Background
Block Transform-Based Coding
Transform coding is a compression technique used in many audio, image and video compression systems. Uncompressed digital image and video is typically represented or captured as samples of picture elements or colors at locations in an image or video frame arranged in a two-dimensional (2D) grid. This is referred to as a spatial-domain representation of the image or video. For example, a typical format for images consists of a stream of 24-bit color picture element samples arranged as a grid. Each sample is a number representing color components at a pixel location in the grid within a color space, such as RGB, or YIQ, among others. Various image and video systems may use various different color, spatial and time resolutions of sampling. Similarly, digital audio is typically represented as time-sampled audio signal stream. For example, a typical audio format consists of a stream of 16-bit amplitude samples of an audio signal taken at regular time intervals.
Uncompressed digital audio, image and video signals can consume considerable storage and transmission capacity. Transform coding reduces the size of digital audio, images and video by transforming the spatial-domain representation of the signal into a frequency-domain (or other like transform domain) representation, and then reducing resolution of certain generally less perceptible frequency components of the transform-domain representation. This generally produces much less perceptible degradation of the digital signal compared to reducing color or spatial resolution of images or video in the spatial domain, or of audio in the time domain.
More specifically, a typical block transform-based codec 100 shown in Figure 1 divides the uncompressed digital image's pixels into fixed-size two dimensional blocks (Xi, ... Xn), each block possibly overlapping with other blocks. A linear transform 120-121 that does spatial-frequency analysis is applied to each block, which converts the spaced samples within the block to a set of frequency (or transform) coefficients generally representing the strength of the digital signal in corresponding frequency bands over the block interval. For compression, the transform coefficients may be selectively quantized 130 (i.e., reduced in resolution,
such as by dropping least significant bits of the coefficient values or otherwise mapping values in a higher resolution number set to a lower resolution), and also entropy or variable-length coded 130 into a compressed data stream. At decoding, the transform coefficients will inversely transform 170-171 to nearly reconstruct the original color/spatial sampled image/video signal (reconstructed blocks
X.....XJ.
The block transform 120-121 can be defined as a mathematical operation on a vector x of size N. Most often, the operation is a linear multiplication, producing the transform domain output y = M x, M being the transform matrix. When the input data is arbitrarily long, it is segmented into N sized vectors and a block transform is applied to each segment. For the purpose of data compression, reversible block transforms are chosen. In other words, the matrix M is invertible. In multiple dimensions (e.g., for image and video), block transforms are typically implemented as separable operations. The matrix multiplication is applied separably along each dimension of the data (i.e., both rows and columns).
For compression, the transform coefficients (components of vector y) may be selectively quantized (i.e., reduced in resolution, such as by dropping least significant bits of the coefficient values or otherwise mapping values in a higher resolution number set to a lower resolution), and also entropy or variable-length coded into a compressed data stream.
At decoding in the decoder 150, the inverse of these operations (dequantization/entropy decoding 160 and inverse block transform 170-171) are applied on the decoder 150 side, as show in Fig. 1. While reconstructing the data, the inverse matrix M' (inverse transform 170-171) is applied as a multiplier to the transform domain data. When applied to the transform domain data, the inverse transform nearly reconstructs the original time-domain or spatial-domain digital media.
In many block transform-based coding applications, the transform is desirably reversible to support both lossy and lossless compression depending on the quantization factor. With no quantization (generally represented as a quantization factor of 1) for example, a codec utilizing a reversible transform can
exactly reproduce the input data at decoding. However, the requirement of reversibility in these applications constrains the choice of transforms upon which the codec can be designed.
Many image and video compression systems, such as MPEG and Windows Media, among others, utilize transforms based on the Discrete Cosine Transform (DCT). The DCT is known to have favorable energy compaction properties that result in near-optimal data compression. In these compression systems, the inverse DCT (IDCT) is employed in the reconstruction loops in both the encoder and the decoder of the compression system for reconstructing individual image blocks.
Lapped Transforms
In the above described block transform-based coding systems, a block transform is a finite length (typically a short length such as 4 or 8) transform that is applied in succession to non-overlapping adjacent blocks of the input signal or image. Thus, signal components straddling block boundaries do not influence the transform of the block across the boundary. Due to quantization of the high frequency components for compression of data, use of block transforms can introduce perceptible artifacts at block boundaries, or blockiness. Blockiness is apparent in highly compressed JPEG images and shows up as square blocks or staircase shapes in the image. In audio, blockiness leads to periodic popping noise. Neither of these is a tolerable artifact.
The lapped transform (LT 210 illustrated in Figure 2) is an alternative means of representing a signal or image that does not suffer from sharp blockiness. In a lapped transform, the input signal components influencing each transform coefficient set are larger than the size of the transform output block. For instance in a ID case, 8 successive signal components may influence the 4 point transform. Likewise for images, an 8x8 area may influence a 4x4 transform block.
Lapped transforms may be formulated in one of two ways. One classical formulation of a lapped transform is a series of block transforms followed by a series of frequency mixers. The block transforms are aligned to the regular grid of N points (N being the transform size), whereas the frequency mixers are spaced symmetrically across the block boundaries. An alternative formulation has a pre filtering operation performed across block edges followed by a block transform.
