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Under Exposed with bad colour cast - GIMP

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  #1  
Old 08-25-2008, 03:28 AM
petrikNZ petrikNZ is offline
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Question Under Exposed with bad colour cast - GIMP

Looking for some gimp specific help if I could (because it's what I use on Ubuntu Linux)

I have not been able to find a GIMP tutorial that really deals with fixing something as bad as this.

I have seen photoshop tutorials for pulling information from the channels and am looking for help to achieve this in gimp. Or is there another way I should be approaching this?
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Old 08-25-2008, 09:30 AM
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0lBaldy 0lBaldy is offline
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Re: Under Exposed with bad colour cast - GIMP

PetrikNZ, welcome to RetouchPRO ..
May not be GIMP specific but may help get you pointed in the right direction..

Found the Gray point first (Circled) then just levels (Black point under car, white point vase) to get the first sample then used this tutorial (Skipping the levels part) for the next image
Chanell settings:
Red chanell 100% Green 32% Blue 0
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Old 08-25-2008, 11:19 AM
des151 des151 is offline
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Re: Under Exposed with bad colour cast - GIMP

petrikNZ,

I also used levels in photoshop. Does GIMP have levels or curves?

Ray
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Old 08-25-2008, 11:52 AM
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Re: Under Exposed with bad colour cast - GIMP

petrikNZ,
I would agree with the others. You don't have to work so hard to pull information from other channels, at least at first. There appears to be plenty of data in the image. But, you need to adjust white, black and gray points first. On your high resolution image (not the thumbnail here) you will see better results, as there is are more bits to play with as the adjustment stretches the levels out. After viewing your results, you can decide if other techniques are required.
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Old 08-25-2008, 06:13 PM
petrikNZ petrikNZ is offline
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Re: Under Exposed with bad colour cast - GIMP

I'm at work having a play with the uploaded image (sshhh don't tell anyone). I'm sort of getting it. Not that I really know what I'm doing. It's quite grainy but I'm putting most of that down to the jpg compression.

GIMP has curves and levels and a channel mixer. It's the channel mixer that really has me confused.

I hear people talking about looking for detail in the channels and then copying that detail to the other channels but in GIMP I can't figure out how to do it.
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Old 08-25-2008, 06:41 PM
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0lBaldy 0lBaldy is offline
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Re: Under Exposed with bad colour cast - GIMP

Quote:
Originally Posted by petrikNZ View Post
1. GIMP has curves and levels and a channel mixer. It's the channel mixer that really has me confused.

2. I hear people talking about looking for detail in the channels and then copying that detail to the other channels but in GIMP I can't figure out how to do it.
Where the confusion may lie is they are 2 separate things:

1 mixes/blends All the channels differently than originally used so you can use the best of two or more in a different proportion.. (The channel mixing is pretty well explained in the tutorial above and I think it is about the same in GIMP)

2 Create a Selection. Feather them using 2 to 4 pixels. View one of the Channels such as Blue and copy it to the clipboard (use command/control C). Then Paste the Blue Channel from the clipboard into one or two of the other Channels such as Red or the Green (use command/control V).
or
you can use the best channel as a luminosity layer.. (duplicate that channel, select the copy, select all, copy, go back to RGB, to layer menu, new layer, paste.. the channel copy should be in the new layer now, change mode to luminosity)

the above is very generalized and meant just to show the differences

Last edited by 0lBaldy; 08-25-2008 at 06:49 PM. Reason: added info
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Old 08-25-2008, 06:51 PM
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Re: Under Exposed with bad colour cast - GIMP

This was a one click third party filter from Kodak--Digital ROC. No other retouching. I don't know if your program can use this filter.

dc
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Old 08-25-2008, 10:29 PM
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Re: Under Exposed with bad colour cast - GIMP

I'm using PS so I'm trying to keep the adjustments to things that GIMP should have too (i.e. no plugins or automatic adjustments).

1. Adjust levels to the best you can get by pushing the white slider over to the left separately for each colour channel. Assume the vase is white and work from there.

2. Use curves to lighten mid-tones.

3. Use a desaturation layer or tool/filter to adjust the saturation and lightness of red, yellow and cyan. Watch out for the plants in the background.

4. Merge layers

5. Make a copy layer and apply in screen blending mode if GIMP can do that. If not, try brightness and contrast.

6. You might need to make a second saturation adjustment after the screen.
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Old 08-26-2008, 09:23 AM
BobZZZ BobZZZ is offline
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Re: Under Exposed with bad colour cast - GIMP

I too use PS but I think all these moves can be made in GIMP.

If you open the channels the green is dark and the blue is almost black and very noisy. so my first step was to replace that useless blue channel with a copy of the red. I did that by copying the layer opening the blue channel on the top layer and applying the red to it at 100%. This makes the image green so I changed the blend mode to luminosity.

Then onto the cast. I used a curves adjustment layer. The cast is not uniform it is much stronger in the shadow areas so I placed 3 measurement point one on the wall in the foreground one on the front pillar and one on the wall in deep shadow back right. I assume these are concrete hence grey. Neutral colours in RGB have equal values so since the red channel looks more or less fine I applied straight line curves to the green and blue until the 2 lighter points more or less balanced out. I then placed points on those curves over the measurement points by cmd click to fix them and moved the shadow end of the curves till the shadows balanced as well.

The image then looked the right colour so I just put another adjustment layer curve on top and sorted out the contrast and brightness with a curve in the RGB.

It will need a bit more local work to get the foliage and car sorted out but its moving in the right direction.
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