may be also applied to an N x N
prediction block, without being limited to the 8 x 8 prediction block.
In Example 1 shown in FIG. 6, the diagonal pixels of the current prediction block
are predicted first based on a reference pixel (Ri0 and/or R0j, 0≤i,j≤8 in 8 x 8 prediction
10 block case) of a reference block neighboring to the current prediction block.
That is, after the diagonal pixels Pii are obtained, other pixel values in the
prediction block may be derived by interpolation or extrapolation using reference pixel
values (Rij) of the neighboring block and the Pii.
FIG. 7 schematically illustrates a method of deriving the other pixel values in the
15 prediction block based on the diagonal pixels.
In the present invention, planar prediction is carried out in consideration of
changes in pixel values. For example, as shown in FIG. 7(A), when the reference pixel
values increase both in an x direction (right) and in a y direction (down), pixel values in
the prediction block are also more likely to increase in a bottom-right direction. In this
20 case, a pixel value of P88 on a bottom right corner of the prediction block may be
predicted first, and other pixels may be predicted based on the pixel value of P88.
To predict the value of P88, defining a pixel value of reference pixel R00 on a top
left corner of the current prediction block as a pixel value of base pixel, a variation from
the base pixel to the prediction target pixel P88 in the prediction block may be applied to
25 the pixel value of the base pixel. For example, a pixel value of the target pixel P88 may
19
be obtained by Equation 1. For convenience of description, Rij or Pij illustrated in the
drawings and the specification are presented as Ri,j and P i,j .
[Equation 1]
5
When P88 is obtained, the other diagonal pixels Pii may be obtained by Equation
2.
[Equation 2]
10
Here, since the present example illustrates the 8 x 8 prediction block, i may be 1,
2, …, 8. Although Example 1 illustrates the 8 x 8 prediction block for convenience of
description, in an N x N prediction block, Pii may be obtained as Pii=R00+(i/N)P88.
As shown in FIG. 7(B), even when the reference pixel value decrease both in the
15 x direction (right) and in the y direction (down), a pixel value of P88 in the bottom right
corner of the prediction block may be derived in consideration of variations in decreasing
pixel values, and the other pixel values may be predicted based on the pixel value of P88.
In this case, P88 may be derived by Equation 3.
[Equation 3]
20
When P88 is obtained, the other diagonal pixels in the prediction block may be
20
obtained by Equation 4.
[Equation 4]
Here, i may be 1, 2, …, 85 .
As shown in FIG. 7(C), when the reference pixel values increase in a top-right
direction, the diagonal pixels located from the bottom left to the top right in the prediction
block are derived first based on variations in pixel values, unlike in FIGS. 7(A) and 7(B).
For example, a pixel value of P81 in a bottom left corner of the prediction block is derived,
10 and the remaining pixel values may be predicted based on the pixel value of P81. In this
case, P81 may be derived by Equation 5.
[Equation 5]
15 When P81 is obtained, the remaining diagonal pixels (bottom left to top left) in
the prediction block may be derived by Equation 6.
[Equation 6]
20 Here, i may be 1, 2, …, 8.
Also, as shown in FIG. 7(D), when the reference pixel values increase in a
bottom-left direction, the diagonal pixels located from the bottom left to the top right in
21
the prediction block are derived first based on variations in pixel values. For example,
the pixel value of P81 in the bottom left corner of the prediction block is derived, and the
remaining pixel values may be predicted based on the pixel value of P81. In this case,
P81 may be derived by Equation 7.
[Equation 75 ]
When P81 is obtained, the remaining diagonal pixels (bottom left to top left) in
the prediction block may be derived by Equation 8.
10 [Equation 8]
Here, i may be 1, 2, …, 8.
In view of calculation loads, approximation of the calculations of square roots to
15 derive the diagonal pixels may be considered as in Equation 9.
[Equation 9]
Subsequently, the other pixel values in the prediction block may be derived by
20 interpolation or extrapolation using the prediction values of the diagonal pixels, upper
reference pixel values and left reference pixel values.
In FIGS. 7(A) and 7(B), the pixels Pij in the prediction block may be derived by
interpolation using the diagonal pixels Pii and reference pixels R of the neighboring block.
22
Here, an interpolation shown in Equation 10 may be used.
[Equation 10]
or
5
Here, d1 is a distance from the pixel R0j or Rj0 of the neighboring block used for
interpolation to the prediction target pixel Pij, and d2 is a distance from the diagonal pixel
Pii used for interpolation to the prediction target pixel Pij.