Inverses of lapped transforms (e.g., ILT 220 of Figure 2) generally are straightforward to compute and implement. The signal flow graph is reversed, with each elementary operation being inverted. One classical forumulation of an inverse lapped transform is a series of frequency mixers followed by a series of block transforms. An alternative formulation comprises a series of block transforms followed by post-filtering operations applied across block boundaries.
In either formulation of lapped transforms, the key components are (i) block transforms and (ii) operators straddling blocks, which may be frequency mixers, pre- or post- filters. These operators (ii) are referred to collectively as overlap filters.
Lapped orthogonal transforms (LOTs) are a subclass of lapped transforms. These have the property that the forward and inverse transforms are transposes. From the compression standpoint, the subclass lapped biorthogonal transforms are more interesting since they can achieve better PSNR than LOTs. Biorthogonality refers to the analysis and synthesis basis functions being biorthogonal (i.e. mutually orthogonal). Summary
A digital media coding and decoding technique and realization of the technique in a digital media codec described herein achieves speed-up of the transform used for encoding and decoding. This technique reformulates a lapped (or other) transform as a set of operations that are largely single instruction, multiple data (SIMD) friendly. This is achieved by remapping the input and output sampling grids of the lapped transform. By this remapping, the input data can be grouped into "vectors" or parallel units. With this rearrangement, many of the lapped transform steps can be executed as vector operations. The few remaining operations that are not vectorizable are performed on the vector components in a sequential manner.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
Brief Description Of The Drawings
Figure 1 is a block diagram of a conventional block transform-based codec in the prior art.
Figure 2 is a flow diagram illustrating an example of a lapped transform.
Figure 3 is a flow diagram of a representative encoder incorporating the adaptive coding of wide range coefficients.
Figure 4 is a flow diagram of a decoder incorporating the decoding of adaptively coded wide range coefficients.
Figure 5 is a flow diagram illustrating an example lapped transform formulation as a pre-filter (or overlap operator) and block transform, where the pre-filter is applied across input boundaries or block edges of the block transform.
Figure 6 is a signal flow graph of a representative lapped transform having the pre-filter and block transform formulation of Figure 5.
Figure 7 is a signal flow graph of a parallelized SIMD version of a representative lapped biorthogonal transform having the pre-filter and block transform formulation of Figure 6.
Figure 8 is a diagram illustrating grouping of one-dimensional data into 2-component vectors used in the parallelized SIMD version of the one-dimensional lapped biorthogonal transform of Figure 7.
Figure 9 is a vector signal flow graph of the one-dimensional lapped biorthogonal transform of Figure 7.
Figure 10 is a diagram illustrating grouping of two-dimensional data into 4-component vectors used in the parallelized SIMD version of the two-dimensional lapped biorthogonal transform.
Figure 11 is a diagram illustrating a vector notation for the two-dimensional data as per the grouping into vectors as shown in Figure 10.
Figure 12 is a diagram illustrating pixel components in the two-dimensional data and corresponding parallelized component vectors over which an overlap operator (pre-filter) portion of the two-dimensional lapped biorthogonal transform is applied, and to which a 2x2 Hadamard operator portion of the overlap operator is applied.
Figure 13 is a diagram illustrating pixel components in the two-dimensional data and corresponding parallelized component vectors over which a block transform portion of the two-dimensional lapped biorthogonal transform is applied, and to which a 2x2 Hadamard operator portion of that block transform is applied.
Figure 14 is a diagram illustrating an overlap operator of the two-dimensional lapped biorthogonal transform.
Figure 15 is a flow diagram illustrating a process implementing the overlap operator of the parallelized two-dimensional lapped biorthogonal transform.
Figure 16 is a flow diagram illustrating a process implementing the block transform of the parallelized two-dimensional lapped biorthogonal transform.
Figure 17 is a block diagram of a suitable computing environment for implementing the parallelized SIMD version of the representative encoder/decoder of Figures 3 and 4. Detailed Description
The following description relates to coding and decoding techniques that provide a faster implementation of lapped transform as parallelized or SIMD operations [hereafter "transform parallelization technique"]. The following description describes an example implementation of the technique in the context of a digital media compression system or codec. The digital media system codes digital media data in a compressed form for transmission or storage, and decodes the data for playback or other processing. For purposes of illustration, this exemplary compression system incorporating this transform parallelization technique is an image or video compression system. Alternatively, the technique also can be incorporated into compression systems or codecs for other 2D data. The transform parallelization technique does not require that the digital media compression system encodes the compressed digital media data in a particular coding format.
1. Encoder/Decoder
Figures 3 and 4 are a generalized diagram of the processes employed in a representative 2-dimensional (2D) data encoder 300 and decoder 400. The diagrams present a generalized or simplified illustration of a compression system incorporating the 2D data encoder and decoder that implement the transform
parallelization technique. In alternative compression systems using the transform parallelization technique, additional or fewer processes than those illustrated in this representative encoder and decoder can be used for the 2D data compression. For example, some encoders/decoders may also include color conversion, color formats, scalable coding, lossless coding, macroblock modes, etc. The compression system (encoder and decoder) can provide lossless and/or lossy compression of the 2D data, depending on the quantization which may be based on a quantization parameter varying from lossless to lossy.
