MP4 Video Compression
MP4s are video files (with audio) compressed using lossy compression.
The lossy compression gives huge savings in file size, needed to:
- Make video streaming services practical (e.g. YouTube, Vimeo, NowTV, on demand TV)
- Let us record and store lots of video files on devices with only modest capacities
Like all lossy compressed files, savings in size are made by deleting data that the computer believes is redundant and will not be missed by the user.
MP4 video compression reduces a file size by spiting the image in each video frame into small blocks of pixels, then:
- Only store data for blocks that have changed from one frame to the next (if a block remains unchanged, all that needs storing is a note to say “nothing has changed”)
- Recognising where a block of pixels has moved location but remains the same (and apply some maths accordingly)
- Reducing the level of detail in a pixel block (blending similar colours into one or smoothing out textures etc)
When compressing a video file it is important to strike the right balance between the overall file size and the video quality. The more aggressively you compress the file to make it smaller, the poorer the video quality will be. This decision is taken out of your hands when uploading to some video streaming websites.