[FFmpeg-user] FFmpeg and libx264 presets and Constant Rate Factor (CRF)

[Bill Crockett]

>> If I choose a CRF of 20, and I use the "veryslow" preset, the quality 
>> will
>> still be the same if I choose a CRF of 20 and use the "superfast" preset?
>>
>> The same with this.  If I choose a CRF of 10, and I use the "veryslow"
>> preset, the quality will still be the same if I choose a CRF of 10 and 
>> use
>> the "superfast" preset?

>> The quality will be (very) similar in both cases, it will not be 100% 
>> identical.

>> Carl Eugen

> It will not be 100% identical?

> Do you mean the "very slow" compared to the "superfast" at a CRF is not
> 100%?  Or, a CRF of 20 compared to a CRF of 10 is not 100%?

> The quality will be (very) similar in both cases, it will not be 100% 
> identical.

I ran a test of a Blu-ray video segment that goes for 2 minutes and 30 
seconds and converted down to 1280 x 720 res.

-c:v libx264 -preset (superfast or veryslow) -crf 20

superfast time = 1:10

 "    bit rate = 6093 kb/s

 "       bytes = 118 MB

 "         CRF = 20

================================

veryslow time = 15:37

 "   bit rate = 4157 kb/s

 "      bytes = 84 MB

 "        CRF = 20

=================================

I looked at both playing together and I could not see any difference in 
quality.  Even frame stepping to clear images and both looked the same.  I 
did notice superfast bframes are 3 and veryslow bframes are 8.  This, and 
the bit rate comparison, indicates that superfast should have the better 
quality.