Further, in FIGS. 7(C) and 7(D), the pixel Pi derived by interpolation among the
10 pixels in the prediction block may be derived by Equation 11.
[Equation 11]
or
15 Here, i+j<9, and d1 is a distance from the pixel R0j or Rj0 of the neighboring
block used for interpolation to the prediction target pixel Pij, and d2 is a distance from the
diagonal pixel Pii used for interpolation to the prediction target pixel Pij. Here, although
Equation 11 is used for interpolation to derive the pixel Pij of the prediction block, various
interpolation methods may be employed in the present invention, without being limited
23
thereto.
Meanwhile, in FIGS. 7(C) and 7(D), there is a pixel Pe derived by extrapolation
among the pixels of the prediction block. Here, an extrapolation shown in Equation 12
may be used to derive the pixel in the prediction block.
[Equation 125 ]
or
In this case, i+j>9 and P is a diagonal pixel used for extrapolation. Further, as
10 described above, d1 and d2 are a distance from the reference pixel to the prediction target
pixel Pij and a distance from the pixel Pii to the prediction target pixel Pij, respecitvely.
Example 2
FIG. 8 schematically illustrates another method of predicting a pixel value
15 considering a base pixel value and a variation from a base pixel. Although FIG. 8
illustrates an 8 x 8 prediction block for convenience of description, the present invention
may be also applied to an N x N prediction block, without being limited to the 8 x 8
prediction block.
FIG. 8 illustrates a reference pixel P00 located on the top left corner of the
20 prediction block as a base pixel. In Example 2, a prediction target pixel Pij is derived by
applying vertical and horizontal variations from the reference pixel to the base pixel value.
24
For example, the target pixel Pij is derived by Equation 13.
[Equation 13]
Here, △y = Ri0 - R00, △x = R0j - R00, and 1≤ i, j ≤8 in the 8x8 prediction bloc5 k
case.
For example, referring to FIG. 8, a pixel P33 is derived by P33=R00+△x+△y
according to Equation 7. Here, △x and △y are variations in pixel value in the x direction
and the y directions from the base pixel R00 to P33.
10 Alternatively, referring to FIG. 8, a pixel P76 is derived by P76=R00+△x'+△y'
according to Equation 13. Here, △x' and △y' are variations in pixel value in the x
direction and the y directions from the base pixel R00 to P76.
Example 3
15 FIG. 9 schematically illustrates another method of deriving diagonal pixels of a
prediction block first and then pixel values of remaining pixels.
Although FIG. 5 illustrates that the diagonal pixels are derived based on an
average value of two pixels in a horizontal/vertical direction of a neighboring block to the
current prediction block, Example 3 shown in FIG. 9 drives the diagonal pixels in
20 consideration of variations.
Referring to FIG. 9(A), the diagonal pixels of the prediction block are predicted
using pixel values of neighboring blocks located on upper and/or left boundaries of the
prediction block. For example, the diagonal pixels Pii is predicted by Equation 14.
[Equation 14]
25
or
For example, referring to FIG. 9(A), P33 may be predicted by P33=R03+△y or
P33=R30+△x according to Equation 14. △x and △y are variations in pixel value in the 5 x
direction from a base pixel R30 to P33 and in the y direction from a base pixel R03 to P33,
respectively.
Referring to FIG. 9(B), other pixels Pij of the current block than the diagonal
pixels may be predicted by linear interpolation using the prediction values of the diagonal
10 pixels and reference pixels R00, R10 to R80 and R01 to R08 of the neighboring blocks on
the upper and left boundaries of the current block.
For example, a pixel value Pij may be predicted by Equation 15.
[Equation 15]
or
15
d1 is a distance from the pixel R0j or Pi0 of the neighboring blocks used for
interpolation to the prediction target pixel Pij, and d2 is a distance from the diagonal pixel
Pii used for interpolation to the prediction target pixel Pij.
20
Example 4
26
FIG. 10 schematically illustrates that diagonal pixels are derived first and other
pixels than the diagonal pixels are derived in the same method as used for the diagonal
pixels.
In FIG. 10, the diagonal pixels may be predicted in the same manner as illustrated
in FIG. 9. Thus, referring to FIG. 10(A), a diagonal pixel P33 of a current predictio5 n
block may be predicted by P33=R03+△y or P33=R30+△x.