The 2D data encoder 300 produces a compressed bitstream 320 that is a more compact representation (for typical input) of 2D data 310 presented as input to the encoder. For example, the 2D data input can be an image, a frame of a video sequence, or other data having two dimensions. The 2D data encoder tiles 330 the input data into macroblocks, which are 16x16 pixels in size in this representative encoder. The 2D data encoder further tiles each macroblock into 4x4 blocks. A "forward overlap" operator 340 is applied to each edge between blocks, after which each 4x4 block is transformed using a block transform 350. This block transform 350 can be the reversible, scale-free 2D transform described by Srinivasan, U.S. Patent Application No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless 2-D Data Compression," filed December 17, 2004, the disclosure of which is hereby incorporated herein by reference. The overlap operator 340 can be the reversible overlap operator described by Tu et al., U.S. Patent Application No. 11/015,148, entitled, "Reversible Overlap Operator for Efficient Lossless Data Compression," filed December 17, 2004, the disclosure of which is hereby incorporated herein by reference; and by Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005, the disclosure of which is hereby incorporated herein by reference. The overlap operator and transform together effect a lapped biorthogonal transform. Alternatively, the discrete cosine transform or other block transforms and overlap operators can be used. Subsequent to the transform, the DC coefficient 360 of each 4x4 transform block is subject to a similar processing chain (tiling, forward overlap, followed by 4x4 block
transform). The resulting DC transform coefficients and the AC transform coefficients are quantized 370, entropy coded 380 and packetized 390.
The decoder performs the reverse process. On the decoder side, the transform coefficient bits are extracted 410 from their respective packets, from which the coefficients are themselves decoded 420 and dequantized 430. The DC coefficients 440 are regenerated by applying an inverse transform, and the plane of DC coefficients is "inverse overlapped" using a suitable smoothing operator applied across the DC block edges. Subsequently, the entire data is regenerated by applying the 4x4 inverse transform 450 to the DC coefficients, and the AC coefficients 442 decoded from the bitstream. Finally, the block edges in the resulting image planes are inverse overlap filtered 460. This produces a reconstructed 2D data output.
In an exemplary implementation, the encoder 300 (Figure 3) compresses an input image into the compressed bitstream 320 (e.g., a file), and the decoder 400 (Figure 4) reconstructs the original input or an approximation thereof, based on whether lossless or lossy coding is employed. The process of encoding involves the application of a forward lapped transform (LT) discussed below, which is implemented with reversible 2-dimensional pre-/post-filtering also described more fully below. The decoding process involves the application of the inverse lapped transform (ILT) using the reversible 2-dimensional pre-/post-filtering.
The illustrated LT and the ILT are inverses of each other, in an exact sense, and therefore can be collectively referred to as a reversible lapped transform. As a reversible transform, the LT/ILT pair can be used for lossless image compression.
The input data 310 compressed by the illustrated encoder 300/decoder 400 can be images of various color formats (e.g., RGB/YUV4:4:4, YUV4:2:2 or YUV4:2:0 color image formats). Typically, the input image always has a luminance (Y) component. If it is a RGB/YUV4:4:4, YUV4:2:2 or YUV4:2:0 image, the image also has chrominance components, such as a U component and a V component. The separate color planes or components of the image can have different spatial resolutions. In case of an input image in the YUV 4:2:0 color format for example, the U and V components have half of the width and height of the Y component.
As discussed above, the encoder 300 tiles the input image or picture into macroblocks. In an exemplary implementation, the encoder 300 tiles the input image into 16x16 macroblocks in the Y channel (which may be 16x16, 16x8 or 8x8 areas in the U and V channels depending on the color format). Each macroblock color plane is tiled into 4x4 regions or blocks. Therefore, a macroblock is composed for the various color formats in the following manner for this exemplary encoder implementation:
1. For a grayscale image, each macroblock contains 16 4x4 luminance (Y)
blocks.
2. For a YUV4:2:0 format color image, each macroblock contains 16 4x4 Y
blocks, and 4 each 4x4 chrominance (U and V) blocks.
3. For a YUV4:2:2 format color image, each macroblock contains 16 4x4 Y
blocks, and 8 each 4x4 chrominance (U and V) blocks.
4. For a RGB or YUV4:4:4 color image, each macroblock contains 16
blocks each of Y, U and V channels.
2. Fast SIMD Lapped biorthogonal Transform Overview One of the more computationally complex operations in the above-described representative encoder 300 (Figure 3) and decoder 400 (Figure 4) is the lapped biorthogonal transform. The complexity of this operation impacts the performance of both the encoder and the decoder.
