[Libav-user] How to pass quality scale to mjpeg encoder

[Tobias Rapp]

On 25.01.2018 09:25, Tobias Rapp wrote:
> Hi,
> 
> trying to do the equivalent of FFmpeg command-line output option "-q:v 
> 3" on library level I came up with the following code (error checking 
> stripped off):
> 
>      AVCodecContext *enc_ctx = NULL;
>      const AVCodec *codec = avcodec_find_encoder_by_name("mjpeg");
>      int ret;
> 
>      enc_ctx = avcodec_alloc_context3(codec);
>      enc_ctx->flags |= AV_CODEC_FLAG_QSCALE;
>      enc_ctx->global_quality = FF_QP2LAMBDA * 3.0;
>      ret = avcodec_open2(enc_ctx, enc_ctx->codec, NULL);
> 
> Unfortunately it doesn't seem to have any effect on the output image 
> size. Is there something obvious that I'm missing?
> 

Answering myself: Apparently setting the encoder option is not 
sufficient for mjpeg. The encoding quality has to be set for each frame 
that is passed to the encoder.

Example:

     frame->quality = enc_ctx->global_quality;
     frame->pict_type = AV_PICTURE_TYPE_NONE;
     ret = avcodec_send_frame(enc_ctx, frame);
     if (ret < 0) {
         // ...
     }

Looks weird but seems to work.

Regards,
Tobias