Subsequently, other pixels Pij of the current block than the diagonal pixels may be
predicted by linear interpolation using the prediction values of the diagonal pixels and
reference pixels R00, R10 to R80 and R01 to R08 of neighboring blocks on upper and left
10 boundaries of the current block.
Here, the same method as used for deriving diagonal pixels may be used. For
example, a pixel Pij may be predicted by Equation 16.
[Equation 16]
or
15
Here, △y = Ri0 - R00, △x = R0j - R00, and 1≤ i, j ≤8 in the 8x8 prediction block
case.
For example, referring to FIG. 10, P37 may be derived by P37=R07+△y or
20 P37=R70+△x according to Equation 16.
Meanwhile, accumulation of minor errors resulting from integer arithmetic by the
encoder or decoder for a long time may cause a serious error. Further, when a
transmission error occurs in a neighboring block to a current block, a mismatch arises
27
between the encoder and the decoder or the error spread. For example, when an error
occurs in the neighboring block, pixel values on a boundary of the neighboring block are
changed. In this case, when the decoder uses a pixel with a changed pixel value as a
reference pixel, the error spreads to the current block. Thus, a tool to prevent such a
problem is needed, for example, an encoding tool such as constrained intra predictio5 n
(CIP).
FIG. 11 schematically illustrates an CIP method.
In the method of FIG. 11, if there is any one inter prediction mode block
neighboring to a current macroblock T, a DC intra prediction mode is used only and a DC
10 prediction value is fixed to 128.
Here, a pixel value of a block predicted by the inter prediction mode among the
neighboring blocks is not used as a reference pixel value. Thus, in this method, a DC
prediction mode is compulsorily employed excluding even available information, for
example, neighboring intra prediction mode pixels.
15 FIG. 12 schematically illustrates an alternative CIP method.
In the method of FIG. 12, a pixel value of a block predicted in the intra prediction
mode among neighboring blocks is used as a reference pixel value, and a pixel value of a
block predicted in the inter prediction mode is derived using neighboring intra prediction
mode blocks. Thus, not only the DC mode but other intra prediction modes may be used.
20 Referring to FIG. 12, among neighboring blocks to a current prediction block T,
pixel values 1210, 1220 and 1230 of blocks A, B, D, E, F, H and I predicted by the inter
prediction mode are derived using pixels of blocks predicted by the intra prediction mode.
For example, when predicted pixels of intra prediction mode are present on both
right and left sides of a target inter-prediction sample, a pixel value PT of a block predicted
25 by the inter prediction mode is derived by Equation 17.
28
[Equation 17]
Here, PT is a target inter-prediction sample, PLB is a left or lower intra-prediction
sample, and PRA is a right or upper intra-prediction sample. Further, when an intra5 -
prediction sample is present on either side of the target inter-prediction sample only, a
pixel value PT of a block predicted by the inter prediction mode is derived by Equation 18.
[Equation 18]
or
10
The method of FIG. 12 utilizes the intra prediction mode more properly than the
method of FIG. 11 but uses an average value of available intra prediction mode pixel
values or an available intra prediction mode pixel value itself as a pixel value of a
15 neighboring block predicted in the inter prediction mode, without considering variation in
pixel values.
Thus, a CIP method considering variations in pixel value is needed.
Example 5
20 FIG. 13 schematically illustrates that a system according to the present invention
performs CIP in consideration of variations in pixel value.
The method of FIG. 13 using variations in pixel values of both pixels for
interpolation achieves more accurate prediction of a target pixel value than the method of
FIG. 12 using an average value of both pixel values as a pixel value to derive. For
29
example, a target pixel PT among pixel values 1310, 1320 and 1330 to be derived may be
derived by Equation 19.
[Equation 19]
5
Here, PT is a target prediction sample, PLB is a left or lower intra-prediction
sample, and PRA is a right or upper intra-prediction sample. Further, as shown in FIG. 13,
d1 is a distance from PLB to PT, and d2 is a distance from PRA to PT.
For instance, referring to FIG. 13, PT1 may be derived by
10 (PLB1*d21+PRA1*d11)/(d11+d21), and PT2 may be derived by
(PLB2*d22+PRA2*d12)/(d12+d22).
If an intra-prediction sample to be used for interpolation is present on either right
and left sides or either upper and lower sides of the target prediction sample PT only, PT =
PLB or PT = PRA. Further, if there is no block predicted in the intra prediction mode
15 neighboring to the target prediction block T, a pixel value in the same position as in a
previous picture may be copied for use as a reference pixel value.
