A SYSTEM AND METHOD FOR ENCODING MOSAICED IMAGE DATA EMPLOYING A REVERSIBLE COLOR TRANSFORM
BACKGROUND Technical Field
This invention is directed toward a system and method for encoding and decoding color image data More particularly, this invention is directed toward a system and method for compressing image data formatted in a mosaiced sampling pattern by employing a reversible color transform
Background Art
Typical digital cameras use a single light sensitive sensor and a color filter array Each pixel element in the color filter array records the intensity information of a single color component, typically red, green or blue In most applications, the captured data is interpolated into a full color image, which is then compressed A Bayer-patterned color filter array is often used as the preferred color filter array In this type of filter, green filters are interlaced with red and blue filters
It has been noted, however, that when color interpolation is performed before compression, instead of after compression, better image quality and higher compression ratios can be achieved Typically the goal in image compression is to increase the compression ratio of the compressed data while maintaining high image quality
There are various problems with known image compression systems, however, even those that interpolate before compressing the data For instance, direct compression of color filter array data with Joint Photographic Experts Group (JPEG) compression produces poor quality images If, however, the Bayer patterned color filter array data is separated into the three primary components (red, green, blue), the red and blue components can be down-sampled into a compact rectangular array and compressed directly To do this it is necessary to find a transformation of the quincunx green pixels typical of the Bayer format into a form suitable for compression Lee and Ortega [3] use a reversible transformation that maps pixel information from the Bayer pattern color filter array into another range The mapping rotates the original interlaced array into a rhombus, packing the data together However, the shape of the data to be compressed after transformation is not rectangular and thus is not suitable for typical JPEG compression Toi and Ohita [2] apply sub-band decomposition to compress the color filter array data using a non-separable two-dimensional diamond filter to process the quincunx green array The sub-bands are then encoded for optimum rate-distortion Reconstruction of the image data is carried out by decoding, synthesizing and interpolating the data to obtain the resultant full color image This method is also somewhat computationally expensive and since it does not allow for exact invertibility in integer arithmetic it is not suitable for lossless compression Koh, Mukherjee and Mitra [4] also devised a method of compressing color filter array data before full color interpolation In this system, image content affected the performance of the compression and interpolation algorithms and sometimes adversely affected the image quality
In general, there are two types of compression—lossy and lossless Lossless compression allows exact original data to be recovered after compression and decompression, while lossy compression allows for the data to be recovered after compression and decompression to vary slightly from the original data There is a tradeoff between these two types of compression in that lossy compression typically
provides for a better compression ratio than lossless compression, while lossless compression provides a better image quality after decompression
It is noted that in the remainder of this specification, the description refers to various individual publications identified by a numeric designator contained within a pair of brackets For example, such a reference may be identified by reciting, "reference [1]" or simply "[1]" A listing of the publications corresponding to each designator can be found at the end of the Detailed Description section
SUMMARY
The system and method of the present invention overcomes the aforementioned limitations of previous image compression schemes by mapping mosaic-patterned pixels, typical of the Bayer data format, to four color channels that correspond to a regular pattern In some embodiments of the invention, direct and inverse transform matrices that have integer magnitudes are employed This provides reduced computational complexity, and lossless compression may be readily achieved A lossless compression system for raw Charged Couple Device (CCD) data from a digital camera may then be constructed using these reversible color transforms and lossless codecs for each color channel
The invention comprises a new color space that maps image pixel values in a mosaiced sampling pattern (such as that generated by a Bayer color filter array [1]) into four color channels that correspond to rectangular sampling patterns Because these new channels correspond to a rectangular sampling grid, they are much more amenable to processing steps such as compression
An additional aspect of the invention is that the transformation from the original mosaic-patterned pixels into the new four-channel color space can be made reversible
in integer arithmetic That allows for the implementation of efficient lossless image compression systems for mosaiced (e g , raw, or raw CCD) images In one working embodiment of the system and method of the invention this transform was used in a Progressive Transform Codec (PTC) [7], with better compression results and lower computational complexity than systems based on pixel value prediction (such as PNG -"portable network graphics") However, the transform may be applied with any conventional codec
When lossless compression is not necessary, one embodiment of the invention also provides for a lossy result, in that case the four-channel transform of the invention can be used in either an integer-reversible or a reduced-complexity form, coupled with any lossy image codec
