How does AutoGK calculate compressibility percentage?

orion44 · 31st July 2023, 05:16

I would like to learn how to do compressibility calculation manually.

From what info I could find on Doom9, AutoGK performs a constant quality pass at Q2 quantizer on 5% of the video,
but I couldn't find the exact formulas that it uses.

How exactly does AutoGK calculate compressibility percentage?

hello_hello · 10th August 2023, 05:26

If I remember correctly it's just a percentage, so if Q2 equals 100% and (based on the sections of video it tests) the total video size at Q2 would be 1000MB, then a file size of 750MB would be 75% quality, which is what it aims for, from memory (or it might be between 70% and 100%). That part would be easy enough to test if you ran a single pass encode at 100% quality and then again at 75% quality (same resolution each time).

What I don't think I've ever known is exactly how it calculates a new resolution after the compression test if need be, when it's in 2 pass auto mode, in order to achieve a 70% to 75% quality. It's possibly a simple formula but I don't know what it is. You could probably ask the same question in the StaxRip thread because StaxRip can (or at least it used to) run a compression test and I think it updates the expected quality when you change the resizing. It's been a while since I've used it.

31st July 2023, 05:16	#1 \| Link
orion44 None Join Date: Jul 2007 Location: The Background Posts: 314	How does AutoGK calculate compressibility percentage? I would like to learn how to do compressibility calculation manually. From what info I could find on Doom9, AutoGK performs a constant quality pass at Q2 quantizer on 5% of the video, but I couldn't find the exact formulas that it uses. How exactly does AutoGK calculate compressibility percentage?

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10th August 2023, 05:26	#2 \| Link
hello_hello Registered User Join Date: Mar 2011 Posts: 4,898	If I remember correctly it's just a percentage, so if Q2 equals 100% and (based on the sections of video it tests) the total video size at Q2 would be 1000MB, then a file size of 750MB would be 75% quality, which is what it aims for, from memory (or it might be between 70% and 100%). That part would be easy enough to test if you ran a single pass encode at 100% quality and then again at 75% quality (same resolution each time). What I don't think I've ever known is exactly how it calculates a new resolution after the compression test if need be, when it's in 2 pass auto mode, in order to achieve a 70% to 75% quality. It's possibly a simple formula but I don't know what it is. You could probably ask the same question in the StaxRip thread because StaxRip can (or at least it used to) run a compression test and I think it updates the expected quality when you change the resizing. It's been a while since I've used it.