[Libav-user] how to set mjpeg encoding quality programmatically?

[YIRAN LI]

2016-06-10 13:32 GMT+10:00 YIRAN LI <mrfun.china at gmail.com>:

>
>
> 2016-06-09 23:29 GMT+10:00 Carl Eugen Hoyos <cehoyos at ag.or.at>:
>
>> YIRAN LI <mrfun.china at ...> writes:
>>
>> > AVCodecContext.flags |= CODEC_FLAG_QSCALE
>> > and AVCodecContext.global_quality  = qscale * FF_QP2LAMBDA,
>> > in my case qscale = 3
>>
>> This is correct, qscale 3 ensures (with mpeg-4 asp) a
>> near-lossless visual quality.
>>
>>
> ​Thansk Carl, I think I can use the same way to set quality for mpeg4
> video, right?
>
> What't the differenct between it and seting qmin qmax in codeccctx?
>
> Thanks
> ​
>
>

​Hi Carl, I asked this because I found whatever qsacale value I set, the
size and quality of output mpeg4 videos are same.

I did a bit debug, and found the cause (I'm using an old version of ffmpeg)

​In the encode2 function, it uses

MpegEncContext *s = avctx->priv_data;
s->qscale ​ as the qsacle value.

So setting it into pVideoCodecCtx->global_quality doesn't pass the value
into priv_data

I tried av_opt_set_int(pVideoCodecCtx->priv_data, "qscale", 3, 0);  but
failed.

Here is the mpeg4 encoder, there's a qscale member in MpegEncContext.

Could you give me a hint how to set it?

Thanks


​


Carl Eugen

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