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Thread started 30 Oct 2008 (Thursday) 08:19
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Removing color cast artifacts after green screen extraction.

 
bwolford
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Oct 30, 2008 08:19 |  #1

If you look closely at this image, I have a color cast on the lower legs that I need to address. It's a reflection off of a green screen. Any thoughts on the best way to selectively remove the color cast artifacts from a green screen?

And I do realize if I had more fill, I could have avoided this, but now that I have it, what should I do?


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Gatorboy
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Oct 30, 2008 08:27 |  #2

That's spill for sure. What tool did you use to extract the green screen?

I recommend the product Primatte Chromakey (external link).


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Oct 30, 2008 09:04 |  #3

I'd duplicate the layer, adjust the colour balance (or colour replace tool would probably work too) so that that foot is fine, then put that layer behind the original, and apply a layer mask and brush out the foot with a low-opacity brush until it looks normal.


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Peano
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Oct 30, 2008 13:03 |  #4


  1. Create a blank layer above the image.
  2. Change blend mode to color.
  3. Sample the skin tone.
  4. Paint.
You might have to sample several times to get what you want, but this is probably the quickest and simplest way to remove spill like this.

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bwolford
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Oct 31, 2008 22:27 as a reply to  @ Peano's post |  #5

I'll give them both a try. I had plenty of light on the back and sides of the subject and under lit the front be cause I wanted to maintain some shadow/definition rather than blow the subject out completely. It's interesting that the most reflection of the green cast is in shadow.


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Damo77
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Nov 01, 2008 22:21 |  #6

What's the advantage of shooting against a green screen? I would have thought a white background would be better - certainly in this case.


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Nov 01, 2008 22:54 |  #7

Damo77 wrote in post #6606171 (external link)
What's the advantage of shooting against a green screen? I would have thought a white background would be better - certainly in this case.

During the film days they used the bluescreen because blue was the complimentary color to the red and yellow skin tones, allowing them to filter the blue then filter for good skin tones. It was all a complicated process.

I've seen two rationales for using a greenscreen with digital: 1) It can be used outdoors without interference from blue skies and 2) the green channel holds more tonal detail, but I'm not sure how those things would apply to our digital photography. At any rate, bright blue and bright green are commonly used because they are more easily masked from "most" subjects, i.e. subjects that don't happen to be wearing bright blue or bright green clothing!

As far as white goes, it could be just fine if there was no white or bright highlights on the subject, especially toward the edges. But imagine a white dress or shirt or even a little highlight from a flash on the side of a person's face -- could turn a quick simple selection into a time-consuming task. Like trying to whitescreen a snowy mountain top:)!


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Nov 01, 2008 23:57 |  #8

Thanks for the explanation. Yes, I've no doubt it's a horses-for-courses situation. But I think this thread proves one of the downsides of the use of green.


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Gatorboy
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Nov 02, 2008 12:42 |  #9

Damo77 wrote in post #6606548 (external link)
Thanks for the explanation. Yes, I've no doubt it's a horses-for-courses situation. But I think this thread proves one of the downsides of the use of green.

... or getting the lighting right to start with.


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bwolford
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Nov 03, 2008 20:55 |  #10

Damo77 wrote in post #6606171 (external link)
What's the advantage of shooting against a green screen? I would have thought a white background would be better - certainly in this case.

White was used to show here just to emphasize the cast. The ultimate background will not be black.


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bwolford
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Nov 03, 2008 20:57 |  #11

Gatorboy wrote in post #6609097 (external link)
... or getting the lighting right to start with.

And yes I didn't get the lighting right as I have limited resources. 3 lights is not enough to prevent a cast in this situation. So, I'll try a couple of the suggestion provided here and elsewhere. Doesn't look like this is a problem easily solved.


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Nov 03, 2008 21:00 |  #12

I know that when green screen is used in broadcast televsion, it's a two step consideration:

1) Get the lighting on the screen itself as even as possible and run it about a stop over the subject
2) You have to get some backlighting on the subject themsevles to cut the hole cleanly. Dark areas are where green screen (Chroma Key .. Ultimatte) will start to go ragged. These are the areas that leave residual green on a still photo subject.


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Nov 04, 2008 09:00 |  #13

I'd duplicate the layer, adjust the colour balance (or colour replace tool would probably work too) so that that foot is fine, then put that layer behind the original, and apply a layer mask and brush out the foot with a low-opacity brush until it looks normal.

You can do that in one step using an Adjustment Layer for local changes. Layer> Adjustment Layer / "whatever" (PS7) or Image/ Adjustments/ "whatever"
Note what's said in the "Advanced Tip".
Adjustment Layers (external link)

Just make the change, click on the mask in the Layers Palette & Edit> Fill with black, & paint in the image, not the mask, with a white brush to just put the change where you want it.


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bwolford
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Nov 04, 2008 10:29 |  #14

FlyingPhotog wrote in post #6618830 (external link)
I know that when green screen is used in broadcast televsion, it's a two step consideration:

1) Get the lighting on the screen itself as even as possible and run it about a stop over the subject
2) You have to get some backlighting on the subject themsevles to cut the hole cleanly. Dark areas are where green screen (Chroma Key .. Ultimatte) will start to go ragged. These are the areas that leave residual green on a still photo subject.

The dark areas are exactly where I see the reflection. I put a rim light to improve separation. I had limited lights so I couldn't get the backdrop as uniform as I should have, but Select->Color Range is working nicely for extraction. Very quick and accurate. I select using that method, modify the extraction and contract a pixel or two and I almost always get a clean extraction...

Seems counter intuitive that if I add more light in the foreground that I'll actually reduce the reflection... But I understand it works.


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Roach711
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Nov 04, 2008 13:49 as a reply to  @ bwolford's post |  #15

There's a tutorial on removing green screen spill on russellbrown.com.

Click on "Photoshop tips & tricks" then scroll down about 1/3 of the way to "Green screen monster alert."

He's often got a different approach to a problem and is entertaining in the bargain.


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