These videos I want to encode have:

• stream 0 is hevc 10-bit video
• stream 2 is “VO DDP 5.1 atmos”
• Stream 5 is english subtitles (ass)

I want to convert that to something my old Roku can play. I only care about those three streams. Video should be 8-bit h264, audio should be ac3.

ffmpeg -i original.mkv -map 0:0 -map 0:2 -map 0:5 -c:v libx264 -crf 18 -vf format=yuv420p -c:a ac3 output.mkv