FFDShow X264 Encoder: What Does 'Constant Quality' Mean?

seunosewa · 26th December 2007, 17:13

When I ask for a video to be encoded at "constant quality, 70%", what does that mean exactly in terms of SSIM or PNSR. Or what does it mean objectively. I can't seem to figure that out at all.

Dark Shikari · 26th December 2007, 17:20

Constant Quality maps (not sure of the formula) to a specific CRF. CRF, like CQP (Constant QP/Quantizer), ranges from 0 to 51.

Constant Rate-Factor (CRF) mode is similar to Constant Quantizer mode, but uses higher quantizers on more complex frames and lower quantizers on simpler frames. Overall, the result is better SSIM and PSNR than one would get with constant quantizer mode. The formula used is very similar to the one used for two-pass ratecontrol, except that since it isn't trying to achieve a specific bitrate, it can do the same in only one pass.

However, it is definitely not at all a "constant PSNR" or "constant SSIM" mode--rather, its a "like CQP, but better" mode.

Blue_MiSfit · 26th December 2007, 18:21

A sidenote, it's probably a bad idea to encode to x264 using ffdshow, unless you really can't manage it any other way. x264.exe or mencoder are surely better options!

~MiSfit

26th December 2007, 17:13	#1 \| Link
seunosewa Registered User Join Date: Nov 2007 Posts: 11	FFDShow X264 Encoder: What Does 'Constant Quality' Mean? When I ask for a video to be encoded at "constant quality, 70%", what does that mean exactly in terms of SSIM or PNSR. Or what does it mean objectively. I can't seem to figure that out at all.

26th December 2007, 18:21	#3 \| Link
Blue_MiSfit Derek Prestegard IRL Join Date: Nov 2003 Location: Los Angeles Posts: 5,957	A sidenote, it's probably a bad idea to encode to x264 using ffdshow, unless you really can't manage it any other way. x264.exe or mencoder are surely better options! ~MiSfit __________________ These are all my personal statements, not those of my employer :)

26th December 2007, 17:20	#2 \| Link
Dark Shikari x264 developer Join Date: Sep 2005 Posts: 8,667	Constant Quality maps (not sure of the formula) to a specific CRF. CRF, like CQP (Constant QP/Quantizer), ranges from 0 to 51. Constant Rate-Factor (CRF) mode is similar to Constant Quantizer mode, but uses higher quantizers on more complex frames and lower quantizers on simpler frames. Overall, the result is better SSIM and PSNR than one would get with constant quantizer mode. The formula used is very similar to the one used for two-pass ratecontrol, except that since it isn't trying to achieve a specific bitrate, it can do the same in only one pass. However, it is definitely not at all a "constant PSNR" or "constant SSIM" mode--rather, its a "like CQP, but better" mode.