[FFmpeg-devel] [PATCH] avfilter: port qp filter from libmpcodecs

[Reimar Döffinger]

Derek Buitenhuis <derek.buitenhuis at gmail.com> wrote:
>On 9/19/2013 12:57 AM, Paul B Mahol wrote:
>> + at table @option
>> + at item qp
>> +Set expression for quantization parameter.
>> + at end table
>> +
>> +The expression is evaluated through the eval API and can contain,
>among others,
>> +the following constants:
>
>This doesn't really make clear what it does of why it's useful...

Out of memory, so there might be inaccuracies:
It modifies the qp table. Those qp values are per macroblock and are for example used by deblocking filters like spp to decide on the strength of deblocking etc. So this fitter allows to fine-tune their behaviour.
It could (but not sure if it is) also be used as a hint for encoders, blocks with high qp are likely to have mostly artefacts as details and wasting bits on representing them is pointless.
For decoders that provide a broken/wrongly scaled qp table this was in MPlayer also a way to make the deblocking filters at least somewhat work.