At 11:36 AM -0500 12/9/09, Tom Barry wrote:
I guess it is time for me to repeat one of my pet lobby/rants that many of you have already heard on this list. To wit: 1) If bit rates are sufficiently constrained then 720p is better than 1080p and maybe 480p is even better than 720p. 2) We never have enough damn bit rate. 3) Some material needs more bits than others and the needs vary continuously. Therefore we should not fix an emission resolution but allow it to vary continuously from moment to moment based upon the economics of the current situation. Let's just design encoders and decoders such that they support anything up to some practical maximum dictated by current resource usage (memory, processor, etc). Then let the resolutions vary along with the bit rates controlled by a stat muxer for the whole stream. At one moment one stream might be 480p and another 720p, but always changing with the sum of all streams optimized. Of course no such encoders, decoders, and muxers are currently available. But I believe we should make some. It is silly to throw away bits to constrain bit rate without also being allow to throw away pixels. - Tom
Actually Tom, this was envisioned when the MPEG standards were created. The profiles and levels only defined performance limits. There has never been a need to constrain MPEG to specific spatial or temporal formats, only to limit the combinations of frame size and frame rate to fit within the performance limits for a level.
I have long advocated changing frames rates dynamically to deal with the motion requirements of each scene. Changing spatial resolution dynamically is a bit more problematic as it affects receiver design, requiring real time scaling changes to the local display resolution. Instead, what has happened is that most encoders now employ closed loop low pass filters which can limit the amount of information reaching the encoder. Thus, for example, a scene may be filtered to the equivalent resolution of 480p when the encoder is stressed, but deliver the full 720P when the scene is not as complex.
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