Average values of intra pixels on the boundary may be used as PLB or PRA value.
For example, in FIG. 3, when PT is located in a lower pixel row 1320 of an E block or D
block, an average value of four lowest pixels of an intra prediction mode C block may be
20 used as PRA, and an average value of eight rightmost pixels of a G block may be used as
PLB. In this case, a reference point of d1 is a top pixel among the rightmost pixels of the
G block, and a reference point of d2 is a leftmost pixel among the lowest pixels of the C
block.
Further, linear interpolation gives smoothing effect on boundary pixels and thus
30
adaptive intra smoothing (AIS) may be off. Here, in the DC prediction mode, filtering
on pixels on a boundary of the prediction block may be on.
FIG. 14 is a flowchart schematically illustrating an operation of the encoder in the
system according to the present invention.
Referring to FIG. 14, a new prediction unit of a current picture is input (S1410)5 .
The prediction unit (PU) may be a basic unit for intra prediction and inter prediction.
The prediction unit may be a smaller block than a coding unit (CU) and may be a
rectangular shape, not necessarily a square shape. Intra prediction of the prediction unit
is basically carried out by a 2N x 2N or N x N block.
10 Subsequently, a reference pixel needed for intra prediction is derived (S1420).
Pixels in a rightmost vertical line of a left block neighboring to a current prediction block
and pixels in a lowest horizontal line of an upper block neighboring to the current
prediction block are used for generating the reference pixel. When the prediction block
has an N size, 2N pixels of the left and upper blocks are used in total as reference pixels.
15 Here, the pixels in the rightmost vertical line of the left block neighboring to the
current prediction block and the pixels in the lowest horizontal row of the upper block
neighboring to the current prediction block may be used as the reference pixels as they are
or via smoothing.
When smoothing is involved, smoothing information may be also signaled to the
20 decoder. For example, when smoothing is performed, an AIS filter may be employed, in
which filter coefficients [1, 2, 1] or [1, 1, 4, 1, 1] may be used. Among these two
coefficients, the latter filter coefficient may provide a sharper boundary. As mentioned
above, information including whether to use a filter, a type of a filter to use and a filter
coefficient may be signaled to the decoder.
25 Meanwhile, when CIP is used for generating the reference pixel, a CIP_flag value
31
is set to 1. When CIP is applied, only pixels of neighboring blocks encoded in the intra
prediction mode are used as reference pixels and pixels of the neighboring blocks encoded
in the inter prediction mode are not used as reference pixels. In this case, as shown in
FIG. 13, pixels (target prediction samples) corresponding to positions of the pixels of the
neighboring blocks encoded in the inter prediction mode are generated as reference pixel5 s
by interpolating the neighboring reference pixels encoded in the intra prediction mode, or
the neighboring reference pixels encoded in the intra prediction mode are copied and used
as reference pixels corresponding to positions of the pixels of the neighboring blocks
encoded in the inter prediction mode.
10 For example, when intra prediction mode prediction pixels are present on both
right and left sides, and upper and lower sides of a target inter-prediction sample, the
target prediction sample PT located in a block predicted in the inter prediction mode may
be derived by Equation 11. Further, when an intra-prediction sample is present only on
either side of the target prediction sample, the target prediction sample PT located in a
15 block location predicted in the inter prediction mode may be derived by Equation 12.
In Equation 11 and/or Equation 12, average values of the corresponding intra prediction
mode pixels may be used as PLB and PRA values. If there is no neighboring block
predicted in the intra prediction mode, a pixel value in the same position as in a previous
picture may be copied for use as a reference pixel value.
20 Since linear interpolation gives smoothing effect on boundary pixels, it may be
effective to turn AIS off in use of CIP.
Subsequently, an intra prediction mode is determined (S1430).
The intra prediction mode is determined by a prediction unit (PU), in which an
optimal prediction mode is determined in view of relationship between required bit rate
25 and amount of distortion.
32
For example, when rate distortion optimization (RDO) is on, a mode to minimize
cost J = R+rD (R is bit rate, D is amount of distortion, and r is a Lagrange variable) may
be selected. Here, thorough local decoding is needed, in which case complexity may
increase.
When RDO is off, a prediction mode to minimize a mean absolute differenc5 e
(MAD) by subjecting a prediction error to Hadamard transform may be selected.
Table 1 illustrates a number of prediction modes with respect to a luma
component according to size of a prediction unit block.
