When you use a constant rate factor, it varies the QP slightly. That means that while the average quality as objectively gauged by PSNR goes slightly down, the perceptible image quality goes up. Constant Rate Factor will increase the Q to, say, 20, for high motion frames (compressing them more) and lower it down to 16 for low motion. Slightly more technical explanationĪ constant QP encode at Q=18 will stay at Q=18 regardless of the frame. Many people always use CRF for single-pass encodes and argue there is no reason to ever use CQP. It least compresses the parts you see the most, and most compresses the parts you see the least. But if you’re a human being, subjectively, the CRF copy will look better. If you were a computer, you would look at a CRF encoding and say it was lower quality than the CQP copy. There is not a purist’s argument to made here. This isn’t even like MP3s cutting off highs and lows in music that are audible on CDs. It just wastes space by compressing less in areas you really won’t notice. If you’re using x264, CRF is used by default for constant quality.īut isn’t that other way, constant QP, really better quality in the end? Subjectively, the video will seem to have higher quality. Because of this, a video compressor can apply more compression (drop more detail) when things are moving, and apply less compression (retain more detail) when things are still. The eye perceives more detail in still objects than when they’re in motion. It does this by taking motion into account. The quantization parameter defines how much information to “throw away” from a given block of pixels.Ĭonstant Rate Factor, on the other hand, will compress different frames by different amounts. In tech speak, that’s maintaining a constant QP (quantization parameter). The way constant quality encoding is usually done, it keeps up a constant quality by compressing every frame of the same type the same amount.