[FFmpeg-devel] How to use threads inside custom encoder

[Andreas Rheinhardt]

Ronald S. Bultje:
> Hi,
> 
> On Thu, Feb 23, 2023 at 1:28 PM Alex <3.14pi at ukr.net> wrote:
> 
>> Hi!
>> I write custom encoder codec and want to use threads to speed up encoding
>> process. I know what ffmpeg have frame level threads and slices threads,
>> but in my case best option is to use frame level threads
>> with FF_CODEC_ENCODE_CB() function.
>> But I have couple of questions:
>>
>> 1) Then I add AV_CODEC_CAP_FRAME_THREADS flag to capabilities of my
>> encoder then ffmpeg call init function of my encoder for each spawned
>> threads (for example 9 times because I have 8 core cpu ). How to prevent
>> this and call init function only once? (I need to reuse encoder context.)
>>
> 
> In frame threading, each "frame instance" has its own context. They can be
> synchronized and use dependent data. See how other codecs do this, e.g.
> h264 decoder (the concept is identical between encoder & decoder). You can,
> for example, use avcodec->internal->is_copy for this. See
> update_thread_context() for synchronization between threads.
> 

This is absolutely wrong: The frame-threaded-encode API is very simple
and is only intended for encoders with only intra packets. There is no
way to indicate progress to threads encoding later packets. It is
different from the decode API.

- Andreas