Hi, Michael Niedermayer wrote: > Hi > > On Fri, Jan 19, 2007 at 11:23:59AM +0100, Martin wrote: > >>Hi, >> >>Michael Niedermayer wrote: >> >>>Hi >>> >>>On Wed, Jan 17, 2007 at 11:14:40AM +0100, Martin wrote: >>>[...] >>> >>> >>>>>>Even better, is there a >>>>>>possibility to change the bitrate without closing and reopening ffmpeg? >>>>> >>>>> >>>>>you can set the quality of frames which affects the bitrate, >>>>>closing and reopening within the same stream is generally not allowed >>>>>doing so means you are on your own >>>> >>>>Ok, how can I set the quality within the same stream without closing and >>>>reopening? I tried to set the AVCodecContext.crf parameter but it >>>>doesn't seem to have any effect on quality and size of outputted frames. >>>>Is there another parameter which I can use? >>> >>> >>>AVFrame.quality >>>and set CODEC_FLAG_QSCALE >> >>Thank you very much, that worked. How is the relationship between >>quality and bitrate? Is there a formula which expresses it? > > > besides encode at quality X and look at the bitrate, no but theres some > approximation, and that is that quality and bitrate are approximatly > inversly proportional so double the quality variable and bitrate should > be approximately cut in half what I measured didn't give this behaviour (double quality -> half bitrate): I used a bitrate of 1280000 bits/sec in AVCodecContext. Measured Bitrate with different qualities: ## Qual Bitrate 1) 600: 3012856 2) 1200: 1002368 3) 2400: 556024 4) 4800: 419112 A function is inversely proportional if x*y = a, where a is some constant. So the products of quality and bitrate are: 1) 1807713600 2) 1202841600 3) 1334457600 4) 2011737600 This is far from approximately constant. There must be some computation that you are doing to get a bitrate from quality, right? I really need to know how to compute it and appreciate any help on this! Thanks, Martin
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