This FFmpeg command combines a video stream from a `.mkv`...

This FFmpeg command combines a video stream from a .mkv file (INPUT0.mkv) and an audio stream from a .aac file (INPUT1.aac) into a new .mp4 file (OUTPUT.mp4) without re-encoding the streams. Here's a detailed explanation:

1. -i INPUT0.mkv: Specifies the input file INPUT0.mkv, which contains at least one video stream (and possibly other streams).

2. -i INPUT1.aac: Specifies the second input file INPUT1.aac, which contains at least one audio stream.

3. -map 0:0: Selects the first stream (index 0) from the first input file (INPUT0.mkv). This is usually the video stream.

4. -map 1:0: Selects the first stream (index 0) from the second input file (INPUT1.aac). This is the audio stream.

5. -c copy: Specifies that both the video and audio streams should be copied directly into the output file without any re-encoding. This ensures fast processing and no quality loss.

6. OUTPUT.mp4: Specifies the name of the output file, in this case OUTPUT.mp4, which has a container format that supports both the selected video and audio streams.

Summary of what the code does:

• It takes the video stream from INPUT0.mkv and the audio stream from INPUT1.aac.
• It combines these two streams into an OUTPUT.mp4 file without modifying (re-encoding) the actual content of the streams.