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MPEG Video (Part 2)

MPEG Video (Part 2). Ketan Mayer-Patel. Last Time. Overall MPEG bitstream organization. I-Frames Examples of many encoding techniques: Subsampling (chrominance planes) Transform Coding (DCT, zig-zag) Run-length Encoding (AC coeffs) Predictive Encoding (DC coeffs)

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MPEG Video (Part 2)

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  1. MPEG Video (Part 2) Ketan Mayer-Patel CS 294-9 :: Fall 2003

  2. Last Time • Overall MPEG bitstream organization. • I-Frames • Examples of many encoding techniques: • Subsampling (chrominance planes) • Transform Coding (DCT, zig-zag) • Run-length Encoding (AC coeffs) • Predictive Encoding (DC coeffs) • Entropy Encoding (Huffman encoding) • Quantization (All coefficients) CS 294-9 :: Fall 2003

  3. This Time • P and B frames • Motion compensation. • Search techniques • The problem with error measurements • Skipped macroblocks • Quantization control • Variable bitrate vs. Constant bitrate • DCT Artifacts • Spider noise • Blockiness CS 294-9 :: Fall 2003

  4. P-Frames • Two types of macroblocks in P-Frames: • I-Macroblocks. • Just like macroblocks in a I-Frame • DC term is differentially encoded from DC predictor • DC predictor is simply last coded DC term. • Predictor reset at slice boundaries. • Encoded as DC size followed by that many bits. • AC terms • RLE’d as (run,value) pairs. Huffman encoded. • P-Macroblocks CS 294-9 :: Fall 2003

  5. Luminance Blocks U Block V Block Block Pattern (3- 9 bits) Motion Vector (variable) Q Scale (5 bits) Macroblock Type (1-6 bits) Macroblock Address Increment (variable) P-Macroblocks Macroblock Type determines if Q Scale, Motion Vector, or Block Pattern exist. One or all of the blocks may be absent in a P-Macroblock. CS 294-9 :: Fall 2003

  6. Address Increment • Each macroblock has an address. • MB_WIDTH = width of luminance / 16 • MB_ROW = row # of upper left pixel / 16 • MB_COL = col. # of upper left pixel / 16 • MB_ADDR = MB_ROW * MB_WIDTH + MB_COL • Decoder maintains PREV_MBADDR. • Set to -1 at beginning of picture. • Set to (SLICE_ROW*MB_WIDTH-1) at slice header. • MB address increment added to PREV_MBADDR provides current macroblock address. • PREV_MBADDR set to current macroblock address. CS 294-9 :: Fall 2003

  7. Address Increment Coding • Address increment coded using Huffman code. • 33 codes for values (1-33). • 1 is smallest (1-bit) • 33 is largest (11-bits) • 1 code for ESCAPE • ESCAPE means add 33 to address increment code that follows. • ESCAPS can be chained allowing any positive value to be encoded as an address increment. • This occurs for I-Frames as well. CS 294-9 :: Fall 2003

  8. MB Type • Huffman coded. • 7 possible codes (1 - 6 bits) • Determine the following: • Intra or non-intra. • Q scale specified or not. • Motion vector exists or not. • Block pattern exists or not. • Not all combinations are possible. • Not all possible combinations are feasible. CS 294-9 :: Fall 2003

  9. Quantization Scale • 5 bits. • Zero is illegal. • Encoded as 1-31 which results in q-scale values of (2-62). • Odd values impossible to encode. • Decoder maintains current q-scale. • If not specified, current q-scale used. • If specified, current q-scale replaced. CS 294-9 :: Fall 2003

  10. Motion Vector • Two components: • Horizontal and vertical offsets. • Offset is from upper left pixel of macroblock. • Positive values indicate right and down. • Negative values indicate left and up. • Offsets are specified in half pixels. • Motion vector is used to define a predictive base for the current macroblock from the reference picture. CS 294-9 :: Fall 2003

  11. Motion Vector Illustrated Previously Decoded I- or P- Frame P-Frame Prediction base does not have to be macroblock aligned. If predictive base is half-pixel aligned, bilinear interpolation is used. Whatever luminance pixels are picked out, corresponding chrominance pixels used to form chrominance prediciton. CS 294-9 :: Fall 2003

  12. Motion Vector Encoding • If no motion vector is present, then motion vector is understood to be (0,0). • Horiz. component followed by vertical. • Decoder maintains motion vector predictor. • Set to 0,0 at beginning of picture or slice or whenever an I-macroblock is encountered. • Difference between predictor and value is Huffman encoded. • Actually a bit more complicated than this. CS 294-9 :: Fall 2003

  13. Predictive Base • P-Macroblocks always specify a predictive base: • Either motion vector picks out an area, or • No motion vector implicitly implies 0,0 (i.e., predictive base is same macroblock in reference frame.) CS 294-9 :: Fall 2003

