>>   There appears to be some problems in this algorithm. I'm certainly no colour
>>   expert, but the weak point seems to be in:
>>   >2. take a color pick of some point that has to be gray (say: 123 / 115 / 139)
>>   How can you know that this colour is supposed to be grey, and not a gray with
>>   a tint of blue, for instance? The problem is finding what is supposed to be
>>   exactly gray :-)
>>   The other problem seems to be that you'll lose some precision, but hopefully
>>   it's not more than a bit or perhaps two.
>>   For the UI adjustments, I'll leave that to the others :-)

Hi Steinar,

In fact you never really know what the correct colors in a photo
should be unless you have a known point of reference (like a color or
grayscale card in the photo).  Even so, the lighting conditions may be
such that the color card is no longer relevant.

Nevertheless, you can often look at a photo and see that it is not
quite right.  When this happens you can experiment with adjusting the
colors so that certain measured pixels in the image are forced to be
neutral (that is, gray).  For most photos of normal scenes it is often
very easy to identify parts that "should" be gray: road surfaces, car
wheel rims, other steel objects, etc.  Adjusting the color of these
parts of the image to a neutral value often makes the whole image look
much better (for examples, you can see Chapter 6 of Grokking the GIMP,
in particular the parts at http://gimp-savvy.com/BOOK/node60.html and
following -- but you may need to wait for the slashdot effect to
subside first ;-).

Also, keep in mind that color correction is largely a function of the
artist's perception.  In the original scene, gray objects may not
appear gray at all due to the spectrum of the illumination.  However,
the artist's objective is not necessarily to make the image appear as
it did when photographed.  Often it is to produce an image which looks
like it was taken under "natural" or "studio" lighting conditions (but
not always).

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]              

Reply via email to