The implementation of the lapped biorthogonal transform that is described in the patent applications (Srinivasan, U.S. Patent Application No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless 2-D Data Compression," filed December 17, 2004; Tu et al., U.S. Patent Application No. 11/015,148, entitled, "Reversible Overlap Operator for Efficient Lossless Data Compression," filed December 17, 2004; and Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005) is designed to minimize complexity. However, the transform parallelization techniques described herein achieve a further speed-up by formulating the lapped transform operations in a SIMD (single instruction, multiple data) or parallel-instruction friendly manner. The SIMD operations can be used to compute multiple instructions in parallel. Such SIMD
instructions are supported on a variety of processors, including the Pentium® family processors from Intel, various x86-compatible processors from AMD, PowerPC® and a variety of other DSPs (digital signal processors).
The transform parallelization technique described herein reformulates a lapped (or other) transform as a set of operations that are largely SIMD friendly. This is achieved by remapping the input and output sampling grids of the lapped transform. By this remapping, the input data can be grouped into "vectors" or parallel units. With this rearrangement, many of the lapped transform steps can be executed as vector operations. The few remaining operations that are not vectorizable are performed on the vector components in a sequential manner.
Although the technique can be applied to lapped transforms in general, a specific application of the technique to the lapped biorthogonal transform of the representative encoder and decoder (i.e., the lapped biorthogonal transform detailed in the above-listed patent applications) is discussed herein below for purposes of illustration. The transform parallelization technique remaps and groups the input sampling grid or lattice of the representative lapped biorthogonal transform such that each group of data samples can be treated as a vector for many of the operations implementing the lapped transform. In this particular lapped biorthogonal transform example, the techniques are applied to formulate SIMD-friendly versions of 4-point overlap operators and 4-point block transforms, but the techniques can be generalized to other transform lengths as well. Further, the technique alternatively can be applied to create SIMD or parallel instruction versions of other lapped transform realizations.
The following sections detail both one- and two-dimensional SIMD-friendly implementations of the representative lapped biorthogonal transform. In the one dimensional case, two elements may be grouped together into a vector, and many of the ID lapped transform operations may be performed using vector operations. In the two dimensional case, two or four elements may be grouped together into a vector, and many of the lapped transform operations may be performed using vector operations.
These vectorization techniques are equally applicable to the forward and inverse transforms (used by the encoder and decoder, respectively).
2.1 SIMD Realization of One-dimensional Lapped biorthogonal Transform
With reference to Figure 5, consider a general case of a lapped transform 500 formulated as a pre-filter (overlap operator) 510 and block transform 520. In the illustrated example case, the block transform 520 has a block size of 4, and the pre-filter 510 has an overlap size of 4 as well. The overlap size is defined as the pre/post filter length. Thus, if the data sequence is numbered x0, xh x2, x3, etc., the lapped transform 500 proceeds as follows:
1. The pre-filter 510 is applied to each set of input data [x4i+2, X4i+3,
x4j+5];and
2. The block transform 520 is applied to each set [x4i, x^+i, x^+2,
In alternative implementations, the lapped transform can be defined with other, different block transform size and overlap size.
Figure 6 illustrates a more specific example of a lapped biorthogonal transform 600 that has the pre-filter and block transform formulation as illustrated in Figure 5. The lapped biorthogonal transform 600 is that described above as being used in the representative encoder 300 (Figure 3) and decoder 400 (Figure 4), whose implementation is detailed more specifically in the patent applications: Srinivasan, U.S. Patent Application No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless 2-D Data Compression," filed December 17, 2004; Tu et al., U.S. Patent Application No. 11/015,148, entitled, "Reversible Overlap Operator for Efficient Lossless Data Compression," filed December 17, 2004; and Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005. For simplicity, the pre-filter and block transform of the encoder 300 are depicted in Figure 6. The post-filter and inverse block transform of the inverse lapped transform for the decoder is an inverse of the forward lapped transform 600. As shown in Figure 6, the pre-filter has an implementation as a set of butterfly or lifting step operations organized as a first butterfly stage 610, rotation/scaling 620, and second butterfly stage 630. The block transform has an implementation as a third butterfly stage 640 and a rotation 650.
One way to parallelize operations for realization using SIMD instructions is by simply grouping together like-indexed signal components across blocks. In other words, the components of the form x^+j for some j are grouped together. For the specific lapped biorthogonal transform 600 example considered here, vectors of 2 components can be: [x,4 x)8], [x,5 x,9], [x,6 x20], and [x,7 x21].
This grouping works well for the pre-filter. However for the block transform, the vectors [x,4 x,g] and [x16 x20] straddle three, and not two blocks. This means that this grouping cannot be used to achieve overall speed up the lapped transform. At the transform stage, the desired grouping is different: [x16 x20], [x,7 *2i], [xi8 x22], and [x19 x23].
Comparing the desired groupings for the pre-filter and block transform, it can be seen that two of the vectors are common to both groupings (i.e., [x]6 x20] and [x)7 x2i]). However, the remaining two vectors are different between the groupings, which would necessitate regrouping of vectors between the pre-filter and block transform. This is not a desirable solution.
