[Ffmpeg-devel] Bug in libavcodec/x264.c? (original) (raw)

Luca Abeni lucabe72
Thu Feb 2 13:15:05 CET 2006

Hi M?ns,

On Thu, 2006-02-02 at 11:41 +0000, M?ns Rullg?rd wrote: [...]

The H.264 quantizer is in the range 1..51. Using a value of 10 or less the results are usually visually indistinguishable from the original. Values higher than 30 start giving a blocky look. Ok. Thanks for the explaination

> The result of this is that using default parameters h264 rate control > does not work very well in ffmpeg (if I encode mpeg4 video, it always > matches the target bitrate, but if I try encoding h264 I often get a lot > of VBV underflows and the final bitrate is wrong). > > So, how can I correctly convert qmin/qmax values in iqpmin/iqpmax > values?

For the rate control to work properly you should not restrict the quantizer range. In fact, the idea was to set qmin at the minimum possible value and qmax at the maximum possible value :)

On the other hand, you really don't want to use values higher than 31 anyway, so if the rate control fails with that limit you should use a higher bitrate. Well, the problem is that I am trying to stream the video across a network with a limited bandwidth... With MPEG4, using -mbd 2 I can obtain a decent quality while remaining under my bandwidth constraints. I tried h264 (using libavcodec's x264.c), and the quality is much better... But the bandwidth constraints are not respected (because of all the VBV underflows) and a lot of packets are dropped by the network. So, when streaming MPEG4 is acceptable, and x264 is not.

Anyway, if I understand you correctly changing the q mapping in x264.c does not make too much sense, and I will have to explicitly set qmax=51 when using x264, right?

BTW: I am trying to encode 352x288 video captured from a tv card... Does anyone have suggestions about proper codec values for MPEG4 and for x264? I hope to be able to encode at 400kbps (with the current default settings, x264 is generally not able able to encode 252x288 at less than 600kbps), with peaks of 500kbps.

        Thanks,
            Luca

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