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correspondingly smaller when chroma subsampling is used the ability to use multiple motion vectors per macroblock one or two per partition with a maximum of 32 in the case of a b macroblock constructed of 16 4 4 partitions the motion vectors for each 8 8 or larger partition region can point to different reference pictures the ability to use any macroblock type in b frames including i macroblocks resulting in much more efficient encoding when using b frames this feature was notably left out from mpeg 4 asp six tap filtering for derivation of half pel luma sample predictions for sharper subpixel motion compensation quarter pixel motion is derived by linear interpolation of the halfpixel values to save processing power quarter pixel precision for motion compensation enabling precise description of the displacements of moving areas for chroma the resolution is typically halved both vertically and horizontally see 4 2 0 therefore the motion compensation of chroma uses one eighth chroma pixel grid units weighted prediction allowing an encoder to specify the use of a scaling and offset when performing motion compensation and providing a significant benefit in performance in special cases such as fade to black fade in and cross fade transitions this includes implicit weighted prediction for b frames and explicit weighted prediction for p frames spatial prediction from the edges of neighboring blocks for intra coding rather than the dc only prediction found in mpeg 2 part 2 and the transform coefficient prediction found in h 263v2 and mpeg 4 part 2 this includes luma prediction block sizes of 16 16 8 8 and 4 4 of which only one type can be used within each macroblock integer discrete cosine transform integer dct 5 22 23 a type of discrete cosine transform dct 22 where the transform is an integer approximation of the standard dct 24 it has selectable block sizes 25 and exact match integer computation to reduce complexity including an exact match integer 4 4 spatial block transform allowing precise placement of residual signals with little of the ringing often found with prior codec designs it is similar to the standard dct used in previous standards but uses a smaller block size and simple integer processing unlike the cosine based formulas and tolerances expressed in earlier standards such as h 261 and mpeg 2 integer processing provides an exactly specified decoded result an exact match integer 8 8 spatial block transform allowing highly correlated regions to be compressed more efficiently than with the 4 4 transform this design is based on the standard dct but simplified and made to provide exactly specified decoding adaptive encoder selection between the 4 4 and 8 8 transform block sizes for the integer transform operation a secondary hadamard transform performed on dc coefficients of the primary spatial transform applied to chroma dc coefficients and also luma in one special case to obtain even more compression in smooth regions lossless macroblock coding features including a lossless pcm macroblock representation mode in which video data samples are represented directly 26 allowing perfect representation of specific regions and allowing a strict limit to be placed on the quantity of coded data for each macroblock an enhanced lossless macroblock representation mode allowing perfect representation of specific regions while ordinarily using substantially fewer bits than the pcm mode flexible interlaced scan video coding features including macroblock adaptive frame field mbaff coding using a macroblock pair structure for pictures coded as frames allowing 16 16 macroblocks in field mode compared with mpeg 2 where field mode processing in a picture that is coded as a frame results in the processing of 16 8 half macroblocks picture adaptive frame field coding paff or picaff allowing a freely selected mixture of pictures coded either as complete frames where both fields are combined for encoding or as individual single fields a quantization design including logarithmic step size control for easier bit rate management by encoders and simplified inverse quantization scaling frequency customized quantization scaling matrices selected by the encoder for perceptual based quantization optimization an in loop deblocking filter that helps prevent the blocking artifacts common to other dct based image compression techniques resulting in better visual appearance and compression efficiency an entropy coding design including context adaptive binary arithmetic coding cabac an algorithm to losslessly compress syntax elements in the video stream knowing the probabilities of syntax elements in a given context cabac compresses data more efficiently than cavlc but requires considerably more processing to decode context adaptive variable length coding cavlc which is a lower complexity alternative to cabac for the coding of quantized transform coefficient values although lower complexity than cabac cavlc is more elaborate and more efficient than the methods typically used to code coefficients in other prior designs a common simple and highly structured variable length coding vlc technique for many of the syntax elements not coded by cabac or cavlc referred to as exponential golomb coding or exp golomb loss resilience features including a network abstraction layer nal definition allowing the same video syntax to be used in many network environments one very fundamental design concept of h 264 is to generate self contained packets to remove the header duplication as in mpeg 4 s header extension code hec 27 this was achieved by decoupling information relevant to more than one slice from the media stream the combination of the higher level parameters is called a parameter set 27 the h 264 specification includes two types of parameter sets sequence parameter set sps and picture parameter set pps an active sequence parameter set remains unchanged throughout a coded video sequence and an active picture parameter set remains unchanged within a coded picture the sequence and picture parameter set structures contain information such as picture size optional coding modes employed and macroblock to slice group map 27 flexible macroblock ordering fmo also known as slice groups and arbitrary slice ordering aso which are techniques for restructuring the ordering of the representation of the fundamental regions macroblocks in pictures typically considered an error loss robustness feature fmo and aso can also be used for other purposes data partitioning dp a feature providing the ability to separate more important and less important syntax elements into different packets of data enabling the application of unequal error protection uep and other types of improvement of error loss robustness redundant slices rs an error loss robustness feature that lets an encoder send an extra representation of a picture region typically at lower fidelity that can be used if the primary representation is corrupted or lost frame numbering a feature that allows the creation of sub sequences enabling temporal scalability by optional inclusion of extra pictures between other pictures and the detection and concealment of losses of entire pictures which can occur due to network packet losses or channel errors switching slices called sp and si slices allowing an encoder to direct a decoder to jump into an ongoing video stream for such purposes as video streaming bit rate switching and trick mode operation when a decoder jumps into