How does green screening technology handle secondary reflection of green?

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DaveC426913

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How does green screening technology handle secondary reflection of green?
I get the basics of green screening. The processing software recognizes a small range of green and can substitute a different image where it occurs.

Here's what I don't get: if a subject is standing in front of a green screen, they will have reflections of green.

1564412153493.png


Notice that, in the pre-processed image, the candle base, her hair scrunchie, the hollow of her cheek and even the nape of her neck all have a noticeable green cast to them. The rest of her face will not have a green cast.

The naive assumption would be that they could compensate for this by reducing the green - but it can't be as easy as that. You'd have to process only the affected portions.

Two different areas of her skin for example, will need different treatments. You'll want to reduce the green in portions that are inclined toward the background, without altering adjacent areas (otherwise she will end up with a magenta cast).
green screen 2.jpg

The right patch of skin is too green, but the left patch is adjacent, and yet should not be processed, or it will be too magenta.


How do they do this?
 
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I do not see anything else green on her so perhaps they just get rid of all the green and not just the specific shade of green used on the green screen background. This would only become a problem if the target itself was wearing / holding anything green which is easy to avoid with some common sense advance planning.
 
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I think the green color has some very specific RGB values. I can't say they are the same for all pixels but highly unlikely to occur in a picture unless its an area you are masking out.

In this wiki article, they mention that the subject may not duplicate that backing color:


and I found this article on how it really works:


and some history too:

 

anorlunda

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In the articles @jedishrfu linked, they mention bright green screens. That suggests that it is not just color, but color plus brightness that is the key.
 
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The technique sometimes fails with leprechauns who prefer a softer nicer green. :-)
 

BillTre

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The two big ways to avoid reflections of the green screen on the subject involve the staging (distance between subject and background) and lighting of the scene.

The green most often used is a specific bright green, not commonly found in human skin tones. Other colors may be substituted (software permitting) if you have problems with particular subjects (like Capt Kirk's green woman).

The subject should not be too close to the green screen. Separation will reduce the strength of any reflected green.

Separate lights should be used for the background (to illuminate the green screen), for the subject (consider classic 3-point lighting: key light and fill light, and backlight (if you want to provide outlining of the subject)).
Bright direct lighting of the subject will usually swamp out any small amounts of reflected green.

The green that is removed by the software is pretty specific and (depending on the software) can be to leave or remove more of particular colors. It does not remove just any color with green components, it is pretty specific (as mentioned above).

You can make yourself a floating head, by wearing a green screen suit in front of a green screen and then using the same software. Or make you hands go away, or whatever.
 
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Here's what I don't get: if a subject is standing in front of a green screen, they will have reflections of green.
Not necessarily, depending on how wide the green screen is. If the green screen were only as wide as what is shown in the images, there isn't a way for light from the screen to reflect off her left cheek, and then travel to the camera.
Notice that, in the pre-processed image, the candle base, her hair scrunchie, the hollow of her cheek and even the nape of her neck all have a noticeable green cast to them. The rest of her face will not have a green cast.
Certainly some of the candle base will reflect light from the green screen, but the hollow of her left cheek shouldn't have a green tinge unless the screen is so wide that points on the screen well off to her left can reflect off her cheek..
 

phinds

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There are programs that identify the outline of anything that is on a constant-color background so my assumption has always been that that's what is used when some of the subject itself is pretty much the same shade of green as the green screen. Since photography (including video) is digital these days, the technique can be applied frame by frame for videos.

I could be wrong and it could be just a simple recognition of color plus brightness.
 

DaveC426913

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Sorry, I don't seem to be explaining myself well.

I'm not suggesting anything on the subject is anywhere near the actual green of the green screen.

What I'm saying is: in real life (i.e. if you went up to the actor and looked at her) you will literally see secondary lighting on her neck and cheekbones. This is not going to be picked up by software since it's still flesh coloured skin - but to the eye, the skin (and other surfaces) will have a greener cast.

Look what happens when I sub in a neutral background, as if the post-processing software put her in a scene.
green screen 3.jpg


The actor (all of her) is not selected by the software, because she's not supposed to be.

But that green cast is still there for real. It's a basic principle of optics that objects will reflect the colour of light cast upon them, and there is some green reflective light cast upon her by the green screen. The post-process can't do anything about this.
 

DaveC426913

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Not necessarily, depending on how wide the green screen is. If the green screen were only as wide as what is shown in the images, there isn't a way for light from the screen to reflect off her left cheek, and then travel to the camera.
Certainly some of the candle base will reflect light from the green screen, but the hollow of her left cheek shouldn't have a green tinge unless the screen is so wide that points on the screen well off to her left can reflect off her cheek..
A good attempt, but a look at the before/after shows that the green is indeed reflected off her cheek.
green screen 5.jpg


Left shows her cheek hollow with green screen; right shows what she presumably should look like without the green cast.


But more to the point, look at an ideal case: a cylindrical object, (such as her neck), will at least partially reflect the colour of the background - it's cylindrical after all. To a greater or lesser extent, at least part of her neck is tangent to a straight line between green screen and camera.
green screen 4.jpg

(The example is cylindrical but it applies to any round shape of the subject, such as her chin, her ear lobes, the top of her head, her shoulders - and apparently - the hollows of her cheeks.)
 
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A good attempt, but this won't work.

Look at an ideal case: a cylindrical object, (such as her neck), will at least partially reflect the colour of the background - it's cylindrical after all. To a greater or lesser extent, at least part of her neck is tangent to a straight line between green screen and camera.
But I didn't mention her neck, and I also mentioned that the width of the green screen could have an effect. What I did mention was her left cheek, and particularly the hollow, which would be blocked from light coming from the screen unless the screen was very wide.
 

