MOSCOW: The Graphics and Media Lab at Moscow State University recently released the results of its comparison of Google’s WebM VP8 codec with MPEG-4/H.264. VP8 is a royalty-free, open-source codec recently released by Google for compressing online video. H.264 encoders are fee-based.
The release of VP8 sparked an ongoing debate about its quality compared to that of H.264. The Moscow lab used an MPEG-4 x264 encoder for the tests, which involved movie clips and HDTV, and used the structural similarity index, or SSIM. (Image is a frame from the movie “Troy,” used for sharp scene changes and difficulty in compressing small details.)
“Bitrate handling for the VP8 encoder for HDTV is quite good, except the ‘Troy’ sequence at low bitrates,” the results said. With regard to movies: “When comparing VP8 and x264, VP8... shows five to 25 [times] lower encoding speed with 20 to 30 percent lower quality at average. For example, x264 high-speed preset is faster and has higher quality than any of VP8 presets at average.”
The VP8 developers note, however, that source material was encoded such that MPEG-4/H.264 had a home-field advantage.
“One issue we noticed in the test is that most input sequences were previously compressed using other codecs,” they wrote. “These sequences have an inherent bias against VP8 in recompression tests. As pointed out by other developers, H.264 and MPEG-like encoders have slight advantages in reproducing some of their own typical artifacts, which helps their objective measurement numbers but not necessarily visual quality. This is reflected by relatively better results for VP8 on the only uncompressed input sequence, ‘mobile calendar.’
“Even with this limitation, VP8 delivered respectable results against other encoders, especially considering this is the first time VP8 has been included in the test and VP8 has not been specifically optimized for SSIM as some other codecs have. To date, WebM developers have focused on the VP8 decoder performance and are only starting to optimize the encoder for speed. The WebM project has only been underway for three weeks, and we believe that our encoder speed will improve significantly in the near future.”
MPEG-4 AVC/H.264 Video Codec Comparison 2010 - Appendices: VP8, x264 and XviD comparision,Full Appendices, and Short Version
Google’s WebM developer page.
CNet’s “Google tackles VP8 video quality question.”
-- Deborah D. McAdams
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