the middle of a video stream using the sp si feature it can get an exact match to the decoded pictures at that location in the video stream despite using different pictures or no pictures at all as references prior to the switch a simple automatic process for preventing the accidental emulation of start codes which are special sequences of bits in the coded data that allow random access into the bitstream and recovery of byte alignment in systems that can lose byte synchronization supplemental enhancement information sei and video usability information vui which are extra information that can be inserted into the bitstream for various purposes such as indicating the color space used the video content or various constraints that apply to the encoding sei messages can contain arbitrary user defined metadata payloads or other messages with syntax and semantics defined in the standard auxiliary pictures which can be used for such purposes as alpha compositing support of monochrome 4 0 0 4 2 0 4 2 2 and 4 4 4 chroma sampling depending on the selected profile support of sample bit depth precision ranging from 8 to 14 bits per sample depending on the selected profile the ability to encode individual color planes as distinct pictures with their own slice structures macroblock modes motion vectors etc allowing encoders to be designed with a simple parallelization structure supported only in the three 4 4 4 capable profiles picture order count a feature that serves to keep the ordering of the pictures and the values of samples in the decoded pictures isolated from timing information allowing timing information to be carried and controlled changed separately by a system without affecting decoded picture content these techniques along with several others help h 264 to perform significantly better than any prior standard under a wide variety of circumstances in a wide variety of application environments h 264 can often perform radically better than mpeg 2 video typically obtaining the same quality at half of the bit rate or less especially on high bit rate and high resolution video content 28 like other iso iec mpeg video standards h 264 avc has a reference software implementation that can be freely downloaded 29 its main purpose is to give examples of h 264 avc features rather than being a useful application per se some reference hardware design work has also been conducted in the moving picture experts group the above mentioned aspects include features in all profiles of h 264 a profile for a codec is a set of features of that codec identified to meet a certain set of specifications of intended applications this means that many of the features listed are not supported in some profiles various profiles of h 264 avc are discussed in next section profiles edit the standard defines several sets of capabilities which are referred to as profiles targeting specific classes of applications these are declared using a profile code profile_idc and sometimes a set of additional constraints applied in the encoder the profile code and indicated constraints allow a decoder to recognize the requirements for decoding that specific bitstream and in many system environments only one or two profiles are allowed to be used so decoders in those environments do not need to be concerned with recognizing the less commonly used profiles by far the most commonly used profile is the high profile profiles for non scalable 2d video applications include the following constrained baseline profile cbp 66 with constraint set 1 primarily for low cost applications this profile is most typically used in videoconferencing and mobile applications it corresponds to the subset of features that are in common between the baseline main and high profiles baseline profile bp 66 primarily for low cost applications that require additional data loss robustness this profile is used in some videoconferencing and mobile applications this profile includes all features that are supported in the constrained baseline profile plus three additional features that can be used for loss robustness or for other purposes such as low delay multi point video stream compositing the importance of this profile has faded somewhat since the definition of the constrained baseline profile in 2009 all constrained baseline profile bitstreams are also considered to be baseline profile bitstreams as these two profiles share the same profile identifier code value extended profile xp 88 intended as the streaming video profile this profile has relatively high compression capability and some extra tricks for robustness to data losses and server stream switching main profile mp 77 this profile is used for standard definition digital tv broadcasts that use the mpeg 4 format as defined in the dvb standard 30 it is not however used for high definition television broadcasts as the importance of this profile faded when the high profile was developed in 2004 for that application high profile hip 100 the primary profile for broadcast and disc storage applications particularly for high definition television applications for example this is the profile adopted by the blu ray disc storage format and the dvb hdtv broadcast service progressive high profile phip 100 with constraint set 4 similar to the high profile but without support of field coding features constrained high profile 100 with constraint set 4 and 5 similar to the progressive high profile but without support of b bi predictive slices high 10 profile hi10p 110 going beyond typical mainstream consumer product capabilities this profile builds on top of the high profile adding support for up to 10 bits per sample of decoded picture precision high 4 2 2 profile hi422p 122 primarily targeting professional applications that use interlaced video this profile builds on top of the high 10 profile adding support for the 4 2 2 chroma sampling format while using up to 10 bits per sample of decoded picture precision high 4 4 4 predictive profile hi444pp 244 this profile builds on top of the high 4 2 2 profile supporting up to 4 4 4 chroma sampling up to 14 bits per sample and additionally supporting efficient lossless region coding and the coding of each picture as three separate color planes for camcorders editing and professional applications the standard contains four additional intra frame only profiles which are defined as simple subsets of other corresponding profiles these are mostly for professional e g camera and editing system applications high 10 intra profile 110 with constraint set 3 the high 10 profile constrained to all intra use high 4 2 2 intra profile 122 with constraint set 3 the high 4 2 2 profile constrained to all intra use high 4 4 4 intra profile 244 with constraint set 3 the high 4 4 4 predictive profile constrained to all intra use cavlc 4 4 4 intra profile 44 the high 4 4 4 predictive profile constrained to all intra use and to cavlc entropy coding i e not supporting cabac as a result of the scalable video coding svc extension the standard contains five additional scalable profiles which are defined as a combination of a h 264 avc profile for the base layer identified by the second word in the scalable profile name and tools that achieve the scalable extension scalable baseline profile 83 primarily targeting video conferencing mobile and surveillance applications this profile builds on top of the constrained baseline profile to which the base layer a subset of the bitstream must conf...
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