Dan Margulis Applied Color Theory

Color Removal

 color removal
    Posted by: "leicamike2000"
    Date: Thu Jul 13, 2006 1:02 pm (PDT)

I have an image that has an un-wanted green cast in some areas.Is it possible to set up the earser tool or paintbrush so that I could run over these green areas to remove the green only?

Mike Sellers
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Re: color removal
    Posted by: "adriano esteves"
    Date: Thu Jul 13, 2006 1:46 pm (PDT)

Mike,
Check the page 198 on Dan's LAB Color book (LAB advantage in Selection and Masking chapter) as a reference for a possible way to solve this cast problem. Of course it depends a lot on the image, but it's a good starting point!

Good luck!
Adriano Esteves
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Re: color removal
    Posted by: "Gene Palmiter"
    Date: Thu Jul 13, 2006 3:32 pm (PDT)

Yes...and I can give you some hints but I forget exactly what I did. I shot using a green screen for the first time and I lit the model but not the screen...didn't have enough lights. So the models skin picked up some of the green on the edges.

Mask out all but the model....go the green channel and lighten those areas that have more green than what is in the areas that look right.

That should get you close. Use a soft edged eraser. You can, if you know how, use a masking layer to avoid making any moves that cannot be undone.


Thanks,
Gene Palmiter
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Re: color removal
    Posted by: "Paco Marquez"
   Date: Thu Jul 13, 2006 7:27 pm (PDT)

Mike,

One easy thing you might try is to select the areas which have too much green and then use the Hue/Sat to remove the Green by desaturating just this color. If these areas also have a lot of yellow, there might be a problem but give it a try.

All the best!

Paco Márquez
661 McKinley St.
MIramar
San Juan, PR 00907
787-721-8554
787-587-7384 Cel.
http://www.pacomarquez.com
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Re: color removal
    Posted by: "leicamike2000"
    Date: Fri Jul 14, 2006 5:57 am (PDT)

Great tips-thanks everyone.
Mike Sellers
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Re: color removal
    Posted by: "Mark Segal"
    Date: Fri Jul 14, 2006 5:58 am (PDT)

Once you start with erasing and painting, it takes time and unless you are VERY good at it, the end result may not look natural because of resulting unrealistic or erroneous borders. Furthermore, if you have a colour cast, removing it does not mean simply eliminating the cast - the affected pixels need to be rendered in the correct colours - hence you are moving the pixels from one colour state to another. Once one decides to avoid heavily manual interventions such as erasers and brushes, the keys are two things: (1) how to make natural selections with smooth transitions, and (2) how to adjust the colours from the wrong state to the desired ones.

I agree with a previous post that recommended Dan's approach from page 198 of the LAB book. However, if that isn't satisfactory to your situation, another approach within RGB: go to the Select menu in Photoshop, select Color Range, click the eyedropper into the area having the offending cast, and then adjust the Fuzziness slider until this tool has selected all the areas that you want affected by the changes you will implement next.

You can best see how the fuzziness slider changes the selected areas by choosing a coloured mask in the "Preview" drop-down menu of the Color Range dialog box. By the way, this will show you also how Color Range partially selects pixels where it finds hints of hues within the colour range you wish to affect. Once you have the degree of fuzziness that you think will work best, click OK, and you will be returned to your image, which now has marching ants all over it approximately showing the areas that have been selected. Then press CTRL (CMD)-J, which will convert this selection to a new layer including only the selected pixels.

For the most refined control over the colour re-balancing process, create a Curves Adjustment Layer with Clipping Path selected (so it affects only the underlying selection-layer), and adjust the colour balance of that layer by moving the curves of the appropriate individual channels. In your case, to remove a green cast, select the green channel (and assuming your curves dialog is set-up with black to the right and white to the left), start by clicking the center point of the curve and very gradually shift it to the right (use the arrow keys to control movement completely).

You will see green gradually being displaced as magenta moves in; stop before introducing a magenta cast. If the cast of the selected areas is not yet completely neutralized or you do not yet have the replacement hues that you require, you can further influence the result by shifting the R and B curves (remembering that increasing R decreases Cyan and reducing B increases Yellow - when using Curves for colour correction it is always best to think in terms of the complimentaries: in the R curve, more R means less C, in the G curve less G means more M, in the B curve, less B means more Y, and vice versa in all cases). These changes will only affect the selected areas.