On the other hand, the transform parallelization technique presents an alternative way to parallelize the ID lapped transform. With the alternative technique, a permutation is added between certain components before or after the lapped transform, such that the groupings of components into SIMD instruction vectors are common to both the pre-filter and block transform stages.
Figure 7 shows a modified realization 700 of the lapped biorthogonal transform of Figure 6, which has been parallelized according to the transform parallelization technique described herein. This modified lapped transform realization 700 is functionally identical to the lapped biorthogonal transform implementation 600 of Figure 6, but includes a twist or permutation 710 of components in the first stage, followed by a slightly different network of butterflies 720, 740 and 750. These butterfly stages can be implemented in parallel with 2 component vectors, since for these stages odd components interact only with odd components and even components interact only with even components. Further, the operations for odd components and even components are identical in these stages. Thus, grouping of adjacent odd and even components realizes a parallel implementation.
Nevertheless, some of the stages of the SIMD realization 700 of the lapped biorthogonal transform still are not parallelizable. The rotation/scaling step 730 in the pre-filter, and the rotation step 760 in the block transform are implemented sequentially.
Figure 9 depicts a realization 900 of the lapped biorthogonal transform 700 (Figure 7) using the arrangement of the data into 2-component vectors as shown in Figure 8. In Figure 9, the data paths are 2-component vector valued, and the bold arrows are in-vector operations (i.e., operations between components of the same vector). The vector grouping shown in Figure 8 is used for the input, which is based on the following component-to-vector mapping rule:
V2i = [X4i
V2i+l = [*4i+
This mapping groups the original signal into 2-component vectors, to which SIMD arithmetic is applied for many of the lapped transform steps, and sequential processing is applied for the remaining steps.
2.2 SIMD Realization of Two-dimensional Lapped biorthogonal Transform
The 2-dimensional lapped biorthogonal transform (2D LBT) can be implemented using the 1-dimensional lapped biorthogonal transform (ID LBT) just described. In such implementation, the ID LBT is applied to each row of the image followed by a ID LBT applied to each column (or vice versa). In this case, two types of vectorization techniques may be used:
1. In the first type of vectorization, the same grouping used in the ID LBT
(as described in section 2.1 above) may be used for both the horizontal
and vertical transforms.
2. In the second type of vectorization, the vectors may be formed by
grouping together like-indexed components of multiple rows while
implementing the ID LBT along rows, and by grouping together like-
indexed components of multiple columns while implementing the ID
LBT along columns.
In both these techniques, the vectorization changes between the row and column transforms. This incurs an additional cost of remapping from one
vectorization format to another during the computation of the transform, which may be expensive. An alternative vectorization technique that does not involve reshuffling between transform stages is described below.
Further, the 2D LBT described in the above listed patent applications (i.e., Srinivasan, U.S. Patent Application No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless 2-D Data Compression," filed December 17, 2004; and Tu et al, U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005) implements the LBT directly in 2 dimensions. This transform cannot be separated into two ID operations.
For a parallelized SIMD version of this direct 2D LBT implementation (and also for the separable 2D implementation), a bidirectionally twisted remapping 1000-1001 is first applied as shown in Figure 10. Each 4x4 block of pixels within an area 1010 is mapped 1000-1001 into four 4-component vectors within area 1020, such that each vector contains pixels from the 2x2 sub-blocks of the 4x4 block. The ordering of components within vectors follows a two dimensional extension of the ID remapping (the permutation 710 shown in Figure 7) described above. Figure 11 shows a vector notation 1100 for the resulting set of 4-component vectors in area 1020.
The 4-component vectors thus formed have the property that groups of 4 pixels to which Hadamard transforms are applied either in the overlap operator stage or in the block transform stage of the direct 2D LBT are aligned in the same position within the vectors. This is illustrated in Figure 12 for the overlap operator and in Figure 13 for the Photon Core Transform, and is explained in detail below.
2.2.1 Parallel Implementation Of The Overlap Operator In The SIMD Realization Of Two-Dimensional Lapped Biorthogonal Transform
With reference again to Figure 5, the overlap operator (pre-filter 510) in a lapped transform is applied across block boundaries. This may be done either before or after the block transform 520.
In the case of the 2D LBT implementation described in the above-listed patent applications (i.e., Srinivasan, U.S. Patent Application No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless 2-D Data Compression,"

filed December 17, 2004; and Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005), the overlap operator is applied prior to the block transform on the encoder side. Likewise, it is applied after the inverse block transform on the decoder side. Disregarding the special cases at boundaries of the image, the overlap operator is applied to a 4x4 area straddling 4 4x4 blocks.
With reference to Figure 14, the overlap operator 1400 of this 2D LBT implementation consists of two 2x2 Hadamard transforms 1410 applied to quads of pixels located symmetrically in the grid, followed by a rotation & scaling stage 1420, and 1430, followed by another 2x2 Hadamard transform 1440 applied to the same pixel quads. Details of the operations are presented by Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005. A further simplification can be used in the 2D LBT formulation as described in this patent application, where the scaling stage and one of the 2x2 Hadamard stages cancel out some operations.
