Do you have a video in MKV file and you can't open it because some program (like Sony Vegas Pro) doesn't support this format? If so, you probably wonder how to convert it into some different format. I just discovered a way to do this.
To understand this method, first you need to know that media file formats are just containers (for example, MKV is Matroska). Each format encapsulates a set of streams, usually one video and one audio stream. Now, each stream is encoded using some specific codec. There may be various codecs used, but for the file I needed to convert, Media Player Classic (my favorite movie player, installed with K-Lite Codec Pack) shows following information after selecting File > Properties:
Video: MPEG4 Video (H264) 720x400 25fps [V: English [eng] (h264 main L4.0, yuv420p, 720x400) [default]]
Audio: AAC 44100Hz stereo [A: aac lc, 44100 Hz, stereo [default]]
MPEG4 Video is the same codec that may be used with MP4 file format! It means we could convert ("repack") the file to just different container format, rewriting streams as-is without actually converting video or audio - which should be very fast (converting a long movie takes only few seconds) and wouldn't cause any quality loss.
To do that, I used VLC media player. This program has its own set of codecs for many video and audio formats, so it doesn't depend on codecs installed in Windows. The player is actually just an overlay on top of a powerful library that can also do different things, like streaming video over network (that's probably where the company name "VideoLAN" comes from) or convert files.
So to convert an MKV file to MP4:
Important update 2020-03-22: I found another way to quickly remux MKV to MP4 using OBS Studio. This is a free and open source app intended mostly for video recording and streaming. The feature we need can be found in menu File > Remux Recordings.