Hi, Michael Niedermayer wrote: > Hi > > On Tue, Jan 30, 2007 at 02:21:19PM +0100, Martin wrote: > >>Hi, >> >>Martin wrote: >> >>>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! > > > to find the exact bitrate for a frame at quality X you have to encode the > frame at that quality thanks for answering! I don't want to find out the exact bitrate of a frame. I want, for example, to half the average bitrate. So the option to set is quality. But if I don't know the relationship of quality and bitrate, I can't half the average bitrate. Martin
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