For the parallelized SIMD version of this overlap operator, the same vectorization procedure described in section 2.2 above and shown in Figures 10 and 11 is first applied. With reference to Figure 15, the parallelized SIMD version of the overlap operator based on this vectorized data is implemented according to the following process 1500:
1. As indicated at action 1510, the image or other 2 dimensional data
working area is vectorized into 4-component vectors as shown in Figures
10 and 11.
2. The overlap operation in actions 1520-1570 is performed on each 4x4
overlap area straddling 4 4x4 blocks 1200 over the image, as illustrated
in Figure 12. For this operation, the vectors identified as [v3 v6 v9 v]2]
using the vector notation shown in Figure 11 are used. These steps are
repeated for all such areas.
3. First, the 2x2 Hadamard operation is performed among these 4 vectors at
action 1530.
4. For the next action 1540, the scaling operation (which is detailed in the
patent applications: Tu et al., U.S. Patent Application No. 11/015,148,
entitled, "Reversible Overlap Operator for Efficient Lossless Data
Compression," filed December 17, 2004; and Tu et al., U.S. Patent
Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre¬
test-Filtering For Lapped Biorthogonal Transform," filed January 14,
2005) is performed between the vectors v$ and v^.
5. Rotations 1550 are performed within components of the vectors, v6, v9
and v)2. These are mostly sequential operations that largely do not
exploit parallelism of data.
6. Finally, the 2x2 Hadamard operation is again performed at action 1560
among the four vectors [v3 V6 vg Vi2] of the overlap area.
In the process 1500, the above operations are performed in-place on the indicated vectors. Further, in practice, there are some cancellations between the steps 3 and 4 above which lead to further simplifications, as detailed in the patent applications: Tu et al., U.S. Patent Application No. 11/015,148, entitled, "Reversible Overlap Operator for Efficient Lossless Data Compression," filed December 17, 2004; and Tu et al., U.S. Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January 14, 2005.
2.2.2 Parallel Implementation Of The Block Transform In The SIMD Realization Of Two-Dimensional Lapped Biorthogonal Transform
After the overlap operator is applied to all 2x2 subblocks within a block, the 4x4 block 1300 (Figure 13) is ready to be block transformed. The block transform operation keeps the same vectorization - hence it is not necessary to shuffle data between the overlap and block transform operations.
With reference to Figure 16, the parallel implementation of the block transform is performed according to the following process 1600. The process begins with the image or working area still vectorized by the action 1510 (Figure 15) for the overlap operator as shown in Figures 10 and 11. On the other hand, in instances where the block transform is being applied to the 2D data alone without
the overlap operator process 1500 being first applied, the process 1600 instead begins by performing the action 1510 to provide the same vectorization.
1. In the loop of actions 1610-1640, the transform is applied to each 4x4
block 1300 of the image. For example, the vectors [VQ v\ \2 v^] shown in
Figure 13 are used for the top left block. These steps are repeated for all
blocks.
2. At a first action 1620, the 2x2 Hadamard operation is performed among
these 4 vectors.
3. At next action 1630, rotations are performed within components of the
vectors, v0, v1; v2 and v3. These are mostly sequential operations that
largely do not exploit parallelism of data. The rotations performed are as
detailed in the patent applications: Srinivasan, U.S. Patent Application
No. 11/015,707, entitled, "Reversible Transform For Lossy And Lossless
2-D Data Compression," filed December 17, 2004; and Tu et al., U.S.
Patent Application No. 11/035,991, entitled, "Reversible 2-Dimensional
Pre-/Post-Filtering For Lapped Biorthogonal Transform," filed January
14, 2005.
In alternative implementations of the SIMD lapped transform, the transform operations applied to the vectors of the block can be those of other DCT-like transforms (instead of the reversible transform described in the above-listed patent applications).
2.3 Extensions
For both the overlap operator 1500 and transform 1600 processes, a four way 2x2 Hadamard transform is a fundamental and repeated operation. With the data components ordered by the vectorization illustrated in Figures 10 and 11, the 2x2 Hadamard is easily performed as SIMD instructions operating on these vectors. Further, for the overlap operator, the scaling operation likewise can be performed as SIMD instructions that operate on these vectors. The rotations (actions 1550, 1630) are partially parallelizable. This is so because some of the rotations involved are identical ID operations that are performed for two pairs of data points within the 4 component vector. These rotations can also be parallelized with multiply and shift operations.
Due to reordering of data components in the vectors, the final output of the transform is also re-ordered. This is typically not an issue because the transform is scanned to order the coefficients as a list for output by the encoder in the compressed bitstream. In the parallel implementation, the scan array takes into account re-ordering and has no negative impact on the algorithm complexity.
The same parallelization technique holds for the inverse lapped biorthogonal transform, except the order of block transform and overlap operator is reversed, and the order of actions 1530-1560 and 1620-1630 in the respective process is reversed. The reordered scan pattern is used to populate the input data array, and the output is afterwards remapped in a manner inverse to the mapping shown in Figure 10.
