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.

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.

- 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?)

- 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.

Uwe Koloska

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right now the web page is in german only
but this will change as time goes by ;-)

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