Value range for AVCodecContex::global_quality, ::compression_level and AVFrame::quality

manpreet Best Answer 3 years ago

I'm trying to compress video using different codecs (WMV, MP4, H264, VP8, VP9) using FFmpeg / libav. I'm unsure on how to set the global and per-frame quality values. Even the range of values is unclear to me. The docs are not helpful. Currently I'm using:

AVCodecContex::global_quality = FF_QP2LAMBDA * quality; // no idea about range, default seems to be 0
AVCodecContex::compression_level = 0; // There is FF_COMPRESSION_DEFAULT (-1), which seems to be the default
AVFrame::quality = FF_LAMBDA_MAX * quality; // docs say range is [1, FF_LAMBDA_MAX], default seems to be 0

quality is in the range [0,1], where 0 means best image quality. Only 0 seems to do anything for me and leads to ok-quality videos. All other values produce low-quality output.
Any clarification on how to set these values is welcome.