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green screen

Posted: Tue, 24th May 2011, 12:21pm

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EJR32123

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Is the green screen that composite lab pro comes with a high quality greenscreen? it seems a lot darker than then other greenscreens, making it harder to key out.
Posted: Tue, 24th May 2011, 2:34pm

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Axeman

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It is a high quality, true chroma green screen. The brightness of the screen comes down to how you light it. Generally the greenscreen should be lit bright enough that it is at least one f-stop overexposed in comparison to your subject. If you can post up an image or a frame pulled from your footage, we can give more specific suggestions for how you might make keying easier or fine-tune your setup, but remember that you will want the greenscreen well-lit, and evenly lit.
Posted: Tue, 24th May 2011, 3:38pm

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EJR32123

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here is a a frame.

could it be my camera does not display the color green well?
Posted: Tue, 24th May 2011, 4:38pm

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Axeman

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Thanks for the image. Its probably not your camera. The reason greenscreens are so much more popular than blue these days is that consumer cameras store twice the data in the green channel that they do in any of the other color channels.

In your image, the exposure of the screen looks decent, its fairly bright. Not perfect, but it should be keyable. It seems quite green to me. What exactly is the problem you run into when keying it out? I do notice that there appears to be a couple of hot spots on either side of the area shown in the pic, where perhaps the lights on the screen are overly bright, but since its outside the pic, its hard to say how much of a problem that is. I think a little more detail about what goes wrong when you try to key it would be helpful at this point.
Posted: Tue, 24th May 2011, 5:09pm

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EJR32123

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I can key it, but videos with the greenscreen like this pic are way easier to key.
Posted: Tue, 24th May 2011, 5:10pm

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EJR32123

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thats the color of the greenscreen used in the example projects for composite lab pro (giant chasing). that green does not no look like mine.
Posted: Tue, 24th May 2011, 5:21pm

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Axeman

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Again, if you can explain what the problem is when you try to key it, we can offer better advice.
Posted: Tue, 24th May 2011, 5:36pm

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EJR32123

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i am having no problem keying, i am saying that the color of my greenscreen makes it more difficult to key out then videos with the other color i showed
Posted: Tue, 24th May 2011, 5:46pm

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EJR32123

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How can they both be chroma green? is that the camera record it differently
Posted: Tue, 24th May 2011, 6:22pm

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Mad Mike

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the first shot looks to be the green screen here offered by tubetape. the second lighter green is also sold elsewhere.
I questioned the very shame thing but found the darker green keys better.
they are both completely different shades and different companies sell either one or the other and always call them green screen.
Posted: Tue, 24th May 2011, 6:38pm

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Axeman

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Sorry, my point was just that if its more difficult to key, then there must be something that doesn't work quite right (i.e. a problem) that you have to adjust or compensate for when keying, right? Why is yours more difficult to key? Where does the difficulty come in? Is it harder to get all of the green to disappear? To get a clean edge? Somewhere in there, something is being more problematic, and that's the problem I'm trying to identify.

The difference in color between the two examples you provide isn't so much the camera as it is the lighting. The exact same screen, filmed with the same camera, can appear to be two very different colors if the lighting changes significantly. The lighting isn't as bright on theirs, but more importantly, its very even, where yours isn't quite as even. Moving your lights farther away may help even the light out, or perhaps try using less directional light, bouncing it off of reflectors or the ceiling or the side walls or something. Whether you are using manual exposure or auto exposure could also impact how the greenscreen appears, as what the camera reads as the correct exposure isn't always going to be right for greenscreen work, depending on the metering mode being used and the particular lighting of the setup. If you'd like a more technical explanation of how the camera can read the same color differently, I can try explain it for you, just let me know.

Which keying filters you are using can also have a huge impact on how easily something keys, so more specifics about where things get more difficult with your footage might also be useful if you want some suggestions on how to key things.

If not, and you just were after a straight answer to your original question, then no worries. The answer is Yes, it is a high quality greenscreen, it is true chroma green, and the differences in color you see are most likely an effect of how the screen is lit and exposed, not necessarily a difference in the color of the screen itself.
Posted: Tue, 24th May 2011, 6:54pm

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Mad Mike

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Posters original question is in relation to two different green screen colours I own both.

The lighter colour I bought cheaply with a very cheap throwaway camera. I only wanted the scree.
My second green screen I got here from but the blockbuster movie kit.
Though I eventually discovered the darker green easier to key.
Posted: Tue, 24th May 2011, 7:02pm

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EJR32123

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thanks guys and yes axeman, could you please explain a more technical explanation of how the camera can read the same color differently? thanks
Posted: Thu, 26th May 2011, 4:21pm

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Axeman

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Sure thing. I've tried to cobble together a few images as well to help with the explanation.

The first factor that comes into play is the Color Temperature of the lights illuminating your scene. If you aren't familiar with color temperature, it is measured in degrees Kelvin (K), and measures the warmness or coolness of a light source, not in actual temperature, but from a color standpoint. Blue is a cool color, and orange is a warm color, and basically the kelvin scale goes from blue to orange. It gets a little confusing in that cool colors have higher kelvin numbers, due to the process used to calculate color temp in the first place, which is explained in that wiki article.


Image borrowed from http://www.mediacollege.com/lighting/colour/colour-temperature.html

Since all light has some color or other, the color of the light will impact the color of the subject, to varying degrees. Our eyes, demonstrating superior design to any camera available, can handle a huge range of color and brightness, and generally compensate for color temperature changes or differences automatically, so we often don't even notice. Cameras have a much more limited dynamic range, though, so these color changes become much more noticeable.

In the case of green, such as in your screen, the color is made up of a mixture of Yellow and Blue, so whether the light shining on it is yellow or blue make a difference in the color, just as adding more yellow or blue to green paint will alter its color. Notice in this pic how much the color of a chroma green can change based on light. The center of the image is pure, unadulterated green, in normal daylight.



The second factor is how exposure in cameras works. Basically, auto exposure in the camera tries to make the value of anything you put in front of it equivalent to 18% gray.


18% grey refers to a middle shade of gray which reflects 18% of the light that hits it back to the camera.

If you take your camera, on auto exposure, and take a picture of a solid black wall, the image won't come out black. It will come out grey. The camera automatically adjusts exposure to brighten the scene, trying to make the black appear as 18%grey in the photo. If you do the same thing with a pure white wall, the image won't come out white, it will come out grey, as the camera again adjusts exposure trying to hit its 18%grey target value. When there is a variety of colors and shades in front of the lens, it averages them all out, then tries to match the resulting value to 18% gray.

There are lots of different metering modes that cameras can use to calculate this exposure though, such as spot meting, center-weighted metering and evaluative metering. These modes will vary from camera to camera, but typically you'll be able to select from an assortment of options there, and each one can impact the final exposure of the scene differently.

Most cameras, by default, use a system where the entire frame is averaged together. This means that if your scene isn't lit consistently, then areas that are too dark (or too bright) can potentially throw off the exposure, and therefore the color, of the areas that are well lit. I realize that this is kind of a complicated subject, and hopefully I made it somewhat understandable. If you have questions about any aspect of it, feel free to let me know.
Posted: Thu, 26th May 2011, 4:42pm

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EJR32123

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thanks