James Morrow
Stunt Coordinator
- Joined
- Jun 1, 2004
- Messages
- 107
Microsoft are pushing for HD on red laser products using WMV at around 8Mbps, and I've had a close look at the dozen or so demos of this they've provided on their website. On an identical resolution display (i.e. 1280 by 1024 pixels for the 720p24 demos and 1920 by 1440 pixels for the 1080p24 demos) they look awesome, considering the bitrate. However, it appears that alignment errors - probably an artifact of the motion vector estimation process which is fundamental to WMV and MPeg4 getting down to the low bitrates they were designed for - cause strange "cogging" movements as the camera pans across trees, and other motion artifacts, particularly on natural objects - manmade objects tend to look fine. If these are a consequence of the decisions made to get down to low bitrates, these artifacts may still appear in higher bitrate WMV/MPeg4 because motion vector estimation is still used. MPeg2 doesn't have these artifacts because it doesn't use this compression technology - but then it doesn't go down to single figure Mbitrates either. :rolleyes
Just because MPeg4/WMV are younger than MPeg2 does not make them better. MPeg2 has been steadily improved, and MPeg4/WMV were not designed to producer better quality than MPeg2, just lower bitrates. You can’t just give them more bits and expect them to improve on HD MPeg2 at, say, 28Mbps. Given that high definition capable discs now exist which can easily support over four hours at 24Mbps or so, it would be nice to see codecs optimised for these rates. Maybe even MPeg4/WMV with an "intelligent" tuning process which restricts or adapts the use of motion vector estimation across natural areas?
Of course, the best way to compare MPeg2 with MPeg4/WMV is with a large high quality screen at full 1920 by 1080 p24 (or higher) resolution, but not too many people have those yet. It would also best show if the motion vector artifacts present in MPeg4/WMV are better suppressed at higher bitrates than 8Mbps or if the precision of the compression algorithm is not currently sufficient when handling natural objects to provide a convincing impression of stability.
The 15, 20, 25 and 30 Mbit tests that John speaks of would show us where we are now, I would guess that a more intelligent use of the MPeg4/WMV algorithms within each image sequence would allow the best of these to avoid such artifacts whilst still delivering a more stable picture than MPeg2 and at a bitrate of around 20Mbps - this would also leave enough room for lossless audio - even Sony's first generation BD recorder could cope with a bitrate of at least 28.6Mbps.
Just because MPeg4/WMV are younger than MPeg2 does not make them better. MPeg2 has been steadily improved, and MPeg4/WMV were not designed to producer better quality than MPeg2, just lower bitrates. You can’t just give them more bits and expect them to improve on HD MPeg2 at, say, 28Mbps. Given that high definition capable discs now exist which can easily support over four hours at 24Mbps or so, it would be nice to see codecs optimised for these rates. Maybe even MPeg4/WMV with an "intelligent" tuning process which restricts or adapts the use of motion vector estimation across natural areas?
Of course, the best way to compare MPeg2 with MPeg4/WMV is with a large high quality screen at full 1920 by 1080 p24 (or higher) resolution, but not too many people have those yet. It would also best show if the motion vector artifacts present in MPeg4/WMV are better suppressed at higher bitrates than 8Mbps or if the precision of the compression algorithm is not currently sufficient when handling natural objects to provide a convincing impression of stability.
The 15, 20, 25 and 30 Mbit tests that John speaks of would show us where we are now, I would guess that a more intelligent use of the MPeg4/WMV algorithms within each image sequence would allow the best of these to avoid such artifacts whilst still delivering a more stable picture than MPeg2 and at a bitrate of around 20Mbps - this would also leave enough room for lossless audio - even Sony's first generation BD recorder could cope with a bitrate of at least 28.6Mbps.