  14. Block Pattern • The goal of motion compensation is to find predictive base that matches most closely with macroblock. • If match is really good, then no appreciable difference will need to be encoded at all. • Block pattern indicates which blocks have enough error to warrant coding. • Absence of block pattern indicates no blocks needed coding. CS 294-9 :: Fall 2003

  15. Block • Difference between pixels in prediction and macroblock is encoded as block: • 9-bit input values • Still produces 12-bit coefficients • Sometimes called error blocks. CS 294-9 :: Fall 2003

  16. Error Block Encoding • Different quantization matrix is used. • Default is “16” in all coefficient positions. • Error blocks have lots of high frequency info. • No good perceptual correlation between frequencies of error coding and artifacts. • DC no longer specially treated. • No differential encoding from predictor. • All terms are zig-zag RLE’d and then (run,value) pairs Huffman encoded. CS 294-9 :: Fall 2003

  17. P-Frame Review • Macroblocks are either I-macroblocks or P-macroblocks. • I-macroblocks just like macroblocks in I-frame. • P-macroblocks define predictive base and encode the difference. CS 294-9 :: Fall 2003

  18. Skipped Macroblocks • If P-macroblock has (0,0) motion vector and no appreciable difference to encode, then can be skipped altogether. • Skipped macroblock detected when address increment for next coded macroblock is detected. • First block and last block of slice must not be skipped. • Last slice must include lower right macroblock. CS 294-9 :: Fall 2003

  19. Decoder State Updates • DC predictors are reset whenever a P-macroblock or skipped macroblock is encountered. • Motion vector predictors reset whenever I-macroblock is encountered. CS 294-9 :: Fall 2003

  20. B-Frames • B-frames have 4 macroblock types: • I-macroblocks • P-macroblocks • Predictive base specified from previous reference frame. • B-macroblocks • Predictve base specified from subsequent reference frame. • Bi-macroblocks. • Predictive base specified from both reference frames. CS 294-9 :: Fall 2003

  21. Skipped Macroblocks • Handled slightly differently than P-frames. • Skipped macroblock implies: • Same macroblock type as last encoded macroblock (i.e., P-, B-, or Bi-). • Motion vectors same a previous encoded macroblock. • Compare to (0,0) assumption in P-frame. • Also means that predictors not reset. • Can’t skip macroblock following an I-macroblock. • Other state changes as per P-frames. CS 294-9 :: Fall 2003

  22. Motion Compensation • Provides most of MPEG’s compression. • Relies on temporal coherence. • Finding a good motion vector essentially a search problem. • Evaluating “goodness” of a motion vector can be a bit tricky. • MC is what makes MPEG asymmetric. • Harder to encode than to decode. CS 294-9 :: Fall 2003

  23. Exhaustive Search • The most obvious and easiest solution. • Encoding time related to size of search window. • Although time consuming, also embarrassingly parallel. CS 294-9 :: Fall 2003

  24. Logarithmic Search • Evaluate the search window with an even sampling of motion vectors. • Take best and reevaluate in region of the motion vector with denser sampling. CS 294-9 :: Fall 2003

  25. Predictive Search • Motion vectors differentially encoded for a reason. • Tend to be correlated from one macroblock to the next. • Use previous macroblocks motion vector as centering point for search. • Or, use motion vector from same block in previous frame as center of search. • My research is looking at using depth and other spatial info to guide encoding. CS 294-9 :: Fall 2003

  26. Error Measurements • Regardless of search algorithm, need to determine which motion vector is best. • Simple measures: • Mean Squared Error • Mean Absolute Error • Minimum Difference Variance • Fundamental problem is no good correlation between any simple metric and perceptual quality. CS 294-9 :: Fall 2003

  27. VBR vs. CBR • Two ways to handle bitrate: • Variable Bit Rate (VBR) • Allows compressed bitrate to vary • Constant Bit Rate (CBR) • Bitrate constant over some averaging window. • MPEG buffer model. • Optional (don’t have to use it). • Provides in the sequence header parameters to a buffer model that can describe bitrate behavior. CS 294-9 :: Fall 2003

  28. VBR Q-scale adjustments • In general, VBR used to maintain quality. • Q scale is adjusted to provide maximum compression given quality limit. • Need some metric for quality. • Same issue for judging perceptual quality crop up here. • Common solution: q scale statically set for I-, P-, and B-frames. • A variation on this is differentiating among macroblock types. CS 294-9 :: Fall 2003

  29. CBR Q-scale adjustments • To achieve CBR, q-scale used to control bitrate. • Higher q-scale provides better compression at the expense of quality. • Lower q-scale provides better quality at the expense of compression. • Algorithms for controlling how q-scale is adjusted can get pretty complicated. • Common solution is to have target I, P, and B frame sizes and then adjust q-scale as macroblocks are encoded to hit the target. CS 294-9 :: Fall 2003

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