The system and method of the invention allows for the efficient compression of raw CCD camera data with a relatively low computational complexity, thus making it attractive to camera manufacturers The system and method of the invention also provides for higher image quality and provides for lower bandwidth requirements to transfer the data to other sites and less storage requirements to save the image data to a storage medium
The system and method of the invention also allows for codecs that support both lossless and lossy compression (such as PTC) to be used in a mosaiced-sensor digital camera to support both high compression (lossy) and maximum-fidelity raw (lossless) formats That is an advantage over current digital cameras that support raw mode, in which different codecs are employed for lossy and lossless compression, increasing firmware size or chip gate count
In addition to the just described benefits, other advantages of the present invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it
DESCRIPTION OF THE DRAWINGS
The file of this patent contains at least one drawing executed in color Copies of this patent with color drawing(s) will be provided by the U S Patent and Trademark Office upon request and payment of the necessary fee
The specific features, aspects, and advantages of the invention will become better understood with regard to the following description, appended claims, and accompanying drawings where
FIG 1 is a diagram depicting a general purpose computing device constituting an exemplary system for implementing the invention
FIG 2 is a typical Bayer mosaic for a color image captured in single-CCD digital cameras, there twice as many green (G) pixels as there are red (R) and blue (B) pixels
FIG 3 depicts the mapping of a typical three-channel Bayer-mosaiced color image, such as that shown in FIG 2, into four regular images of V* the original size, each for one of the channels {Y, Dg, Co, Cg}
FIG 4 is an example of the color transformation according to the system and method of the present invention Top left full-resolution original color image, with three colors (RGB) per pixel location Top right Bayer-mosaiced version of the original, according to the sampling pattern of FIG 2, it simulates the output of a single-CCD camera Bottom left a zoom of the Bayer-mosaiced image, where one can clearly see the jump intensities from each of the R, G, and B locations Bottom right the four sub-images formed by the YDgCoCg transform of the invention (clockwise from top left Y, Dg, Cg, Co)
FIGs 5A and 5B show an exemplary compression system for raw CCD images using the YDgCoCg reversible transform of the system and method of the invention
FIG 5A depicts an encoder, while FIG 5B depicts the corresponding decoder One lossless image codec that can be used is a PTC codec [7] However, the reversible transform of the system and method of the invention can be used with any lossless compression system
FIG 6A depicts a flowchart of a lossless encoding process according to one embodiment of the invention
FIG 6B depicts a flowchart of a lossless decoding process according to one embodiment of the invention
FIG 7 is an exemplary implementation of the integer-reversible YDgCoCg-R color space transform via ladder operators according to the system and method of the invention Top the 2x2 basic direct and inverse ladder operators Middle direct YDgCoCg-R transform Bottom inverse transform
FIGs 8A and 8B show an exemplary lossy compression system for raw CCD images using the YDgCoCg reversible transform of the system and method of the invention Various lossy compressors can be used with the system and method according to the invention, if so desired, to obtain higher compression ratios FIG 8A depicts an encoder, while FIG 8B depicts a corresponding decoder
FIG 9A is a flowchart of the lossy encoding process according to one embodiment of the invention
FIG 9B is a flowchart of the lossy decoding process according to one embodiment of the invention
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following description of the preferred embodiments of the present invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention
1 0 EXEMPLARY OPERATING ENVIRONMENT
FIG 1 illustrates an example of a suitable computing system environment 100 on which the invention may be implemented The computing system environment 100 is only one example of a suitable computing environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention Neither should the computing environment 100 be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment 100
The invention is operational with numerous other general purpose or special purpose computing system environments or configurations Examples of well known computing systems, environments, and/or configurations that may be suitable for use with the invention include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like
The invention may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer Generally, program modules include routines, programs, objects, components, data structures,
etc that perform particular tasks or implement particular abstract data types The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices
With reference to FIG 1, an exemplary system for implementing the invention includes a general purpose computing device in the form of a computer 110 Components of computer 110 may include, but are not limited to, a processing unit 120, a system memory 130, and a system bus 121 that couples various system components including the system memory to the processing unit 120 The system bus 121 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus
Computer 110 typically includes a variety of computer readable media Computer readable media can be any available media that can be accessed by computer 110 and includes both volatile and nonvolatile media, removable and nonremovable media By way of example, and not limitation, computer readable media may comprise computer storage media and communication media Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by computer 110 Communication media
typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media The term "modulated data signal" means 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 includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media Combinations of the any of the above should also be included within the scope of computer readable media
The system memory 130 includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) 131 and random access memory (RAM) 132 A basic input/output system 133 (BIOS), containing the basic routines that help to transfer information between elements within computer 110, such as during start-up, is typically stored in ROM 131 RAM 132 typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit 120 By way of example, and not limitation, FIG 1 illustrates operating system 134, application programs 135, other program modules 136, and program data 137
The computer 110 may also include other removable/non-removable, volatile/nonvolatile computer storage media By way of example only, FIG 1 illustrates a hard disk drive 141 that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive 151 that reads from or writes to a removable, nonvolatile magnetic disk 152, and an optical disk drive 155 that reads from or writes to a removable, nonvolatile optical disk 156 such as a CD ROM or other optical media Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like The hard disk drive 141 is typically connected to the system bus 121 through anon-removable memory interface such as
interface 140, and magnetic disk drive 151 and optical disk drive 155 are typically connected to the system bus 121 by a removable memory interface, such as interface 150
The drives and their associated computer storage media discussed above and illustrated in FIG 1, provide storage of computer readable instructions, data structures, program modules and other data for the computer 110 In FIG 1, for example, hard disk drive 141 is illustrated as storing operating system 144, application programs 145, other program modules 146, and program data 147 Note that these components can either be the same as or different from operating system 134, application programs 135, other program modules 136, and program data 137 Operating system 144, application programs 145, other program modules 146, and program data 147 are given different numbers here to illustrate that, at a minimum, they are different copies A user may enter commands and information into the computer 110 through input devices such as a keyboard 162 and pointing device 161, commonly referred to as a mouse, trackball or touch pad Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like These and other input devices are often connected to the processing unit 120 through a user input interface 160 that is coupled to the system bus 121, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB) A monitor 191 or other type of display device is also connected to the system bus 121 via an interface, such as a video interface 190 In addition to the monitor, computers may also include other peripheral output devices such as speakers 197 and printer 196, which may be connected through an output peripheral interface 195 Of particular significance to the present invention, a camera 163 (such as a digital/electronic still or video camera, or film/photographic scanner) capable of capturing a sequence of images 164 can also be included as an input device to the personal computer 110 Further, while just one camera is depicted, multiple cameras could be included as an input device to the personal computer 110 The images 164 from the one or more cameras are input into the computer 110 via an appropriate camera interface 165 This interface 165 is connected to the system bus 121, thereby
allowing the images to be routed to and stored in the RAM 132, or one of the other data storage devices associated with the computer 110 However, it is noted that image data can be input into the computer 110 from any of the aforementioned computer-readable media as well, without requiring the use of the camera 163
The computer 110 may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer 180 The remote computer 180 may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer 110, although only a memory storage device 181 has been illustrated in FIG 1 The logical connections depicted in FIG 1 include a local area network (LAN) 171 and a wide area network (WAN) 173, but may also include other networks Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet
When used in a LAN networking environment, the computer 110 is connected to the LAN 171 through a network interface or adapter 170 When used in a WAN networking environment, the computer 110 typically includes a modem 172 or other means for establishing communications over the WAN 173, such as the Internet The modem 172, which may be internal or external, may be connected to the system bus 121 via the user input interface 160, or other appropriate mechanism In a networked environment, program modules depicted relative to the computer 110, or portions thereof, may be stored in the remote memory storage device By way of example, and not limitation, FIG 1 illustrates remote application programs 185 as residing on memory device 181 It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used
The exemplary operating environment having now been discussed, the remaining parts of this description section will be devoted to a description of the program modules embodying the invention
2 0 A SYSTEM AND METHOD FOR ENCODING MOSAICED IMAGE DATA EMPLOYING A REVERSIBLE COLOR TRANSFORM
The system and method according to the invention is described in detail in the following sections
2 1 General Description of the Invention
The invention comprises a new color space that maps image pixel values in a mosaiced sampling pattern (such as that generated by a Bayer color filter array [1]) into four color channels that correspond to rectangular sampling patterns Because these new channels correspond to a rectangular sampling grid, they are much more amenable to processing steps such as compression
