Performance Analysis and Comparison of H 264 and VP6 Siddhartha Mukkamala 1000571314 Siddhartha mukkamala mavs uta edu Under guidance of Dr K R Rao What is VP6 VP6 codec was first introduced in May 2003 and was officially released in October 2003 It is a proprietary video codec developed by On2 Technologies as a successor to earlier efforts such as VP3 and VP5 Can provide higher visual quality when using at lower bitrates Macromedia has licensed this for its Flash family products VP6 is the preferred video compression format for use with Flash Player 8 and higher Frames in VP6 Only two frame types Intra or I frames may be reconstructed from their compressed representation with no reference to other frames in the sequence I frames provide entry points into the bitstream that do not require preceding frames to be decoded providing a method for fast random access Inter prediction or P frames are encoded differentially with respect to a previously encoded reference frame in the sequence This reference frame may either be the reconstruction of the immediately previous frame in the sequence or a stored previous frame known as the Golden Frame The alternative prediction or Golden Frame is a frame buffer that by default holds the last decoded I frame but it may be updated at any time A flag in the frame header indicates to the decoder whether or not to update the Golden Frame buffer Coding Profiles in VP6 Two Different Profiles Simple and Advanced Each Frame header contains a flag Vp6Profile which indicates the profile that was used to code it In both profiles the BoolCoder is used for encoding The BoolCoder is a simplified binary arithmetic coder allowing tokens to be encoded with fractions of a bit It is much more efficient in terms of compression performance than the Huffman coder but this comes with a significantly increased computational complexity H 264 H 264 was developed by ITU T Video Coding Experts Group together with the ISO IEC Moving Picture Experts Group in 2003 Key video compression codec scheme in MPEG 4 part 10 format for digital media exchange H 264 provides better image quality compared to previous standards when reaching its limits It does not break into blocks but degrades much more smoothly making the image softer H 264 2 H 264 attains the MPEG 2 quality compression at a lower bit rate but with increased computational effort which is not a big deal in modern super computers age It will provide the same quality as MPEG 2 at half the date rate as shown in Figure below Video quality MPEG 2 Vs H 264 20 AVC Encoder H 264 Encoder 1 AVC Decoder H 264 Decoder 1 How does an H 264 codec work Encoder Prediction Motion estimation and compensation Intra Prediction 2 Inter Prediction 2 How does an H 264 codec work 2 Intra Prediction Intra prediction uses the macroblocks from the same image for prediction Two types of prediction schemes are used for the luminance component These two schemes can be referred as INTRA 4x4 and INTRA 16x16 16 In INTRA 4x4 a macroblock of size 16x16 samples is divided into 16 4x4 subblocks Intra prediction scheme is applied individually to these 4x4 subblocks There are nine different prediction modes supported as shown in Fig below 4x4 Luma prediction intra prediction modes in H 264 10 How does an H 264 codec work 3 In mode 0 the samples of the macroblock are predicted from the neighboring samples on the top In mode 1 the samples of the macroblock are predicted from the neighboring samples from the left In mode 2 the mean of all the neighboring samples is used for prediction Mode 3 is in diagonally down left direction Mode 4 is in diagonal down right direction Mode 5 is in vertical right direction Mode 6 is in horizontal down direction Mode 7 is in vertical left direction Mode 8 is in horizontal up direction The predicted samples are calculated from a weighted average of the prediction samples A to M How does an H 264 codec work 4 As shown in below figure four modes are used for prediction of 16x16 intra prediction of luminance components The three modes mode 0 vertical mode 1 horizontal and mode 2 DC are similar to the prediction modes of 4x4 block In the fourth mode the linear plane function is fitted in the neighboring samples How does an H 264 codec work 5 Inter Prediction Inter prediction is used to reduce the temporal correlation with the use of motion estimation and compensation algorithms An image is divided into macroblocks each 16x16 macroblock is further partitioned into 16x16 16x8 8x16 8x8 sized blocks A 8x8 sub macroblock can be further partitioned in 8x4 4x8 4x4 sized blocks Figure in the next slide illustrates the partitioning of a macroblock and a sub macroblock The input video characteristics govern the block size A smaller block size ensures less residual data however smaller block sizes also mean more motion vectors and hence more number of bits required to encode theses motion vectors How does an H 264 codec work 6 Macroblock partitions for motion estimation motion compensation 16x16 16x8 8x16 and 8x8 9 Macroblock sub partitions for motion estimation motion compensation 8x8 8x4 4x8 and 4x4 9 How does an H 264 codec work 7 Deblocking Filter H 264 suffer from blocking artifacts due to block based transform in intra and inter prediction coding and quantization of transform coefficients Deblocking filter reduces the artifacts at the block boundaries and prevents the propagation of accumulated noise The presence of the filter however adds to the complexity of the system as shown in the figure below Filtering is applied to horizontal or vertical edges of 4x4 blocks in a macroblock Boundaries in a macroblock to be filtered luma boundaries shown with solid lines and chroma boundaries shown with dotted lines 1 How does an H 264 codec work 8 Subtracts the prediction from the current block residuals Residual samples are transformed using 4x4 or 8x8 integer transform DCT The output of the transform a block of transform coefficients is quantized Encoding Variable length coding Arithmetic coding H 264 Profiles and levels Profile Set of coding tools or algorithms that can be used in generating a bit stream specific encoding tech Level Places constraints on certain key parameters of the bit stream H 264 Profiles 1 H 264 Profiles and levels 2 Baseline Profile BP Primaryly for lower cost applications with limited computer resources In BP macro blocks need not to be in raster scan order this profile is used for real time conversational services such
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