On Nov 23, 2007 6:03 AM, GSR - FR <[EMAIL PROTECTED]> wrote:
> [EMAIL PROTECTED] (2007-11-22 at 1544.46 +1030):
> > 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.
> And then you see it changes them, in a destructive way.
> > 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.
> No, it does not. It sets min and max "inside" the histrogram used
> range (play with Lin/Log setting if this is not clear). There is a way
> anybody can test it:
> 1 Create new image, 512*512
> 2 Render plasma, with seed 0 and turbulence 1
> 3 Run Levels, click the auto button, look at all the channels.... OOPS!
> You get:
> R 9 1.0 229 | 0 255
> G 12 1.0 194 | 0 255
> B 25 1.0 241 | 0 255
> Those settings are destructive, Red 0-9 becomes 0, Red 229-255 becomes
> 255, and so on. You can look for such pixels before applying the
> operation, mark with guides or points, then look what they are after.
> You can also look at the code and see how it iterates over the
> histrogram until it decides it has "eaten" enough. It does not stop
> when it reaches the first non zero, which would be non destructive (as
> in any input channel value gets mapped to a new, different, output
> value, rounding and precission issues aside).
Okay, well this is not what I expected and I won't be using Levels
Auto in the future - I hate clipping.
I had thought that Auto was supposed to perform the same function as
Colors->Auto->Stretch contrast -- to stretch the current R,G,B range
ends to 0,255, clearly it doesn't.
It would be nice to have an Auto button in Curves that did achieve this result.
> > 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.
> If the input range is not full, there is loss, as shadows and
> highlights are lost and become flattened, grouped in the same result.
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