DaveC426913

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But I didn't mention her neck, and I also mentioned that the width of the green screen could have an effect. What I did mention was her left cheek, and particularly the hollow, which would be blocked from light coming from the screen unless the screen was very wide.
Sorry. I edited to add a clarifying image. See above.
 

BillTre

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If you are picking up some green on the front of the subject, reflected from the green screen,then it should be a secondary reflection from the green screen, then off of something else (perhaps a white surface) and then onto the subject, from which the camera will be able to pick-up reflected green.
To better control light, many studios will be painted black, similar to the inside of a camera or telescope.
My previous comments about lighting and distance from the green screen also apply to this kind edge reflection situation.
Direct lighting of the subject should swamp out the reflected green light.
A decent fill light on the subject should fill in the shadows to swamp the green also (the fill light is to fill in shadows with some light.

Film noire (dark, with dramatic shadows) lighting of your subject would make this more difficult.

If desperate, you should be able to process the subject layer of your composite separately from the other components (like the background) to remove residual green.
 
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Yes, I saw the new image. That's exactly why I qualified what I said, that a very wide green screen could cause the green tint to sort of wrap around.
 

DaveC426913

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Yes, I saw the new image. That's exactly why I qualified what I said, that a very wide green screen could cause the green tint to sort of wrap around.
Sorry, not to be argumentative, but if any part of the green screen is visible behind the actor, then it will reflect light on the edges of curved surfaces. Okay, the hollows of her cheek may constitute an exception, I'll grant that, but the reflection will still apply to every other curved surface adjacent to green screen.
 

DaveC426913

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If you are picking up some green on the front of the subject, reflected from the green screen,then it should be a secondary reflection from the green screen, then off of something else (perhaps a white surface) and then onto the subject, from which the camera will be able to pick-up reflected green.

Direct lighting of the subject should swamp out the reflected green light.
A decent fill light on the subject should fill in the shadows to swamp the green also (the fill light is to fill in shadows with some light.
I wondered about this. I think this may be zeroing in on the answer.

If they added a faint green fill light directly from the front, then it would even out the green reflected light from the back. The subject would now be evenly lit with a green cast all around - front and sides.

Then it's a simple matter of compensating for the green cast of the entire scene with a touch of magenta, balancing to neutral.

That makes a lot more sense than some hypothetical intelligent software that can edge detect the subject and preferentially compensate for too much green only at the periphery of the subject.
 

BillTre

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If they added a faint green fill light directly from the front, then it would even out the green reflected light from the back. The subject would now be evenly lit with a green cast all around - front and sides.

Then it's a simple matter of compensating for the green cast of the entire scene with a touch of magenta, balancing to neutral.
That seems to me to adding a layer of complexity unless you actually want the green tint (which would not be difficult to do).
However, if you don't want it and your subject is well lit, it should be easy to swamp out the green light with stronger white light.

Then it's a simple matter of compensating for the green cast of the entire scene with a touch of magenta, balancing to neutral.
I have never heard of this done in this way (I have taken several lighting and video classes), but there are a lot of weird things you can do with different color lights.
Any good quality video editing software will allow you to just work on your subject layer, after the green screen has been removed. Not sure if you are referring to that or the whole composited final product here.

This approach would make it more difficult to match the green screened clip color-wise, with any other clips, not shot with a green screen.
My personal approach would be to try to get things as good as possible with lighting and staging, without color manipulations of the images via software.
 

DaveC426913

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However, if you don't want it and your subject is well lit, it should be easy to swamp out the green light with stronger white light.
At the risk of putting too much faith in the accuracy of a picture I Googled, the example I've been using seems to suggest that the effect can easily stand out beyond the lighting of the subject.


Any good quality video editing software will allow you to just work on your subject layer, after the green screen has been removed. Not sure if you are referring to that or the whole composited final product here.
Yes. You'd just apply the correction to the subject layer, not the green screened background.

But your suggestion doesn't solve the problem. If you don't compensate with a green fill light during shooting then you're still left with a subject layer that has varying degrees of green cast that need careful attention (software or human) to remove.

This approach would make it more difficult to match the green screened clip color-wise, with any other clips, not shot with a green screen.
Yeah, I studied cinematography in college.
Colour matching elements of a scene and between scenes is a whole industry unto itself. It already exists, independent of green screen technology, so it wouldn't require any additional processing technology.
 

FactChecker

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Many of the proposed solutions are ignoring the example given, where there clearly is some green reflected onto the skin and successfully removed. They must have done that some way. I think that the suggestions should focus on that example instead of proposing alternative situations.
 
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I think that the suggestions should focus on that example instead of proposing alternative situations.
I agree. The fact that there are secondary green reflections in some circumstances is clear. It may be that they could be avoided sometimes, but when they are not avoided, then what?
 

Baluncore

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, but when they are not avoided, then what?
You know the colour balance of normal skin, so you can identify green tinted skin and remove the excess green illumination to restore the colour balance.
 

DaveC426913

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You know the colour balance of normal skin, so you can identify green tinted skin and remove the excess green illumination to restore the colour balance.
It is not nearly that simple.
  1. There is no such thing as normal coloured skin. There are as many skin colours as there are people on the planet.
  2. The only time you would see a given colour of skin as its natural colour is in a neutral grey environment. In other words, every environment will have elements that shed more green light and/or less green light on a subject. You can't just unilaterally remove - or leave - any green.
  3. What about every other object in the subject? 'Normal colour' for hair? 'Normal colour' for cream dress? 'Normal colour' for candlestick? etc...
 

Baluncore

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It is not nearly that simple.
You have a colour sample of everything you need in that photo. If you can see green bleed then you can see where there is no bleed, so you can apply the colour correction.
 

FactChecker

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This may answer some questions. It is about the Advanced Spill Suppressor that @DavidSnider mentioned.
 
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