The parallelization technique also holds for alternative implementations using other versions of lapped orthogonal / biorthogonal transforms. As noted in the discussion of the block transform process 1600, the parallelization may be used for block transforms by themselves (i.e. without the overlap operator) as well. Transform and overlap sizes other than 4, and dimensions greater than 2 may also be accommodated with straightforward extension of the parallelization logic.
The cost of vectorization is minimized by performing the remapping to the twisted lattice on the encoder, and remapping from the twisted lattice on the decoder, during the stage of color conversion. Color conversion in the decoder is generally implemented sequentially due to several reasons including (i) multitude of color formats, (ii) lack of word alignment due to 24 bit pixel boundaries of many color formats, (iii) need to perform clipping on decoder side, and etc. The additional cost of remapping over and above color conversion is minimal and facilitates use of this parallelization technique for overall performance improvement. Further, when the input image is presented in a rotated and/or laterally inverted orientation or when the output image is desired in a rotated and/or laterally inverted orientation, this can be achieved with almost no increase in the overall computational complexity.
3. Computing Environment
The above described representative encoder 300 (Figure 3) and decoder 400 (Figure 4) incorporating the Lapped Biorthogonal Transform implemented using the transform parallelization techniques can be performed on any of a variety of
devices in which digital media signal processing is performed, including among other examples, computers; image and video recording, transmission and receiving equipment; portable video players; video conferencing; and etc. The digital media coding techniques can be implemented in hardware circuitry, as well as in digital media processing software executing within a computer or other computing environment, such as shown in Figure 17.
Figure 17 illustrates a generalized example of a suitable computing environment (1700) in which described embodiments may be implemented. The computing environment (1700) is not intended to suggest any limitation as to scope of use or functionality of the invention, as the present invention may be implemented in diverse general-purpose or special-purpose computing environments.
With reference to Figure 17, the computing environment (1700) includes at least one processing unit (1710) and memory (1720). In Figure 17, this most basic configuration (1730) is included within a dashed line. The processing unit (1710) executes computer-executable instructions and may be a real or a virtual processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. The memory (1720) may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two. The memory (1720) stores software (1780) implementing the described digital media encoding/decoding and transform parallelization techniques.
A computing environment may have additional features. For example, the computing environment (1700) includes storage (1740), one or more input devices (1750), one or more output devices (1760), and one or more communication connections (1770). An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing environment (1700). Typically, operating system software (not shown) provides an operating environment for other software executing in the computing environment (1700), and coordinates activities of the components of the computing environment (1700).
The storage (1740) may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, CD-RWs, DVDs, or any
other medium which can be used to store information and which can be accessed within the computing environment (1700). The storage (1740) stores instructions for the software (1780) implementing the described encoder/decoder using the transform parallelization techniques.
The input device(s) (1750) may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing environment (1700). For audio, the input device(s) (1750) may be a sound card or similar device that accepts audio input in analog or digital form, or a CD-ROM reader that provides audio samples to the computing environment. The output device(s) (1760) may be a display, printer, speaker, CD-writer, or another device that provides output from the computing environment (1700).
The communication connection(s) (1770) enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, compressed audio or video information, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media include wired or wireless techniques implemented with an electrical, optical, RF, infrared, acoustic, or other carrier.
The digital media processing techniques herein can be described in the general context of computer-readable media. Computer-readable media are any available media that can be accessed within a computing environment. By way of example, and not limitation, with the computing environment (1700), computer-readable media include memory (1720), storage (1740), communication media, and combinations of any of the above.
The digital media processing techniques herein can be described in the general context of computer-executable instructions, such as those included in program modules, being executed in a computing environment on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program
modules may be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules may be executed within a local or distributed computing environment.
For the sake of presentation, the detailed description uses terms like "determine," "generate," "adjust," and "apply" to describe computer operations in a computing environment. These terms are high-level abstractions for operations performed by a computer, and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.
In view of the many possible variations of the subject matter described herein, we claim as our invention all such embodiments as may come within the scope of the following claims and equivalents thereto.

We claim:
1. A method of encoding digital media data, the method comprising:
re-mapping (710, 1000, 1001) components of blocks of input digital media
data (1010) into a set of vectors (1020, 800) on which operations (720, 730, 740, 750, 760, 1410, 1420, 1430, 1440) of a transform can be applied across the components of blocks on a single instruction, multiple data basis;
applying the transform (350) to blocks of the digital media data to produce a set of transform coefficients (360, 362) for the respective blocks, wherein applying the transform comprises performing at least one operation on a single instruction, multiple data basis on the vectors of components for a block; and
encoding (380) the transform coefficients in a compressed bitstream.
2. The method of claim 1, wherein the transform is a lapped biorthogonal
transform comprising an overlap filter and a block transform, the block transform
being applied to blocks of the input digital media data and the overlap filter being
applied to overlap areas overlapping adjoining blocks; and
wherein said re-mapping groups components into vectors on which at least one operation of the overlap filter and at least one operation of the block transform can be applied across the components on a single instruction, multiple-data basis; and
wherein said applying the transform comprises applying said at least one operation of the overlap filter and said at least one operation of the block transform on a single instruction, multiple data basis on the vectors.