[Table 1]
Block size Number of prediction modes
4 x 4 17
8 x 8 34
16 x 16 34
32 x 32 34
64 x 64 3
10
FIG. 15 illustrates a prediction direction of an intra prediction mode. Referring
to FIG. 15, a mode number 0 is a vertical mode in which prediction is conducted in a
vertical direction using a pixel value of a neighboring block. A mode number 1 is a
horizontal mode in which prediction is conducted in a horizontal direction using a pixel
15 value of a neighboring block. A mode number 2 is a DC mode in which a prediction
block is generated using an average pixel value of a current prediction target block, for
example, a luma value in the case of luma pixels and a chroma value in the case of chroma
pixels. In other modes shown in FIG. 15, prediction is conducted using pixel values of
neighboring blocks depending on corresponding angles.
33
In the DC mode, top prediction pixels and leftmost prediction pixels may be
filtered to enhance prediction efficiency. Here, intensity of filtering may become higher
for a smaller block. The other internal pixels in the current prediction block may not be
filtered.
Meanwhile, a planar mode to reflect directionality may be used instead of the D5 C
mode. In the planar mode, a Planar_flag value among information transmitted from the
encoder to the decoder is set to 1. When the planar mode is used, the DC mode is not
used. Thus, when the DC mode is used instead of the planar mode, the Planar_flag value
is set to 0.
10 When the planar mode is employed, the same prediction methods as described
above in FIGS. 6 to 10 may be used. Here, the decoder may perform an RDO operation
described above so as to select optimal method. If necessary, two or more methods of
the foregoing methods may be used together. The encoder signals to the decoder
information on which method the encoder selects among the prediction methods in the
15 planar mode illustrated in FIGS. 6 to 10.
Regarding a reference pixel of a chroma component, unified directional intra
(UDI) of a luma block may be employed as it is in a mode number 4, which is referred to
as a DM mode. In a mode number 0, a prediction block is generated using linear
relationship between a luma and a chroma, which is referred to as a linear model (LM)
20 mode. A mode number 1 is a vertical mode, in which prediction is carried out in the
vertical direction, and corresponds to the mode number 0 of the luma. A mode number 2
is a horizontal line, in which prediction is carried out in the horizontal direction, and
corresponds to the mode number 1 of the luma. A mode number 3 is a DC mode, in
which a prediction block is generated using an average chroma value of a current
25 prediction target block, and corresponds to the mode number 2 of the luma.
34
Referring back to FIG. 14, the encoder encodes a prediction mode of the current
block (S1440). The encoder encodes a prediction mode for a luma component block and
a chroma component block of the current prediction block. Here, since the prediction
mode of the current prediction target block highly correlates with a prediction mode of a
neighboring block, the current prediction target block is encoded using the predictio5 n
mode of the neighboring block, thereby reducing bit amount. Further, a most probable
mode (MPM) of the current prediction target block is determined, and accordingly the
prediction mode of the current prediction target block may be encoded using the MPM.
Subsequently, a pixel value of the current prediction block and a differential value
10 by a pixel for the pixel value of the prediction block are derived, thereby generating a
residual signal (S1450).
The generated residual signal is transformed and encoded (S1460). The residual
signal may be encoded using a transform kernel, wherein the transform encoding kernel
has a 2 x 2, 4 x 4, 8 x 8, 16 x 16, 32 x 32 or 64 x 64 size.
15 A transform coefficient C is generated for the transform, which may be a 2D
block of transform coefficients. For example, for an n x n block, a transform coefficient
may be calculated by Equation 20.
[Equation 20]
20
Here, C(n, n) is an n * n transform coefficient matrix, T(n, n) is an n * n transform
kernel matrix, and B(n, n) is an n * n matrix for a prediction target block.
When m=hN, n=2N and h=1/2, a transform coefficient C for an m * n or n * m
differential block may be obtained by two methods. First, the m * n or n * m differential
25 block is split into four m * m blocks and a transform kernel is applied to each block,
35
thereby generating the transform coefficient. Alternatively, a transform kernel is applied
to the m * n or n * m differential block, thereby generating the transform coefficient.
The encoder determines which to transmit among the residual signal and the
transform coefficient (S1470). For instance, when prediction is adequately performed,
the residual signal may be transmitted as it is without transform encoding5 .