An additional aspect of the invention is that the transformation from the original mosaic-patterned pixels into the new four-channel color space can be made reversible in integer arithmetic This allows for the implementation of efficient lossless image compression systems for mosaiced (e g , raw, or raw Charged Couple Device (CCD)) images This transform has been used in the PTC codec [7], with better compression results and lower computational complexity than systems based on pixel value prediction (such as PNG - "portable network graphics") However, the system and method of the invention can be used for both lossless and lossy compression
FIG 2 shows a typical pattern of pixel values 202 from a single-CCD digital camera Different cameras may use different patterns, but almost all follow the Bayer design of twice as many green (G) pixels as red (R) and blue (B) pixels, and a diagonal adjacency (a quincunx pattern) for green pixels and for each red-blue pair
If one wants to compress an image where the pixels are patterned as in FIG 2, one cannot just feed the pixel values to a standard compressor designed for a single-
channel (e g grayscale) image The jumps in values (because adjacent pixels are not of the same colors) produce high-frequency components that significantly increase the size of the compressed data
The present invention maps a patterned image such as the one in FIG 2 into four images of a quarter of the size of the original Each of the four images can be seen as an independent color channel, which is independently compressed The four images are smooth, because their pixels correspond to a regular sampling pattern in each of the four channels Therefore, they compressed well with any lossless image compressor
The invention is a method for mapping the mosaic-patterned pixels such as those in FIG 2 to four color channels that correspond to a regular pattern The basic idea comes for the observation that a Bayer sampling grid such as that in FIG 2 is regular for every 2x2 block of pixels, which we call a "macropixel" So, for each 2x2 macropixel of the original mosaiced imaged, one maps its G1, G2, R, and B values into four new pixels, corresponding to four channels Y, Dg, Co, Cg Thus, each of these channels generates a regularly-sampled new image, as shown in FIG 3 More specifically, the original three-channel Bayer-mosaiced image 302 is mapped into four regular images of VA the size of the original, each for one of the channels {Y, Dg, Co, and Cg} 304, 306, 308 and 310 If the original mosaiced image has NxN pixels, then each of the color channel images has N/2xN/2 pixels
The mapping from a set of macropixel G1, G2, R, and B values into the four pixels values Y, Dg, Co, and Cg of the four transformed color channels is given by
(1) (Table Removed) It is easy to see that the inverse transform is given by
(Table Removed) (2)
The YDgCoCg color space has several interesting properties
• The Y channel is just an average of all four original values in a macropixel, with a 50% green contribution and 25% contributions of R and B Thus, Y can be seen as a luminance channel, that is, it contains the grayscale information of the image The dynamic range of Y is the same as that of each of the original G1, G2, R, and B pixels
• Dg, Co and Cg are all color channels If a macropixel has values G1 = G2 = R = B, then the macropixel is just a gray level, for which Dg = Co = Cg = 0
• Dg is a "difference green" channel The smoother the pixels values of the original image, the smaller the values of the Dg pixels, as desired
• Like in the previous YCoCg color space design [9], Cg is an "excess green" channel, as one can see in Equation (2), the original green values can be reconstructed from just the luminance Y, difference green Dg, and excess green Cg Co is like an orange channel (although not quite), because the value is Co is largest when the input pixels have R = maximum value and B = minimum value, corresponding to a color hue between red and yellow (depending on the green values), whose midpoint is orange
One interesting aspect of the direct and inverse transform matrices in Equations (1) and (2) is that their entries have magnitudes equal to 0, ¼, ½, or 1 That leads to a reduced computational complexity, since multiplications by those factors can be implemented by right-shift operators, if the pixel values are integer
Usually Bayer-mosaiced (raw) images are encoded via nonlinear predictors that predict the value of a particular pixel in the raw image based on previously-encode neighboring values, taking the Bayer pattern into account Some publications have considered the use of color space transforms to map the Bayer-mosaiced {R,G1,G2,B} data into luminance and chrominance channels [3], [4] but they use two luminance channels and two color channels, leading to a higher entropy representation than the four-channel space of the system and method according to the invention (because it has only one luminance channel) Furthermore, the transformation matrices in [3],[4] have higher complexity than those in Equations (1) and (2), because their entries are fractional numbers Finally, the constructions in [3], [4] do not allow for exact invertibility in integer arithmetic, so they are not suitable for applications such as lossless compression
FIG 4 shows an example of using the color space transformation in the system and method according to the invention The top left image 402 is the full-resolution original color image, with three colors (RGB) per pixel location The top right image 404 Bayer-mosaiced version of the original, according to the sampling pattern of FIG 2, it simulates the output of a single-CCD camera The bottom left image 406 is a zoom of the Bayer-mosaiced image, where one can clearly see the jump intensities from each of the R, G, and B locations The bottom right image 408 shows the four sub-images formed by the YDgCoCg transform of the invention (clockwise from top left Y, Dg, Cg, Co) The discontinuities in pixel values in the Bayer-mosaiced image are clear, so that the image should not be fed directly to a single-channel compressor The four sub-images generated by the four-channel color space transformation of the invention are quite smooth, and thus are amenable to compression Furthermore, one can see that most of the information is in the Y (luminance channel) The Dg image shows just details where the gradient in green is largest, and the Co and Cg images are quite soft, because they contain only color information Therefore, the Dg, Co, and Cg sub-images usually can each be compressed to significantly smaller sizes than that of the compressed version of the Y sub-image
