Hi Liam,

On Nov 22, 2007 9:45 AM, Liam R E Quin <[EMAIL PROTECTED]> wrote:
> Evidence that auto levels loses details --
> take a photograph (or a scanned engraving, or whatever) and open
> Levels, and press autl.  Note that the little triangles marking
> the end-points are not under the ends of the black part - in

The 'Value' controls are not effected by 'Auto'.
Look at the R,G,B,(A) controls instead.

All 'Auto' does, is:
   1. Set the input range to min(CHANNEL), max(CHANNEL) -- ie the
lowest used value of that channel in the picture, and the highest used
value of that channel in the picture.
   2. Set the output range to 0, 255
 for each CHANNEL in R,G,B (and possibly A)

Because the output range is full, there is literally no way that this
operation can reduce detail.

> other words there are multiple pixel values that are used in the
> image that end up having the same value after "levels".  I'll
'Value' is invalid after applying Auto, cause you either use Value or
R,G,B, not both. If you change the Value controls, then adjusting RGB
individually is meaningless, and vice versa.
GIMP makes some effort at showing sensible values for all controls
though -- the range you show is probably the average of the R,G,B

> attach a screenshot that shows the triangle isn't at the end of
> all the values, but some way in.  This behaviour is good in some
> cases and not others.
> Liam
> --
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