Determining which to transmit among the residual signal and the transform
coefficient may be carried out by RDO or the like. Cost functions before and after
transform encoding are compared to immunize costs. When a signal type to transmit,
that is, the residual signal or transform coefficient, for the current prediction block is
10 determined, a type of the transmitted signal is also signaled to the decoder.
Subsequently, the encoder scans the transform coefficient (S1480). A quantized
2D block of transform coefficients may be changed into a 1D vector of transform
coefficients by scanning.
The scanned transform coefficient and the intra prediction mode are entropy15
encoded (S1490). The encoded information is formed into a compressed bit stream,
which may be transmitted or stored through an NAL.
FIG. 16 is a flowchart schematically illustrating an operation of the decoder in the
system according to the present invention.
Referring to FIG. 16, the decoder entropy-decodes a received bitstream (S1610).
20 Here, a block type may be obtained from a variable length coding (VLC) table, and a
prediction mode of a current decoding target block may be derived. When the received
bit stream may include side information needed for decoding, such as information on a
coding unit, a prediction unit and a transform unit, information on AIS filtering,
information on limitation of a prediction mode count, information on unused prediction
25 modes, information on rearrangement of prediction modes, information on transform
36
methods and information on scanning methods, the side information is entropy-decoded
along with the bit stream.
The decoded information may confirm whether a transmitted signal for the current
decoding target block is a residual signal or a transform coefficient for a differential block.
A residual signal or 1D vector of transform coefficients for the differential block i5 s
obtained for the current decoding target block.
Subsequently, the decoder generates a residual block (S1620).
The decoder inversely scans the entropy-decoded residual signal or transform
coefficient to generate a 2D block. Here, a residual block ismay be generated from the
10 residual signal, and a 2D block of transform coefficients may be generated from the
transform coefficient.
The transform coefficients are dequantized. The dequantized transform
coefficients are inversely transformed, and the residual block for the residual signal is
generated via inverse transformation. Inverse transformation of an n * n block may be
15 expressed by Equation 11.
The decoder generates reference pixels (S1630). Here, the decoder generates the
reference pixel by referring to information on whether AIS filtering is applied and on a
used filter type signaled and transmitted by the encoder. Likewise in the encoding
process, pixels in a rightmost vertical line of a left block already decoded and
20 reconstructed and neighboring to the current decoding target block and pixels in a lowest
horizontal line of an upper block neighboring to the decoding target block are used for
generating the reference pixel.
Meanwhile, when a CIP_flag value received by the decoder is set to 1, which
means that the encoder uses CIP for a target picture, the decoder generates the reference
25 pixel accordingly. For example, only pixels of neighboring blocks encoded in the intra
37
prediction mode are used as reference pixels, whereas pixels of the neighboring blocks
encoded in the inter prediction mode are not used as reference pixels. In this case, as
illustrated in FIG. 6, pixels (target prediction samples) corresponding to positions of the
pixels of the neighboring blocks encoded in the inter prediction mode are generated as
reference pixels by interpolating the neighboring reference pixels encoded in the intr5 a
prediction mode, or the neighboring reference pixels encoded in the intra prediction mode
may be copied and used as reference pixels corresponding to positions of the pixels of the
neighboring blocks encoded in the inter prediction mode.
For example, when intra prediction mode prediction pixels are present on both
10 right and left sides and upper and lower sides of a target inter-prediction sample, the target
prediction sample PT located in a block predicted in the inter prediction mode is derived
by Equation 17. Further, when an intra-predicted sample is present only on either side of
the target prediction sample, the target prediction sample PT located in a block location
predicted in the inter prediction mode may be derived by Equation 18. In Equation 17
15 and/or Equation 18, average values of the corresponding intra prediction mode pixels may
be used as PLB or PRA values. If there is no neighboring block predicted in the intra
prediction mode, a pixel value in the same position as in a previous picture may be copied
for use as a reference pixel value.
When the encoder employs AIS filtering, that is, when smoothing is applied and
20 thus AIS is on, the decoder also performs AIS filtering in generating the reference pixel
according to the reference pixel generation method used by the encoder. The decoder
may determine a filter coefficient based on filter type information among the received
information. For example, when there are two filter coefficients [1, 2, 1] or [1, 1, 4, 1, 1],
a filter coefficient indicated in the filter type information may be used among the two filter
25 coefficients.
38
Next, a prediction block for the decoding target block is generated using the
reference pixel and the entropy-decoded prediction mode of the current decoding target
block (S1640).
A process of generating the prediction block is the same as a process of
determining the prediction mode and generating the prediction block by the encoder5 .