2 2 Exact Inversion in Integer Arithmetic
One of the main applications of the new four-channel color space transformation of the invention is lossless compression of raw (Bayer-mosaiced) images For that, one needs to be able not only to map the original {R,G1,G2,B} pixels into the transformed {Y,Dg,Co,Cg} pixels for compression, but after decompression one needs to map the decoded {Y,Dg,Co,Cg} pixels exactly back to the original {R,G1 ,G2,B} values
In most cases, the original pixel G1, G2, R, and B values are integer, because the analog/digital converters used in essentially all digital cameras produce integer-valued outputs One usually wants the transformed pixel values Y, Dg, Co, and Cg to be integer-valued, also, especially if one wants to feed them to a lossless compressor If one uses integer arithmetic, the multiplications by factors equal to !4 or Vz in Equations (1) and (2) lead to small truncation errors (e g 14 x 333 = 333 » 1 =166, in integer arithmetic, whereas the exact value would have been 166 5) Although small, these errors would preclude exact reconstruction of the original integer G1, G2, R, and B values from the transformed integer Y, Dg, Co, and Cg values Thus, even though Equation (2) represents a mathematically exact inverse for real numbers, the inverse is not exact in integer arithmetic
However, this invention also comprises a method for computing the direct and inverse transforms in Equations (1) and (2) in such a way that exact inversion under integer arithmetic is achieved This allows for the implementation of a lossless image compression system for raw CCD (mosaiced) images, for example, by the block diagrams in FIGs 5A and 5B
The encoder 502 of one lossless embodiment of the invention is shown in FIG 5A It includes a channel splitter 504, a 4-channel direct color transform module 506, four lossless encoders, 508a, 508b, 508c, 508d and a multiplexer 510 An exemplary flowchart of the encoding process is shown in FIG 6A As shown in FIG 6A, process
action 602, raw CCD data is input into the channel splitter The channel splitter splits the raw CCD data (in Bayer format) into four channels, R, G1, G2 and B on a macro-pixel basis (e g , for every set of R, G1, G2 and B adjacent pixels in the Bayer format) Then, as shown in process action 604, the 4 channel direct color transform converts the R, G1, G2 and B channels into a Y, Dg, Co, and Cg color space or Y, Dg, Co, and Cg channels (process action 606) This results in four sub-images, each 14 the size of the originally input data, for the Y, Dg, Co and Cg components The four sub-images are each input into a lossless encoder (process action 608) The four encoded channels, one for each sub-image, are then combined (process action 610) to yield an encoded file or bit stream
The decoder 514 is shown in FIG 5B It includes a demultiplexer 516, four lossless decoders 518a, 518b, 518c, 518d, a 4-channel inverse color transform 520, and a channel combiner 522 An exemplary flowchart of the decoding process is shown in FIG 6B As shown in FIG 6B, process action 612, an encoded file or bit stream is input into the demultiplexer which separates the encoded file or bit stream into separate encoded channels (process action 614) The separate encoded channels are each input into a lossless decoder which decodes each encoded channel (process action 616) into Y, Dg, Co, and Cg components The 4-channel inverse color transform then transforms the Y, Dg, Co, and Cg components into R, G1, G2 and B channels (process action 618) A channel combiner then combines the R, G1, G2 and B channels into the decoded CCD data (in Bayer format), as shown in process action 620
The key for an integer-reversible implementation of the direct transform / inverse transform pair in Equations (1) and (2) is a precise control of the sequence of truncation errors in the multiplications by Vfe and 14 In one implementation this construction is based on the reversible 2x2 ladder structures in [7], [9]
2 3 YDqCoCq-R Direct and indirect Transforms
The integer-reversible four-channel color transform of the invention is called the YDgCoCg-R transform, similarly to the YCoCg-R transform used in the PTC codec [7], [9] Considering that all original and transformed pixel values are integer, the direct and inverse YDgCoCg-R transforms are specified by the pseudo-code below
(Table Removed) 3,4
To see that the inverse transform in Equations (4) is the exact inverse of the direct transform in Equations (3), note that the inverse transform recovers the same intermediate values, and from them the original values That is because it performs the reverse of each equation in exactly the reverse order So, the truncation errors in the inverse transform equation are identical to those of the direct transform equation, but with reversed signs Thus, the truncation errors cancel out
The computations in Equations (3) and (4) can be represented by the flow graphs in FIG 7 One sees that the direct and inverse transform are computed via
repeated applications of a simple 2-input, 2-ouput ladder operator, for which two inputs x and y are mapped to two outputs (x+y)/2 and x-y That 2x2 ladder transform is called the "average/difference ladder" [10], the average output (x+y)/2 has a small truncation error, which is canceled out by the inverse ladder That ladder construction has been the basis of many techniques for reversible transforms, including reversible block transforms [6], [7] and reversible wavelet transforms [8]
