Hello everyone,

i use gimp quite regurarly but i have a problem with color correction.

In fact, with some digital photo i make, my camera ( and not only mine, i have tried other cameras and they all have the same problem ) creates a very innatural shade of what i would call green ( see sample ).

I know it is a false color since it appears very strange for me that i have seen the real one, and also i have tried with a traditional camera with several slide films ( both fuji and kodak of various kinds ) and it is very different from the digital one.

A sample of the problem: http://www.webalice.it/giulio.canevari/tmp/IMG_1348.JPG ( 4.8 MB, if someone wants a resized one let me know and i'll provide ).

The official logo on the web, with the right shade of green: http://www.ferroviedellostato.it/ffss/img/Logo.gif

As you can see, the difference is huge.

A photo with the right green: http://www.photorail.com/phr3-gli%20updates/images/5fotomese/6683112exxx-040706torralba.jpg

I have asked here and there but after all a friend of mine has showed me how to fix this color, but he knows only photoshop.

In fact, he adds a color correction layer, and adding black to the cyan channel seems to fix the problem.

Is there a simple way to do this with the gimp??

Thank you in advance.

Hi !

Just one comment, before you try to change any colour...

To my mind your picture is over exposed, I think your canon EOS 350D allows you to control the exposure... so try it ! I think you were in an automatic mode that has measured the overall luminosity looking at the center of the picture and unfortunately, this center is dark compared to the rest of the picture... you probably have other modes... or you can try to use manual mode...

Thank you for the reply. Ok, that is a first point, btw photo is not mine. Anyway ok, if i set -1/3 or -2/3 the color shift is sometimes ( let's say 25% of the times ) less noticeable.

I just downloaded your picture and played with the luminosity and contrast (the most simple thing we can do, I think!) in gimp and I obtain "approximatively" the same colour you have in the logo... (I don't have checked the result looking at the precise colour with gimp but just comparing by eye).

Could you please tell me what values have you used?

Here i have tried for example -30 lum and + 15 contrast, and the result is still much less green and much more cyan than the real one. I am not able to mathematically measure it, unfortunatly, but believe me.

An other point is that the picture you take as a reference is, to my mind an other time... too blue. Perhaps the green is correct? but the overall colour seems to be blue...

I have seen it too. The strange rectangular table with a shape in the front of the train ( over the left red light ) contains, together with the orange slice, a pure black and a pure white, so yes, it is too blue.

But balancing color like i often do ( i use curves, picking the near white color, and i put r,g and b channels all the same value, and the same for near black ) still have the "green" problem that is false.

By the way, this strange color cast happens only with trains of this color ( 99% ;) ), all other things have not this problem or at least it is not noticeable.

I have upped 2 more samples: one that has the right green ( http://www.webalice.it/giulio.canevari/tmp/ALe582-zzz-MilanoGrecoPirelli-19052007-IMG_8443.JPG ) and one with the wrong color ( http://www.webalice.it/giulio.canevari/tmp/E655-207-Cicognola-20052007-merci-IMG_8582.JPG ).

Thank you again. --
Giulio Canevari

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