Terje: mystery solved. Andrew's response jostled my brain and here is the answer for Cinelerra usage.

You ran the command using the operating system (Leap 15) ffmpeg package.

If you run the command using cd to cinelerra path /thirdpackage/ffmpeg4.1 then ./ffmpeg -h encoder=libx264 see:

Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p nv12 nv16 nv21 yuv420p10le yuv422p10le yuv444p10le nv20l

This is exactly why cinelerra likes to include the thirdparty packages with it so as to know what is actually being used as opposed to what the O/S has installed.

For x265, I have:

Supported pixel formats: yuv420p yuv422p yuv444p gbrp gray

Might be different for CINX. gg/Phyllis