# Re: [Gimp-user] What is the scale of Levels?

```On Sun, Aug 23, 2009 at 1:33 PM, <dar...@chaosreigns.com> wrote:
> I'm looking for a mathematical definition of this scale.
>
> http://docs.gimp.org/en/gimp-tool-levels.html says 0 is black and 255 is
> white.  What's in between?  Is 7 double the brightness of 6?```
```
Have you not experimented with it? 7 is nowhere near being double the
brightness of 6.

>
> What is the luminance / brightness of each of the levels?

luminance (n) = n (or n/255. if you want luminance on a 0..1 scale)
>
> Is it a logarithmic scale?

no.
It's a nominally linear scale, which is then modified by gamma
parameter (if gamma == 1.0, that's no change). 0..255 simply
correspond to sRGB pixel intensity values directly. So if you want a
truly linear measurement of intensity, you need to apply the inverse
of the sRGB curve.

scipy used to provide an implementation of that -- see
http://mail.scipy.org/pipermail/scipy-svn/2007-February/000703.html
(rgbp2rgb function)
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