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by Aditya

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I've got a vehicle green-screen room. Generally speaking, it works great. However, on candy-apple red there is a noticeable problem in post production:

Post Production Green Screen

See the green outline on the trunk and hood of the car? How do I solve this problem? Is this a problem in lighting? A white-balance problem (using a Cannon T3-I with 18% gray card)? Or, is it a problem that can only be solved in post-production on a car-by-car basis? What method would you use to eliminate the green-screen reflection in the photo?

Here is the original if you want to play with a post-production filter, or think you have something that might work:

Green Screen Red Car

Note: You can see the full gallery here.

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Have a look at this: photo.stackexchange.com/questions/9418/… which is my very similar problem with chroma key and portraits. It may fix it for you. –  John Cavan Sep 10 '11 at 0:08
    
I actually always wondered why this type of thing isn't a bigger problem when using green screen. –  Joel Glovier Sep 10 '11 at 14:48

3 Answers 3

The green is there and is being reflected by the car. Eliminating it requires post-production which is exactly how its done with for visual effects in movies.

You use a tool called 'Color-Curves' (or similar name depending on the S/W) and basically reduce the amount of green in green areas until it looks natural.

A change in WB is not what you are looking for as it impacts all the image, so you may loose the green but end-up with everything else magenta. Lighting makes no difference since the object you are shooting is reflective.

A polarizer helps reduce the intensity of the reflection in some directions but given the angles and curves of the vehicle, it will not remove the reflection in most areas.

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@evan not quite, that method is for correcting white balance and color cast issues. There are various methods for adjusting just one color channel - Curves is one, the Channel Mixer is another. Google will give you plenty of tutorials. –  ElendilTheTall Sep 10 '11 at 7:05
    
I am using Sony Vegas, do you know the name of the tool? I'm kind of lost with this answer because you're telling me the name of the tool without telling me what it does or how to do it. Does this work for the whole image? Or, just the part that has excessive green in it? –  Evan Carroll Sep 12 '11 at 15:40
    
Sorry but I have not used that S/W and each one does it differently. The principal of Color Curves is that you select a color to work on (Green in your case) and then you interact to control what happens to that color (in your case, you lower the amount of Green). So basically you are removing green from green areas. Yes, I know, it sounds a little strange. –  Itai Sep 12 '11 at 15:58
    
Can you watch this clip really quick -- is this what you're talking of youtube.com/watch?v=MbS_Taf7rfI&feature=related#t=56 –  Evan Carroll Sep 12 '11 at 16:12

Why don't you take one shot with the green screen and another (from exactly the same position) with a white screen covering the green screen? The green screen will identify pixels to be replaced by scenery in the white screen picture. The white screen doesn't have to be perfectly white or even uniform; its role is to supply the desired lighting for the car itself.

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+1 for solving the problem in reality while keeping it easy, fast and cheap and (I would say) getting better results. Instead of white (if that gave strong undesired light reflection from the background) one could use black or grey, or some color - except green on cherry red ;-) –  MattiaG Sep 12 '11 at 22:48

In addition to Itai's post production setting, two things come to mind for in the studio that are commonly used for video green screen:

  1. distance. 8 to 10 feet of it between the green screen and the object. MINIMUM.
  2. a linear polarizer, rotate it around to find the best angle to kill the bounce.
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Note: Whether you use a linear of circular polarising filter depends on what kind of camera you use. –  Guffa Sep 10 '11 at 12:30
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Assuming that OP needs a specific perspective, that is, a set distance from the subject and a reasonably large green background, then I am not sure a larger separation of the background will help here. The reason is that the further away it is, the larger it needs to be to allow for enough scenery background superimposition. This way, the relative angles from all parts of the green screen to the car and to the camera remain the same, and thus the reflections remain the same. –  ysap Sep 10 '11 at 14:12
    
@guffa: huh? I have both for my canon, it's just a difference in what kind of polarity the glass in the filter has. –  cabbey Sep 10 '11 at 16:41
    
@cabbey: No, the difference is that a circular polarisation filter has a depolarisation filter behind the polarisation filter so that the light that comes out is not polarised. You need that for some SLR cameras. Se: photo.stackexchange.com/questions/1368/… –  Guffa Sep 10 '11 at 17:58
    
mechanics of polarization aside, I still don't get what that has to do with what king of camera you have. –  cabbey Sep 10 '11 at 19:21

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