[x264-devel] x264 SSIM analysis

[edgar.soldin at web.de]

On 24.02.2014 19:12, BugMaster wrote:
> On Mon, 24 Feb 2014 19:00:00 +0100, Niccolò Belli wrote:
>> Hi, I did analyse SSIM from two videos compressed with x264 using 
>> CRF=18 and different presets. I expected to get a similar SSIM 
>> considering x264 tunes for SSIM by default and not for PSNR. 
>> Unfortunately SSIM differs quite a lot (see attached image).
> 
>> I did calculate the SSIM using qpsnr (http://qpsnr.youlink.org/):
>> ./qpsnr -a avg_ssim -o blocksize=16:fpa=24 -r ~/placebo-lossless.mkv 
>> ~/placebo-cfr18.mkv ~/veryfast-cfr18.mkv > output.csv
> 
>> Why did I get such a strange result?
> 
>> Niccolò
> 
> Because nowhere and never there was any official statemantes that same
> CRF will produce same PSNR/SSIM or same subjective quality with
> different settings (presets). If you thought so than you was wrong or
> was misleaded. All what CRF mode mean is constant ratefactor (notice
> it is ratefactor and not quality which are not equal thing). And
> constant ratefactor means more or less constant tradeoff bitween
> quality/bitrate along the the source (not between different sources or
> different options).
> 

good to know, very well explained!

thx .. ede