Comment by ChadNauseam
Comment by ChadNauseam 16 hours ago
That's true for lossless compression. Is there a generalization for lossy compression?
Comment by ChadNauseam 16 hours ago
That's true for lossless compression. Is there a generalization for lossy compression?
Thanks! That's really enlightening. Maybe this can help me settle a question I've had. If I have 4k video and I want a smaller file size (but suppose I still have a 4k tv), I always felt like I should get better quality at that file size by compressing it further than by reducing the resolution. Rather than deciding for myself what data to throw away, why not let the compression algorithm do it?
Lowering the resolution to match the typical TV resolutions is sensible but beyond that trusting the codec is always the better way. The codecs will adaptively compress portions of the video differently. The top left area that's in shadow for the next 30seconds? Maybe that areas effective resolution can be lowered? Etc. They can do things like this!
If you change the resolution or color space of the entire file you do that without consideration to where the extra details might have been needed.
So resolution should match typical output resolutions exactly and from there it's all on the codec.
Lossy and lossless are essentially fundamentally the same actually. All state of the art lossy compressors use arithmetic coding as an example and they still do prediction. Eg. your favourite video codecs predict not only the next bit in the 2D frame, but also the next bit when modelling past frames (becomes a 3D cube of data at that point) and they also do things like motion prediction of individual objects in frame to help make a better prediction. They all use arithmetic encoders to encode the data.
Where the lossy part comes in is the point at which humans notice/don't notice data being thrown away. Got a bit that was waaay out of line in the prediction and going to cost you 10bits to correct? Perhaps to humans it doesn't matter? Can we throw it away? This throwing away of data is often done before the prediction+compression stage (eg. maybe quantizing the color space to 8bits from 32bits?) but from there on it's the same thing.