Fast Transcoding of Compressed Bitstreams by Reusing Incoming Motion Vectors

Transcoding will be an important activity in the digital studio. People will transcode MPEG-2 bitstreams to resize images, to support progressive or interlaced receivers, to optimize for editability or storage/transmission efficiency, etc. Transcoding involves decoding a bitstream, possibly modifying the decoded pictures, and then re-encoding. Transcoding is expensive because encoding is expensive, and the major cost of encoding is ME (motion estimation). — We have developed a program, MVREX, that reuses incoming motion vectors during transcoding. MVREX (Motion Vector Reuse by EXtrapolation) assumes that motion is linear, and extrapolates known motion from input macroblocks to output macroblocks, avoiding ME. MVREX has transcoded both progressive and interlaced video between different GOP structures. It supports all MPEG-2 prediction modes for frame pictures, except dual prime. — In the cases examined, MVREX produces video whose PSNR is usually within 1 dB of that produced by a full search, but with orders of magnitude less computation. It continues to work well after multiple generations of transcoding, and appears to be a promising approach to transcoding. — After a brief review of MPEG-2, this paper describes the concept of motion extrapolation and derives basic formulas. It then presents several results and observations, and concludes.

Published

1998-10

Content type

Original Research

DOI

10.5594/M00270

ISBN

978-1-61482-929-4