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Chroma key color conundrum

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John OrchardChroma key color conundrum
by on Feb 23, 2012 at 10:55:04 pm

Hey guys!

I'd consider myself a noobie when it comes to chroma key work. I've always had control over what my subjects were wearing, what colors the props were, etc. In light of that, I never had issues with keying in post because the colors were very controlled. Here's my dilemma: I'm in prepro for a spoof of the Wizard of Oz, the scene in which the Wicked Witch sends off her flying monkeys. This will be done completely on (green/blue/magenta?) screen and the environment inserted in post. The Witch's face is green, and the monkey's faces are blue and there is red on their suits. CRAP. As I'm only in prepro, I need to figure out what color screen I should use. Anyone ran into this problem and have a solution? Will a magenta screen key out the reds of the monkey's suits? Help!!



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Conrad OlsonRe: Chroma key color conundrum
by on Feb 23, 2012 at 11:00:41 pm

Just go for a greenscreen and allow time for roto around the faces. If you try and get a key that covers everything you are likely to get nothing.


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Dave LaRondeRe: Chroma key color conundrum
by on Feb 23, 2012 at 11:01:15 pm

Do you have the luxury of running tests?

Off the top of my head I'm thinkin' it might be best to change the witch's makeup to blue. Then after you pull the key, isolate her face to change the blue back to green.

That's an easy test to run: no costumes needed.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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John OrchardRe: Chroma key color conundrum
by on Feb 24, 2012 at 12:08:04 am

Thanks for the responses.

Roto'ing the face would also require roto'ing the hands = super pain.

I thought about changing the makeup color, that's probably my best bet. I have a few days to run tests, so that pretty much sums up my weekend. Is blue a good option to change to? I thought orange might work best as it might match the tone map of green a little better.

I will update you guys with my results.

Thanks again.


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Barend OnneweerRe: Chroma key color conundrum
by on Feb 24, 2012 at 2:47:10 pm

I'd go with Dave's suggestion of changing the make-up on the face. A bright blue would probably work, as would orange.


Raamw3rk - independent colourist and visual effects artist

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Andrew SomersRe: Chroma key color conundrum
by on Feb 25, 2012 at 1:35:09 am

Green Screen is the color of choice for most applications where a color-type key is needed particularly when shooting with a digital camera.

For one, Digital cameras with bayer sensors have better resolution in green (there are twice as many green pixels as red or blue at the sensor level), and more importantly, the green pixels are of a much lower noise than the blue.

But to be successful, the green used should be of a sufficiently narrow band that there are several stops difference between the green and the red/blue channel exposure.

On the other hand, the green makeup on the witch's face is probably not going to be so narrow-band. Provided there is enough image data in the red (or blue) channel, you may still be able to get a good clean key.


I can not stress this enough. Make some tests with your intended camera, under intended lighting and against the specific green color intended for the screen, and a stand-in with the makeup on. Then load that footage into after effects, and examine the color of the screen using the eyedropper tool.

In a 32 bit linear colorspace, You should see values like this on the greenscreen portion of the image:

Note the very large differences in value between the red and blue channels. You need this kind of channel separation for the green screen.

After you do tests on the witch makeup, you may find that you need to make no real changes. Or you may find that you need to adjust it.

My suggestion is to make the witch makeup green but with a substantial red component. Then later in color correction you can do a secondary color correction on that specific color, and rotate the hue back toward green.

In the below example (not a real world test obviously), a small shift in the tone of the makeup was enough to keep the face out of the "green screen" and yet was very easy to selectively adjust the face color with nothing more than a 45 degree rotation of HUE, selectively applied only to the yellow/green color.

NOTE: For some reason, Creative Cow is altering the color green in this image when uploaded here, so this may not look like the correct green on your monitor. This despite the fact that the image is tagged with am sRGB profile - it seems that this website does some transform and ignores the tag.

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