When the prediction mode of the current block is a planar mode, a planner prediction
method used to generate the prediction block may be identified by analyzing signaled
information. Here, the decoder may generate the prediction block based on the identified
information according to a used mode among the planner modes illustrated in FIGS. 6 to
10 10.
Next, a block reconstructed by adding, by a pixel, a pixel value of the prediction
block and a pixel value of the differential block, that is, a reconstructed block, is generated
(S1670).
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I/We claim:
1. An intra prediction method of an encoder, the method comprising:
generating reference pixels for intra prediction with respect to an input prediction
unit;
determining an intra mode for the prediction unit5 ;
generating a prediction block based on the reference pixels and the intra mode; and
generating a residual block for the prediction unit and the prediction block,
wherein at least one of the reference pixels and pixels of the prediction block is
predicted based on a base pixel, and a pixel value of the predicted pixel is a sum of a pixel
10 value of the base pixel and a variation in pixel value from the base pixel to the generated
pixel.
2. The intra prediction method of claim 1, wherein a reference pixel of a
neighboring block disposed on a top left corner of the prediction block is set as a first base
15 pixel, a value obtained by applying a variation in pixel value from the first base pixel to a
lowest pixel among reference pixels of a neighboring block disposed on a left boundary of
the prediction block and a variation in pixel value from the first base pixel to a rightmost
pixel among reference pixels of a neighboring block disposed on an upper boundary of the
prediction block to the base pixel is set as a pixel value of a second base pixel as a
20 diagonal pixel in a bottom right corner of the prediction block, and pixel values of
diagonal pixels of the prediction block are predicted from the first base pixel and the
second base pixel.
3. The intra prediction method of claim 2, wherein non-diagonal pixels of
25 the prediction block are predicted by interpolation or extrapolation using the diagonal
40
pixels and the pixels of the neighboring blocks on the upper and/or left boundaries of the
prediction block.
4. The intra prediction method of claim 1, wherein a reference pixel of a
neighboring block disposed on a top left corner of the prediction block is set as the bas5 e
pixel, and a value obtained by applying a variation in pixel value from the base pixel to a
neighboring pixel disposed in the same row as a prediction target pixel among reference
pixels of a neighboring block disposed on a left boundary of the prediction block and a
variation in pixel value from the base pixel to a neighboring pixel disposed in the same
10 column as the prediction target pixel among reference pixels of a neighboring block
disposed on an upper boundary of the prediction block to the base pixel is predicted as a
pixel value of the prediction target pixel.
5. The intra prediction method of claim 1, wherein a pixel disposed in the
15 same row or column as a prediction target pixel among pixels of neighboring blocks
disposed on left or upper boundary of the prediction block is set as the base pixel, and a
value obtained by applying a variation in pixel value from the base pixel to the prediction
pixel to the base pixel is predicted as a pixel value of the prediction target pixel.
20 6. The intra prediction method of claim 5, wherein the prediction target
pixel is a diagonal pixel of the prediction block, and a non-diagonal pixel of the prediction
block is predicted by interpolation using the diagonal pixel and the pixels of the
neighboring blocks.
25 7. The intra prediction method of claim 1, further comprising generating a
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reference pixel disposed on a boundary between an inter-mode block and the prediction
unit when a block neighboring to the prediction unit is the inter-mode block, wherein a
pixel disposed on a boundary of the prediction unit among pixels of an intra-mode block
disposed on a left side or lower side of the reference pixel is set as a first base pixel, a
pixel disposed on the boundary of the prediction unit among pixels of an intra-mode bloc5 k
disposed on a right side or upper side of the reference pixel is set as a second base pixel,
and the reference pixel is generated based on a distance from the first base pixel to the
reference pixel and a distance from the second base pixel to the reference pixel.
10 8. The intra prediction method of claim 7, wherein a pixel value of the first
base pixel is an average pixel value of pixels disposed on the boundary of the prediction
unit among the pixels of the intra-mode block that the first base pixel belongs to, and a
pixel value of the second base pixel is an average pixel value of pixels disposed on the
boundary of the prediction unit among the pixels of the intra-mode block that the second
15 base reference belongs to.
9. The intra prediction method of claim 7, wherein a pixel value of the first
base pixel is a pixel value of the reference pixel when an intra-mode block is disposed
only on the left side or lower side of the reference pixel, and a pixel value of the second
20 base pixel is a pixel value of the reference pixel when an intra-mode block is disposed
only on the right side or upper side of the reference pixel.