3. The method of claim 2, wherein the at least one operation of the overlap
filter and the at least one operation of the block transform each comprise a 2x2
Hadamard transform.
4. The method of claim 2, wherein the overlap filter and the block transform
each comprise a rotation operation applied to the components on a sequential
instruction basis.
5. The method of claim 2, wherein the vectors are 4-component vectors.
6. The method of claim 1, wherein the transform is a one-dimension lapped
transform comprising an overlap filter and block transform, the block transform
being applied to blocks of the input digital media data and the overlap filter being applied to overlap areas overlapping adjoining blocks; and
wherein said re-mapping groups components into vectors on which at least one operation of the overlap filter and at least one operation of the block transform can be applied across the components on a single instruction, multiple-data basis; and
wherein said applying the transform comprises applying said at least one operation of the overlap filter and said at least one operation of the block transform on a single instruction, multiple data basis on the vectors.
7. The method of claim 6, wherein the at least one operation of the overlap
filter and the at least one operation of the block transform each comprise a butterfly
stage.
8. The method of claim 6, wherein the overlap filter and the block transform
each comprise a rotation operation applied to the components on a sequential
instruction basis.
9. The method of claim 6, wherein the vectors are 2-component vectors.
10. The method of claim 6, wherein the digital media data is two-dimensional
data and the transform is a one-dimensional transform, the method further
comprising:
performing said re-mapping and applying the transform to rows of the two-dimensional media data; and
performing said re-mapping and applying the transform to columns of the two-dimensional media data.
11. A method of decoding digital media data encoded according to the method
of claim 1, the decoding method further comprising:
decoding the transform coefficients from the compressed bitstream;
ordering the decoded transform coefficients in an arrangement of vectors on which operations of an inverse of the transform can be applied across the transform coefficients on a single instruction, multiple data basis;
applying the inverse of the transform to blocks of the decoded transform coefficients to reconstruct a representation of the digital media data in blocks,
wherein applying the transform comprises performing at least one operation on a single instruction, multiple data basis on the vectors of transform coefficients; and
re-mapping components of the vectors to an initial arrangement of the digital media data.
12. At least one computer-readable recording medium carrying the compressed
bitstream encoded according to the method of claim 1.
13. A digital media encoder and/or decoder comprising:
a data storage buffer for storing digital media data (310, 320) to be encoded and/or decoded;
a processor (1710) programmed to:
order elements of blocks (1010) of digital media data to/from a set of vectors (800, 1020) on which at least some operations (720, 730, 740, 750, 760, 1410, 1420, 1430, 1440) of a transform can be applied across the components of blocks on a single instruction, multiple data basis; and
apply the transform (350) to the blocks of the digital media data, wherein applying the transform comprises performing the at least some operations on the single instruction, multiple data basis on the vectors for the block; and
encode (380)/decode (420) the digital media data to/from a compressed bitstream.
14. The digital media encoder and/or decoder of claim 13 wherein the transform
is a lapped biorthogonal transform having a block transform applied to adjacent
blocks of the digital media data and an overlap filter applied on overlap areas
straddling the adjacent blocks, wherein said processor orders the elements of the
blocks into vectors on which at least some operations of both the overlap filter and
the transform can be applied on the single instruction, multiple data basis.
15. The digital media encoder and/or decoder of claim 14 wherein the lapped
biorthogonal transform is one-dimensional, and wherein the overlap filter and the
block transform each comprise butterfly stages whose operations are applied to the
vectors on the single instruction, multiple data basis.
16. The digital media encoder and/or decoder of claim 14 wherein the lapped
biorthogonal transform is two-dimensional, and wherein the overlap filter and the
block transform each comprise 2x2 Hadamard transforms whose operations are applied to the vectors on the single instruction, multiple data basis.
17. The digital media encoder and/or decoder of claim 14 wherein the processor
is further programmed to perform said ordering of elements during a stage of color
conversion of the digital media data between color formats.
18. At least one computer-readable recording medium carrying a computer-
executable digital media processing program thereon for performing a method of
processing digital media data, the method comprising:
re-mapping (710, 1000, 1001) components of blocks of digital media data (1010) into a set of vectors (1020, 800) on which operations (720, 730, 740, 750, 760, 1410, 1420, 1430, 1440) of a transform can be applied across the components of blocks on a single instruction, multiple data basis;
applying the transform (350) to blocks of the digital media data to produce a set of transform coefficients (360, 362) for the respective blocks, wherein applying the transform comprises performing at least some operations on a single instruction, multiple data basis on the vectors of components for a block; and
encoding (380) /decoding (420) the digital media data to/from a compressed bitstream.
19. The at least one computer-readable recording medium of claim 18 wherein
the transform is a lapped biorthogonal transform comprising an overlap filter and a
block transform, and said applying the transform comprises performing at least
some operations of both the overlap filter and block transform on a single
instruction, multiple data basis on the vectors.
20. The at least one computer-readable recording medium of claim 19 wherein
said applying the transform comprises performing at least some rotation operations
of both the overlap filter and block transform on a sequential basis.