>>   I just tried to mimik some algorithm for enhancing the colors of an image taken
>>   by a digital camera (or a scanner, or ...) that was explained for photoshop
>>   (and works very well -- far better than the automatic).  By trying this I found
>>   some things that have to be discussed (and maybe later reported as errors). 
>>   But first I describe the algorithm.  It is taken from an excerpt of a book
>>   about color correction:
>>       http://www.daton.de/wargalla/    (german)
>>       auf dieser Seite gibt es einen Link zu "das BUCH" und dort diesen
>>       Algorithmus als Probekapitel
>>   1. make the level of each channel (R, G, B -- _not_ RGB or value) spread  from
>>      the beginning of the hills to their end (don't know the right english words)
>>   2. take a color pick of some point that has to be gray (say: 123 / 115 / 139)
>>   3. adjust the curves for each channel (again R, G, B, -- _not_ value) with
>>      only one aditional point, that all three values meet the middle one (123
>>      in the example):  R 123 -> 123, G 115 -> 123, B 139 -> 123
>>   Hope this was clear enough to be valuable.

These techniques are described in detail in "Grokking the GIMP"
sections 6.1 and 6.2.  See:


>>   The problems that arise:
>>   - for levels and curves there are four channels R, G, B, value. Is the fourth
>>     one really "value" (from HSV) or is it the combination of the other three??
>>     The latter one is called "RGB" in photoshop and I think this is clearer.

Yes, the fourth one is really value; the functional form is
V=max(R,G,B).  This is the same as what's produced by the Decompose
function (found in the Image:Image/Mode menu) when used with the HSV

>>   - in curves it is not easy to discover what is the input (x or y) that is
>>     matched by the shown curve to the output.  As someone a little bit familiar
>>     with maths, I can determine that x is input and y output but maybe it would
>>     be better to give these two terms directly (or are there problems with i18n
>>     strings in a picture?)

I suspect that you are using GIMP version 1.0.4 which has no feedback
about the mouse position in the Curves tool dialog.  The developers'
version of the GIMP has fixed this by providing a positional
information field which gives the exact mouse position in the Curves
tool dialog while manipulating the curve.  Again, this is explained in
detail in section 6.2 of "Grokking the GIMP".

>>   - if the other two points are really bugs this one is not and though has to be
>>     delayed after 1.2:
>>     It would be very nice to enter the value for x (input) and y (output)
>>     numerical and not by carfully driving the mouse.  Photoshop has (albeit since
>>     5.0) two entrys that show up if a point from the curve is selected.

I agree that this would be a nice feature.  For this to work with
multiple control points on a control curve there needs to be the
notion of the "active" control point which shouldn't be too hard to

Carey Bunks

Dr. Carey Bunks                     
Senior Scientist                    
BBN Corp.                           
70 Fawcett St, 15/2A                
Cambridge,  MA 02138                
tel: 617-873-3028  fax: 617-873-2918
email:  [EMAIL PROTECTED]              

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