The new reversible YDgCoCg-R color space transform is thus an extension of the previous YCoCg-R reversible transform [9] While YCoCg-R is useful for mapping full-resolution {R,G,B} data into luminance/chrominance channels in a reversible way, YDgCoCg-R is useful for mapping Bayer-mosaiced {R,G,B} data into smooth luminance/chrominance channels, in a reversible way
2 4 Scaled Transform
In some applications one may not need to preserve exact invertibility in integer arithmetic, but one may still want an efficient mapping between a Bayer-mosaiced image and an equivalent four-color-channel representation One example is lossy compression, that is, a system similar to that in FIGs 5A and 5B, but with the encoders and decoders being lossy Such a system could be an efficient way of performing lossy compression of images produced by a Bayer-mosaiced CCD sensor in a digital camera, it could lead to better results than the traditional approach of first applying a demosaicing (interpolation) filter and then encoding the resulting three-channel RGB image with a lossy compressor, such as JPEG
If one does not need exact invertibility in integer arithmetic, one can simplify the direct and inverse transform matrices by just scaling rows of the direct transform and columns of the inverse transform appropriately For example, one can scale the Co,
Cg, and Dg channels by a factor of two, so that the direct transform computation becomes
(Table Removed) 5
for which the inverse transform is given by
(Table Removed) 6
In this form, the inverse transform is very simple it involves only additions and subtractions, no multiplications or shifts So, the form above is attractive in applications where decoder complexity is more important than encoder complexity
Similarly, one could scale the matrices in such a way as to make the direct transform simpler to compute The simplest form is
(Table Removed) 7
for which the inverse transform is given by
(Table Removed) 8
The original formulation in Eqns (1) and (2) lead to balanced implementations, in the sense that the direct and inverse transforms have the same computational complexity Plus it allows for exact reversibility in integer arithmetic (a lossless transform) as shown before The scaled versions in Eqns (5)-(6) and (7)-(8) allow for a simpler inverse transform or a simpler direct transform, respectively
One should note that the scaling in Eqns (1)-(2) is not the only one that can produce lossless transforms Other scahngs can produce lossless transforms, with appropriate modifications in the ladder networks in FIG 7 Such variations are just trivial extension of the present invention, so that all of them do not need to be explicitly described
A schematic of an implementation of a lossy compression system for raw CCD (mosaiced) images is shown by the block diagrams in FIGs 8A and 8B For such implementations, one can use one of the scaled versions of the YDgCoCg direct and inverse transforms described above, depending on which of the encoder or decoder should have the lower complexity
The encoder 802 for a lossy compression system according to the invention is shown in FIG 8A It includes a channel splitter 804, a 4-channel direct color transform 806, four lossy encoders, 808a, 808b, 808c, 808d and a multiplexer 810 An exemplary flowchart is shown in FIG 9A As shown in FIG 9A, process action 902, raw CCD data is input into the channel splitter The channel splitter splits the raw CCD data (in Bayer format) into four channels, R, G1, G2 and B on a macro-pixel basis Then, as shown in process action 904, the 4 channel direct color transform converts the R, G1, G2 and B into a Y, Dg, Co, and Cg channels (process action 906) This results in four sub-images, each 14 the size of the originally input data, for the Y, Dg, Co and Cg components The four sub-images are each input into a lossy encoder (process action 908) The four encoded channels, one for each sub-image, are then combined (process action 910) to yield an encoded file or bit stream
The decoder 814 shown in FIG 8B It includes a demultiplexur 816, four lossy decoders 818a, 818b, 818c, 818d, a 4-channel inverse color transform 820, and a channel combiner 822 An exemplary flowchart is shown in FIG 9B As shown in FIG 9B, process action 912, an encoded file or bit stream is input into the demultiplexer which separates the encoded file or bit stream into separate encoded channels (process action 914) The separate encoded channels are each input into a lossy decoder which decodes each encoded channel (process action 916) into Y, Dg, Co, and Cg components The 4-channel inverse color transform then transforms the Y, Dg, Co, and Cg components into R, G1, G2 and B channels (process action 918) A channel combiner then combines the R, G1, G2 and B channels into the decoded CCD data (in Bayer format) as shown in process action 920
The foregoing description of the invention has been presented for the purposes of illustration and description It is not intended to be exhaustive or to limit the invention to the precise form disclosed Many modifications and variations are possible in light of the above teaching It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto
References
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Wherefore, what is claimed is:
1. A method for encoding mosaiced image data comprising the process actions of:
inputting raw charged couple device (CCD) image data;
splitting the raw CCD data into red (R), a first green (G1), a second green (G2) and a blue (B) channel on a macro-pixel basis, wherein each macro-pixel comprises a set of a first green and a second green diagonally adjacent pixels and a red-blue pair of pixels R, B that is diagonally adjacent;
transforming the red, first green, second green and blue channels into Y, Dg, Co and Cg color space channels, each representing an image one quarter the size of the original image wherein for each macro-pixel the Y channel represents luminance, Dg represents the difference between the first green pixel and the second green pixel, and the Co and Cg channels are color channels; and
separately encoding each of the Y, Dg, Co and Cg channels with an encoder.
