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Old 11th February 2021, 01:45   #2  |  Link
Asmodian
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Join Date: Feb 2002
Location: San Jose, California
Posts: 4,406
No, it will not just shift the average up or down. All that does is clip the value if the encoder wants to go above or below the QP limits.

It would limit how far above or below the average could go but to have any effect with real content it would need to be set too restrictive. You would get a lot of blocks that looked terrible (difficult to compress) while also allocating extra bits to blocks that don't need it (easily compressed). The encoder will also use much lower QPs for blocks that get referenced a lot and much higher QPs for non-reference B-frames.

These QP values will be much lower and higher than the average, so to have clipping the min/max QP significantly affect the file size you need to set them very restrictive which then destroys most of the benefit of the rate control. It moves you toward a constant QP rate control, i.e. less quality for larger file sizes.

If you want a specific size use two pass. If not use CRF. That the file size is entirely unpredictable with crf is a feature, you don't want to waste bits or have any videos look terrible do you?

Edit: An example for why raising the minimum QP just a bit does not really change the average QP. The few I frames that would have used that lower QP now get forced to use a higher one. This means future blocks that use that frame as a reference will have more significant difference blocks, requiring a lower QP for the same quality in all those blocks. This value will not get clipped because it is still much higher than the QP the I frame would have used. This can reduce the final quality with no change in the final size (or the final size could be slightly smaller or even increase, it depends on the source, but it will always be lower quality per bit).
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Last edited by Asmodian; 11th February 2021 at 01:57.
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