To understand why this is necessary, it's important to learn the difference between video codecs and containers. In this case, the codecs will be adapted to the output container. If this command does not work, and if your input uses the wrong codecs for the output container, you will probably have to re-encode. Also, subtitle format support for MP4 is different from MKV, and actually quite restricted, so this command may fail. However, there's a big caveat: this only works if the audio and video codecs are supported in the target (MP4) container, which is the case for H.264/H.265 and AAC, for example, but not for many others. There are also tools like MP4Box which can also create MP4 containers - the same exists for MKV with MKVtoolnix.

You can swap containers easily with FFmpeg – you just have to tell it to copy the video and audio streams: ffmpeg -i input.mkv -c copy -map 0 output.mp4 This doesn't lose quality, and it'll be a much faster process. Since passthrough is not possible, ask yourself: Do I need to re-encode? If you only want to change the container from MKV to MP4, you usually don't need to encode anything, you just change the 'wrapping' around the video. Do I have to re-encode? Couldn't I just swap the container?

So, anytime you're changing containers with Handbrake, your video is going to get re-encoded, which means it a) takes time and b) may introduce quality loss. HandBrake is designed to be a video transcoder. Sorry, adding video passthrough is not planned. See also the respective feature request that would have enabled video passthrough:

MKV to MP4 in your case), Handbrake will re-encode the video. When you're using Handbrake to convert from one container format to

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