Performance improvement: double-buffer

[time4tea]

There's a new command line switch in 0.92.. --double-buffer which may give big performance improvements.

It works on Linux at least, and in some configurations can give massive speed up.

If you have tried it, please do feed back whether it was successful, and if it gave any speed up.

It is likely to give best improvement when ffmpeg not running at full capacity... this will be 1) using powerful gfx card and using nvidia encoding 2) when using --generate overlay

Using a "null" encoding I get about 150fps on my 2016 desktop computer, and between 50-100fps when using other overlay modes.

It would be great to hear other user feedback

Thanks!

James