Improve parallelization of effects rendering on CPU-only (no GPU) systems

[Timwi]

I am currently rendering a long video that uses a lot of visual effects such as drop shadows. I created it in kdenlive but I also tried the command line (melt) with equivalent results. I am rendering on a PC with an 8-core CPU but no GPU.

According to Task Manager and Process Hacker, melt is only using 12% CPU, which indicates that it uses only one of my 8 cores, even though the input file specifies threads="8". It should be possible to speed up the process a lot by parallelizing the rendering of visual effects in each frame.

Note that I’m not talking about the video encoding done by ffmpeg. I fully understand that it needs to process the frames in sequential order to encode the video file. I’m talking about the visual effects rendering melt does before it sends the rendered frames to ffmpeg.

[remorses]

Try passing the env variable MLT_AVFORMAT_THREADS=8, this way you make sure you add more threads both in ffmpeg consumers and producers