CRF vs CQP

RF=Rate Factor
QP=Quantizer Parameter

When you use CQP (constant quantizer parameter ), you compress every frame the same amount, regardless of the content.

When you use CRF (constant rate factor), it varies the QP slightly. When a scene has a lot of action and motion, it will raise the QP (compressing more). This is because your eye will be distracted by everything going on, and won't have time to see the heavier compression. When a frame doesn't have a lot of motion, it will lower the QP, compressing it less. This is because your eye will have more time to look at the image, so you want it to be as much like the source as possible.

If you were a computer, you would look at a CRF encoding and say it was lower quality than the CQP copy. And it would be. But if you're a human being, subjectively, the CRF copy will look better. It least compresses the parts you see the most, and most compresses the parts you see the least.

Both CRF and CQP are for single-pass encoding.

When you do a CRF or CQP encoding, you don't have any control over the output's size or bitrate. All you control is how heavily compressed each frame is.

If you need to get a particular size, you can do a 2-pass ABR encoding.


Personally, I always use CRF single-pass encoding. I don't see any reason to use CQP or 2-pass encoding. The latter is much longer, but still can give the result that is worse than CRF encoding. Anyway it might be a good idea to take a video sample and use different encoding methods to compare the results.