THESIS PROPOSAL Pooja Vasant Agawane Student I D 1000 522 697 Date August 20 2007 IMPLEMENTATION AND EVALUVATION OF THE RESIDUAL COLOR TRANSFORM OBJECTIVE The objective of the thesis is to implement and evaluate the performance of Residual Color Transform RCT 9 applied to the High Definition test sequence 16 This thesis aims to compare the performance of the RCT with the lossless coding tools in the JM reference software 16 MOTIVATION The 4 4 4 video sampling format is gaining a lot of attention due to its significance in the professional applications Both the industry and the academia are actively involved to achieve better compression efficiency and high coding gain in RGB red green and blue color space In contrast to typical consumer applications the high quality of the video is demanded in the applications such as professional digital video recording or digital cinema large screen digital imagery These applications require all three color components to be represented with identical spatial resolution Moreover for this kind of applications sample values in each color component of a video signal are expected to be captured and displayed with a precision of more than 8 bits These specific characteristics are posing new questions and new challenges especially regarding the choice of an optimal color space representation Typically both for video capture and display purposes the RGB color space representation can be considered as the natural choice From a coding point of view however the RGB domain is often not the optimum color space representation mainly because for natural source material usually significant amount of statistical dependencies among the RGB components can be observed Thus in order to take advantage of these statistical properties the usage of a decorrelating transformation from the original RGB domain to some appropriate color space is often recommended 2 DISADVANTAGES OF YCbCr AND INTRODUCTION OF YCgCo COLOR SPACE Typically a video is captured and displayed using the RGB Red Green and Blue color space The disadvantages of encoding the video in RGB domain are Color components in the RGB domain are highly correlated The response of the human visual system HVS is better matched to the luminance and chrominance components rather than RGB The HVS is very sensitive to the luminance information in the image It is less sensitive to the chrominance components The YUV color space represents this luminance and chrominance information in a given RGB image Hence the color conversion from the RGB domain to the YUV domain for encoding is performed This conversion can be performed as follows 20 Y U V 0 299R 0 587G 0 114B 0 147R 0 289G 0 436B 0 615R 0 515G 0 100B In the YUV domain the chrominance samples can be subsampled This leads to compression Then the inverse transform is performed from the YUV to RGB for display YCbCr is a family of color spaces Y stands for Luminance Cb represents the blue chromna and Cr represents the red chroma The conversion from RGB to YCbCr can be performed as follows 7 with e g KR 0 2126 KB 0 0722 There are two problems with this approach The samples are actually represented using integers The rounding error is introduced in both the forward and inverse color transformations The above transformation was not originally designed for digital video compression It uses a sub optimal trade off between the complexity of the transformation with difficult to implement coefficient values such as 0 2126 and 0 0722 and coding efficiency Considering the second problem a new color space called YCgCo where the Cg stands for green chroma and the Co stands for orange chroma has been introduced This is much simpler and typically has equal or better coding efficiency The conversion from the RGB to the YCgCo color space can be performed as follows 7 This conversion reduces the complexity of conversion from the RGB domain to YCbCr and also increases the coding efficiency RESIDUAL COLOR TRANSFORM The residual color transform maps RGB to the YCoCg color space The characteristics of the YCgCo color space can be explained as follows 8 This color transform has been shown to be capable of achieving a decorrelation that is much better than that obtained by various RGB to YCbCr transforms and which in fact is very close to that of the Karhunen Loeve transform 23 The transform is reversible in the sense that each original RGB triple can be exactly recovered from the corresponding YCoCg triple if the color difference components Co and Cg are represented with one additional bit accuracy relative to the bit depth used for representing RGB and if furthermore no information loss in any subsequent coding step is assumed Both the forward and inverse RGB to YCoCg transforms require only a few shift and add operations per triple which in addition can be performed inline i e without the need of some extra memory apart from one single auxiliary register The operator denotes the bitwise right shift operator OVERVIEW OF H 264 H 264 MPEG 4 AVC is the latest video coding standard It is noted for achieving very high data compression H 264 is aimed at achieving high quality video at low bit rates as compared to previous standards of MPEG 2 H 263 and MPEG 4 part 2 The price to be paid is an increase in complexity where the decoder complexity is about four times that of MPEG2 and two times that of MPEG 4 Part 2 Visual 6 The basic coding structure of H 264 is similar to the previous standards of MPEG 1 MPEG 2 H 263 etc 3 This coding structure is referred to as motion compensated transform coding structure A video is a group of pictures and it is coded by considering one picture at a time A picture is considered as a group of slices A picture can have one or more slices A slice consists of a sequence of macroblocks MB Each MB is 16 16 pixels of luminance component Y and 8 8 pixels of two chrominance components Cb and Cr for the 4 2 0 sampling format This 16 16 luminance macroblock can be partitioned into sub blocks of 16 8 8 16 and 8 8 Each 8 8 luminance can be further partitioned in 8 4 4 8 and 4 4 sub blocks The hierarchy of video data organization is shown in Fig 1 Figure 1 Hierarchy of video data organization Figure 2 illustrates the various macroblock and sub macroblock partitions supported by H 264 6 Figure 2 Partitioning of a macroblock and a sub macroblock for motion compensated prediction 6 The H 264 MPEG 4 AVC standard introduced new coding tools and concepts in order to achieve high
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