10. An intra prediction method of a decoder, the method comprising:
entropy-decoding a received bitstream;
25 generating a reference pixel used for intra prediction of a prediction unit;
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generating a prediction block from the reference pixel based on a prediction mode
for the prediction unit; and
reconstructing a picture from a residual block obtained by the entropy-decoding
and the prediction block,
wherein at least one of the reference pixels and pixels of the prediction block i5 s
predicted based on a base pixel, and a pixel value of the predicted pixel is a sum of a pixel
value of the base pixel and a variation in pixel value from the base pixel to the generated
pixel.
10 11. The intra prediction method of claim 10, wherein a reference pixel of a
neighboring block disposed on a top left corner of the prediction block is set as a first base
pixel, a value obtained by applying a variation in pixel value from the first base pixel to a
lowest pixel among reference pixels of a neighboring block disposed on a left boundary of
the prediction block and a variation in pixel value from the first base pixel to a rightmost
15 pixel among reference pixels of a neighboring block disposed on an upper boundary of the
prediction block to the base pixel is set as a pixel value of a second base pixel as a
diagonal pixel in a bottom right corner of the prediction block, and pixel values of
diagonal pixels of the prediction block are predicted from the first base pixel and the
second base pixel.
20
12. The intra prediction method of claim 11, wherein non-diagonal pixels of
the prediction block are predicted by interpolation or extrapolation using the diagonal
pixels and the pixels of the neighboring blocks on the upper and/or left boundaries of the
prediction block.
25
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13. The intra prediction method of claim 10, wherein a reference pixel of a
neighboring block disposed on a top left corner of the prediction block is set as the base
pixel, and a value obtained by applying a variation in pixel value from the base pixel to a
neighboring pixel disposed in the same row as a prediction target pixel among reference
pixels of a neighboring block disposed on a left boundary of the prediction block and 5 a
variation in pixel value from the base pixel to a neighboring pixel disposed in the same
column as the prediction target pixel among reference pixels of a neighboring block
disposed on an upper boundary of the prediction block to the base pixel is predicted as a
pixel value of the prediction target pixel.
10
14. The intra prediction method of claim 10, wherein a pixel disposed in the
same row or column as a prediction target pixel among pixels of neighboring blocks
disposed on a left or upper boundary of the prediction block is set as the base pixel, and a
value obtained by applying a variation in pixel value from the base pixel to the prediction
15 pixel to the base pixel is predicted as a pixel value of the prediction target pixel.
15. The intra prediction method of claim 14, wherein the prediction target
pixel is a diagonal pixel of the prediction block, and a non-diagonal pixel of the prediction
block is predicted by interpolation using the diagonal pixel and the pixels of the
20 neighboring blocks.
16. The intra prediction method of claim 10, further comprising generating a
reference pixel disposed on a boundary between an inter-mode block and the prediction
unit when a block neighboring to the prediction unit is the inter-mode block, wherein a
25 pixel disposed on a boundary of the prediction unit among pixels of an intra-mode block
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disposed on a left side or lower side of the reference pixel is set as a first base pixel, a
pixel disposed on the boundary of the prediction unit among pixels of an intra-mode block
disposed on a right side or upper side of the reference pixel is set as a second base pixel,
and the reference pixel is generated based on a distance from the first base pixel to the
reference pixel and a distance from the second base pixel to the reference pixel5 .
17. The intra prediction method of claim 16, wherein a pixel value of the first
base pixel is an average pixel value of pixels disposed on the boundary of the prediction
unit among the pixels of the intra-mode block that the first base pixel belongs to, and a
10 pixel value of the second base pixel is an average pixel value of pixels disposed on the
boundary of the prediction unit among the pixels of the intra-mode block that the second
base reference belongs to.
18. The intra prediction method of claim 16, wherein a pixel value of the first
15 base pixel is a pixel value of the reference pixel when an intra-mode block is disposed
only on the left side or lower side of the reference pixel, and a pixel value of the second
base pixel is a pixel value of the reference pixel when an intra-mode block is disposed
only on the right side or upper side of the reference pixel.
20 19. The intra prediction method of claim 10, comprising acquiring an
instruction to generate the pixels of the prediction block based on the base pixel through
the entropy-decoding.
25
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20. The intra prediction method of claim 10, comprising acquiring an instruction to
generate the reference pixels based on the base pixel through the entropy-decoding.