x264_quality - beandog's mostly dvds multimedia wiki

====== x264 Quality ======

  * [[x264]]

  * [[http://forum.doom9.org/showthread.php?t=150118|Meaning of PSNR and SSIM scores]] - doom9, source of formula by Dark Shikari

Use SSIM or PSNR to gauge differences between encoding settings.

==== SSIM ====

When comparing two values, here is the formula to discern what percentage amount something has improved.

First of all, let's look at a two-pass encode of a file:

<code>
x264 -o movie.264 --ssim --tune ssim --profile high --level 3.1 --bitrate 1024 --preset medium --keyint 30 --pass 1 movie.y4m
x264 -o movie.264 --ssim --tune ssim --profile high --level 3.1 --bitrate 1024 --preset medium --keyint 30 --pass 2 movie.y4m
</code>

Find this value in the output, and use the Y variable.

<code>
x264 [info]: SSIM Mean Y:0.9613678 (14.131db)
</code>

Run the same tests, against a 2048k bitrate instead of a 1024k one:

<code>
x264 -o movie.264 --ssim --tune ssim --profile high --level 3.1 --bitrate 1024 --preset medium --keyint 30 --pass 1 movie.y4m
x264 -o movie.264 --ssim --tune ssim --profile high --level 3.1 --bitrate 1024 --preset medium --keyint 30 --pass 2 movie.y4m
</code>

With separate SSIM output:

<code>
x264 [info]: SSIM Mean Y:0.9751283 (16.043db)
</code>

Here's the formula:
<code>
(((1 - old) / (1 - new)) - 1) * 100
</code>

Here's a simpler visualization:

<code>
1 - 0.9613678 = 0.0386322
1 - 0.9751283 = 0.0248717
0.0386322 / 0.0248717 = 1.55325932686547361057
1.55325932686547361057 - 1 = 0.55325932686547361057
0.55325932686547361057 * 100 = 55.325932686547361057
</code>

Rounding it out to an integer, would be 55, for a result of 55%.  So when encoding this particular video at double the bitrate, the SSIM increases by 55%.  Not bad!

Here's a PHP function to do the same thing:

<code>
function ssim_improvement($old_ssim, $new_ssim) {

        $a = 1 - $old_ssim;
        $b = 1 - $new_ssim;

        $c = $a / $b;

        $d = $c - 1;

        $e = $d * 100;

        return $e;

}
</code>

=== PSNR ===

Using the doom9 forum post as a reference, the equation is:

<code>
(new - old) / 0.05 = % improvement
</code>
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