2. The method of Claim 1 further comprising the process action of combining the Y, Dg, Co and Cg channels after encoding.
3. The method of Claim 2 further comprising the process action of outputting the combined Y, Dg, Co and Cg channels to a stored file.
4. The method of Claim 2 further comprising the process action of outputting the combined Y, Dg, Co and Cg channels as a bit stream.
5. The method of Claim 1 wherein the red, first green, second green and blue channels are transformed into Y, Dg, Co and Cg color space channels using the transform:
(Table Removed)
6. The method of Claim 1 wherein the transform is computed in such a way that exact inversion under integer arithmetic is achieved.
7. The method of Claim 6 wherein the transform is computed for each macro-pixel by the process actions of:
computing Co as the red pixel minus the blue pixel; computing Dg as the second green pixel minus the first green pixel; computing a first intermediate variable u as the blue pixel plus Co; computing a second intermediate variable v as the first green pixel plus Dg; computing Cg as the second intermediate variable v minus the first intermediate variable u; and
computing Y as the first intermediate pixel u plus Cg.
8. A method for decoding mosaiced image data comprising the process actions of:
inputting combined Y, Dg, Do and Cg channels;
splitting the combined channels into separate Y, Dg, Co and Cg channels;
decoding each of the Y, Dg, Co and Cg channels with a decoder;
transforming the Y, Dg, Co and Cg channels into red, first green, second green and blue channels; and
combining the red, first green, second green and blue channels to output a mosaiced color image.
9. The method of Claim 8 wherein the Y, Dg, Co and Cg channels are
transformed by using the transform:
(Table Removed)
10. The method of Claim 8 wherein the transform is computed for each
macro-pixel by the process actions of:
computing a first intermediate variable u as Y minus Cg; computing a second intermediate variable v as the first intermediate variable u plus Cg;
computing a first green pixel as the second intermediate variable v minus Dg; computing a blue pixel as the first intermediate variable u minus Co; computing a second green pixel as the first green pixel plus Dg; and , computing a red pixel as the blue pixel plus Co.
11. A method for converting an image in three color space to four color space comprising:
inputting an image in 3 color space with twice as many green (G) pixels as red (R) and blue (B) pixels, and a diagonal adjacency for green pixels and for each red-blue pair;
dividing said image in 3 color space into macro-pixels wherein each macro-pixel comprises a set of green diagonally adjacent pixels G1, G2 and a red-blue pair of pixels R, B that is diagonally adjacent;
for each macro-pixel, mapping the R, G1, G2 and B pixels to a four color space Y, Dg, Co and Cg to obtain four sub-images each made entirely of Y, Dg, Co and Cg pixels respectively wherein Y represents luminance, Dg represents the difference between the G1 and G2 pixels, and Co and Cg represent color;
compressing each of the Y, Dg, Co and Cg sub-images separately; and combining the compressed sub-images.
12. The method of Claim 11 wherein each of the sub-images are compressed with a lossless encoder.
13. The method of Claim 11 wherein the R, G1, G2 and B pixels are mapped to Y, Dg, Co and Cg color space by a multiple of: (Table Removed)
14. The method of Claim 11 wherein the R, G1, G2 and B pixels are mapped to Y, Dg, Co and Cg color space by: (Table Removed)
15. The method of Claim 11 wherein the G1, G2, R and B pixels are mapped to Y, Dg, Co and Cg color space using the transform: (Table Removed) so as to simplify the transformation.
16. The method of Claim 11 further comprising the process action of saving the combined compressed sub-images to a file.
17. The method of Claim 11 further comprising the process action of outputting the combined compressed sub-images to a bit stream.
18. A computer-readable medium having computer-executable instructions for performing the method recited in Claim 11.
19. A method for converting an image in four color space to three color space comprising:
inputting an image in Y, Dg, Co, Cg color space;
splitting the image in Y, Dg, Co and Cg color space into Y, Dg, Co and Cg channels;
decoding each of the Y, Dg, Co and Cg channels;
transforming the decoded Y, Dg, Co and Cg channels into R, G1, G2 and B channels; and
combining the R, G1, G2 and B channels to obtain a color image.
