1. Real Time Transcoding Soam Acharya July 30, 1997 Cornell University

2. Why Transcode? n Random access vs. Good compression n M-JPEG vs. MPEG n Editing n Processing n Gateways

3. Roadmap n Brief introduction to MPEG and JPEG n Transcoding:Spatial Domain Approach n Transcoding:Compressed Domain n Experiments n Results n CMT/Demo

4. JPEG Summarized DCTQuantize Zigzag Scan Run-length Code Huffman Code Block 0101.

5. MPEG Summarized n Sequence of images/frames n reference frames: past, future n frames consist of various types of blocks

6. MPEG Summarized (cont) I t o P p r o c e s s i n g Compressed block Residual block: correction coefficients Predicted Block Decompressed Block

7. Spatial Domain Transcoding n Simple Method –Decompress each MPEG frame to pixel image –JPEG compress each pixel image n public MPEG decoders, JPEG encoders readily available n “straw man” for comparisons MPEG decompress JPEG compress

8. Spatial Domain Transcoding II n Optimizations –I frames: direct conversion –lazy decompression, cache decompressed blocks n Quality: OK n Speedup: a few percent (except for I frames) I PPI B BBBBB

9. Source Block Extraction Problem predicted block overlaps reference blocks portion of reference image reference blocks required to reconstruct predicted block

10. Compressed Domain Transcoding n Develop compressed domain translation Compressed Domain Source Block Extraction Process

11. Using Translation For Block Extraction Translating on each axis separately dx dy Add dx Add dx Add A

12. Deriving the Translation Operator 011011... RLE vector IDCT, etc Entropy Decoding 72717073 7273 75747372 70 69 011011... FDCT, etc Entropy Coding 72 74 73 7475 72747375 71706968 36.2 1.3 0.4 -0.8 0.0 36 1 0 0 36 1 0 0 0 1 0 36 1 0 0 0 1 0

13. Translation Transform Calculation n Techniques to –skip multiplies –avoid unnecessary computations –unroll loops –hardcode multiplies

14. n All transcoding implementations run on HP 735 PA-RISC machine n MPEG decoder from Berkeley n Performance measured in terms of frames transcoded per second n Frames were grayscale n Run on various MPEG sequences n Quality in PSNR –20 is bad quality, 30 is good, 40 is perfect Experiments

15. Experiments on “bike.mpg” frames/sec PSNR PSNR PSNR Speedup I P B Method I 5.7 36.3 37.5 37.8 1.0 Method II 5.9 36.3 37.4 37.7 1.0 Method III maxerr=0 6.5 36.3 35.3 35.7 1.1 maxerr=5 8.1 36.3 35.0 35.3 1.4 maxerr=10 9.6 36.3 34.5 34.7 1.7 maxerr=15 10.5 36.3 33.5 33.6 1.8

16. Picture Quality Comparisons Decoded to grayscale Maxerr = 0Maxerr = 10

17. Results On Various Streams

18. Performance Charaterizations n Sparseness of MPEG stream –sparse: us.mpg, cannon.mpg –dense: bike.mpg, alesi.mpg –non-standard quantization tables: bike.mpg n Good for sparse streams (conferencing) n Potential for improvement: –50% of code spent on matrix multiplies –rest divided between Huffman encoding and run length encoding matrices to vectors

19. Integration into CMT MPEG File Transcoder JPEG MixerJPEG Player