Understanding video compression

[cooljethro]

I'm having a hard time understanding how programs such as Handbrake make file sizes so much smaller?

I compressed a video of over 1.0 GB to about 0.09 GB, going from 1080p to 720p.

I thought to myself, surely going from 1080p to 720p couldn't make the video approximately ten times smaller?

So I tried converting a video from an original 1080p resolution to the 1080p preset in Handbrake, and still, the file size reduced by around 33%. What is the technical wizardry behind this? I don't understand it.

 

[Outdated]

Essentially what you need to read about for the technical details is Jpeg Compression because that's how it works.

To simplify - the computer sees an image as a single line of code. If there's a lot of pixels all in a line that are exactly the same colour then the computer can make the line of code shorter by saying repeat this colour 76 times. The trouble is that you don't get many pixels in a row that are all exactly the same colour so the computer doesn't get to save much space by that alone.

Compression works by letting the computer say "OK, all these colours are so close to each other that no one will notice the difference, I'll just make them all the same colour.". The more compression you include the less difference there is between all the different tones that make up the picture. You'll notice this first in darker areas of images and in people's skin tones.

Also, you mentioned that you downsized the image, so this is naturally going to take up a lot less space too.