20. The method of Claim 19 wherein the Y, Dg, Co and Cg pixels are decoded using the transform: (Table Removed)
21. The method of Claim 20 wherein the Y, Dg, Co and Cg pixels are decoded to simplify the decoding using the transform:
(Table Removed)
22. The method of Claim 21 wherein the Y, Dg, Co and Cg pixels are decoded using the transform:
(Table Removed)
23. A method for decoding a colorhnage encoded by the process actions of:
inputting raw charged couple device (CCD) image data;
splitting the raw CCD data into red (R), a first green (G1), a second green (G2) , and a blue (B) channel on a macro-pixel basis, wherein each macro-pixel comprises a set of a first green and a second green diagonally adjacent pixels and a red-blue pair of pixels R, B that is diagonally adjacent;
transforming the red, first green, second green and blue channels into Y, Dg, Co and Cg color space channels, each representing an image one quarter the size of the original image wherein for each macro-pixel the Y channel represents luminance, Dg represents the difference between the first green pixel and the second green pixel, and the Co and Cg channels are color channels; and
separately encoding each of the Y, Dg, Co and Cg channels with an encoder; and combining the Y, Dg, Co and Cg channels, comprising the process actions of:
inputting an image in Y, Dg, Co, Cg color space;
splitting the image in Y, Dg, Co and Cg into Y, Dg, Co and Cg channels;
separately decoding each of the Y, Dg, Co and Cg channels by using the inverse of the encoding used;
transforming the decoded Y, Dg, Co and Cg channels into R, G1, G2 and B channels; and
combining the R, G1, G2 and B channels to obtain a color image.
24. The method of Claim 23 wherein the image was transformed from R, G1, G2 and B to Y, Dg, Co and Cg using the transform: (Table Removed)

wherein the Y, Dg, Co and Cg channels are transformed into R, G1, G2 and B channels using: (Table Removed)
25. The method of Claim 23 wherein columns of a transform matrix are scaled to simplify decoding computations.

26. The method of Claim 23 wherein the image was transformed from R, G1,
G2 and B to Y, Dg, Co and Cg using the transform:
(Table Removed)
and wherein the Y, Dg, Co and Cg channels are transformed into R, G1, G2 and B channels using:
(Table Removed)
27. The method of Claim 23 wherein rows of a transform matrix are scaled to simplify encoding computations.
28. The method of Claim 23 wherein the image is transformed from R, G1, G2 and B to Y, Dg, Co and Cg using the transform: (Table Removed)
and wherein the Y, Dg, Co and Cg channels are transformed into R, G1, G2 and B channels using the transform: (Table Removed)
29. A system for encoding a raw Bayer image, the system comprising:
inputting raw Bayer image data;
splitting the raw Bayer image data into red (R), a first green (G1), a second green (G2) and a blue (B) channel on a macro-pixel basis, wherein each macro-pixel comprises a set of a first green and a second green diagonally adjacent pixels and a red-blue pair of pixels R, B that is diagonally adjacent;
transforming the red, first green, second green and blue channels into Y, Dg, Co and Cg channels, each representing an image one quarter the size of the original image, wherein Y is luminance, Dg is the difference between the first green and the second green pixels and Co and Cg are color channels; and
encoding each of Y, Dg, Co and Cg channels.
30. The system of Claim 29 wherein the red, first green, second green and blue channels are transformed into Y, Dg, Co and Cg color space channels using the transform:
(Table Removed) 31. The system of Claim 29 wherein the R, G1, G2 and B channels are transformed into Y, Dg, Co and Cg channels by, for each macro-pixel:
computing Co as the red pixel minus the blue pixel;
computing Dg as the second green pixel minus the first green pixel;
computing a first intermediate variable u as the blue pixel plus Co;
computing a second intermediate variable v as the first green pixel plus Dg;
computing Cg as the second intermediate variable v minus the first intermediate variable u; and
computing Y as the first intermediate pixel u plus Cg.
32. The system of Claim 31 wherein the channels are transformed using
repeated applications of a two input, two output ladder operator for which two inputs x
and y are mapped to two outputs (x+y)/2 and x-y.
33. A system for decoding a raw Bayer image, comprising,
inputting an image in Y, Dg, Co, Cg;
splitting the image into Y, Dg, Co and Cg channels;
decoding each of the Y, Dg, Co and Cg channels using the reverse of the encoding process;
transforming the decoded Y, Dg, Co and Cg channels into red, a first green, a second green and a blue channel; and
combining the red, first green, second green and blue channel to obtain a color image.
34. The system of Claim 33 wherein the transform applied to the decoded Y, Dg, Co and Cg channels comprises:
(Table Removed)
35. The system of Claim 33 wherein the decoded Y, Dg, Co, and Cg channels are transformed into R, G1, G2 and B channels by, for each macro-pixel:
computing a first intermediate variable u as Y minus Cg;
computing a second intermediate variable v as the first intermediate variable u plus Cg;
computing a first green pixel as the second intermediate variable v minus Dg;
computing a blue pixel as the first intermediate variable u minus Co;
computing a second green pixel as the first green pixel plus Dg; and
computing a red pixel as the blue pixel plus Co.
36. The system of Claim 35 wherein the channels are transformed using repeated applications of a two input, two output ladder operator for which two inputs x and y are mapped to two outputs (x+y)/2 and x-y.