This approach may not be satisfactory in all situations, but it is easy, intuituve, and can produce natural-looking results, mainly because the Color Range tool is a very refined selection tool, and Curves gives you alot of custom-control over the extent of the hue adjustment in any part of the tonal range.

Mark Segal
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Re: color removal
    Posted by: "Lee Clawson"
    Date: Fri Jul 14, 2006 7:57 am (PDT)

Mike,
How about posting the image so we can see what you're looking at ??

Lee Clawson
2/\V/\7 Studio
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 color removal
    Posted by: "John Ruttenberg"
    Date: Fri Jul 14, 2006 9:17 am (PDT)

As others have mentioned, this depends on the image. One thing to try,  though, is to move the image to LAB, make a curves adjustment layer, adjust the curves (probably negative part of the A curve and postitive part of the B curve) to remove the cast, and then use the layer's blending options and/or a layer mask to limit the effect.  If there is no vegetation in the image, it may be that you can just play with the green half of the blend-if slider for the A channel.  Move to the right until you exclude what most of the image without the cast.  Use option-click to split the slider to make the blend more natural.  If this gets you most of where you want, you can then clean up any parts of the image with unwanted blend by painting black on the layer mask. This is easy, but doesn't work for every image.  Another possibility is to look at the A and B channels for clear delineation of the areas with the cast.  The A and B channels have subtle variations, don't be fooled at this step.  If you find that the areas with the cast are a different shade than the rest of the image in one of these channels, use Apply-Image to copy into the adjustment layer's layer mask.  After that you can use Image->Adjustments->AutoLevels to improve the contrast of the mask, Image->Adjustments->Invert if your areas of cast are dark instead of light, and a very steep curve.  You may have to paint on the mask to clean it up at the very end.  A slight blur is also always a good idea.

This is all covered in Dan's Canyon book and summarized here: http://www.dgrin.com/showthread.php?t=22331

John Ruttenberg
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Re: color removal
    Posted by: "Jim Rich"
    Date: Fri Jul 14, 2006 10:30 am (PDT)

I guess you can do everything you are suggesting, but why not keep it simple at first.

Why not try a some type of color adjustments or fill method to get rid of the color problem and then use the Histories to take a Snapshot. Then paint using the History brush from that new Snapshot.

Then if that method does not work try something that is more complex.

My. 02.

Jim Rich
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Re: color removal
    Posted by: "John Ruttenberg"
    Date: Fri Jul 14, 2006 11:56 am (PDT)

I guess we really have to see the image.  Right now we have really put the THEORY in color theory.  Need more practice in this particular case.

The LAB techniques are very easy once you understand them.  It's just that the explanation takes a while to sink in.  They have the advantage of being global adjustments which tend to look more natural than anything that involves a brush or a selection.  There is the added advantage for me that I'm really terrible with a brush of any kind.

John Ruttenberg
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Re: color removal
    Posted by: "Paco Marquez" paco@pacomarquez.com pacomarquezfoto
    Date: Fri Jul 14, 2006 11:58 am (PDT)

Mike,

After all these suggestions which range from the most complex to the most simple, I'd be very interested in finding out how you finally solved the problem (if at all).

Please do tell!

Wishing you all the best!

Paco Márquez
661 McKinley St.
MIramar
San Juan, PR 00907
787-721-8554
787-587-7384 Cel.
http://www.pacomarquez.com
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Re: color removal
    Posted by: "Andrew S. Webb" \
    Date: Fri Jul 14, 2006 4:55 pm (PDT)

Mark's advice is great. If you have any film or video experience, it may help to think of his Color Range technique as a color keying operation. In fact, if you have After Effects or Shake, you can probably use the keying tools in those apps to solve your greenspill problem.

In the future, light your greenscreens brightly and evenly (unless you're going to comp to a dark BG, then light it medium and evenly) and keep your talent away from the screen.

_andrew webb

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