kEYING Greenscreen RGB Luminance?

[Amey]

Hi,
Does anyone know how to key a greenscreen with the RGB luminance (specially in Combustion) ?
i heard about this technique :
substract the blue and the red from the green channel and then do a key in luminance to extract the green which at that point is supposed to be white?

thanks

[Michael Pekic]

Hi there.

Are you sure, you want to do it that way? General speaking, it doesn’t matter what process you use, channel manipulation, using a designated “keyer”, rotoscoping, or whatnot, what count at the end is that you have a good matte to separate your fg-object from your bg/green-/bluescreen. I hope you don’t mind if I start with some basics (the way you phrased your question leads me to the assumption that you just started out with combustion and/or compositing).
Ok. Color images can be represented by a number of color spaces. One of them is the RGB color space. The color captured is split into three color channels: red,green and blue. Each of these channels can be thought of as either a picture with shades of the respective channel, or as a black and white (grey scale) image. if you use the short-key “ctrl+shift+5” the red channel of your viewport image will be displayed. “…+6” is the green channel and “…+7” is the blue channel. Each channel as a grey scale image.
the following is a coarse description of the process. if you have difficulties following it, it’s either i have some trouble finding the right words, or you’d better do the tutorials which are shipped with combustion.
Say you have a green-screen shot. load that in a viewport and look at the green channel. you’ll see that the bg is almost white and the fg almost black, or at least a dark grey (maybe except from highlights on the fg object). what you try to achieve is a black bg and a white fg. so, you inverse the image. since you want to have a solid black bg, you can either add an histogram-op or a curve-op. manipulate the green channel until you get your wanted result. use a compound alpha arithmetic op. for the main input of that operator use the original footage. as secondary input use the manipulated one. in the controls of that op set operator to “set” and input to “green”. the result of all that should be a more or less (more likely) good key of your original footage.
this is, all in all, the concept of a (very basic) chroma keyer.
a more evolved technique is that you determine which of the original red and blue channel is brighter and take only the brighter pixel for further proceeding (take a look at the compound rgb arithmetic op) and subtract those from the pixel in the green channel. of course the result of the subtraction, as well as, each channel prior any operation can be scaled using curve- or histogram-ops. this later concept is the one which is behind a color-difference keyer.
you might want to read steve wrights book “digital compositing for film and video” (focal press).

[Jonas Ussing]

i know i sound basic , it’s because a i don’t speak english well, i speak french :/
i tried all the techniques i know, even the one you suggested me.
i know there’s a mathematical operation in Fusion using the suppression of the red and blue channel from the green or something like that and it does a really good result with the kind of shot i have to key.
i wish i could show you just one frame of my shot, it is a desastre. the greenscreen is dark and very grainy and the foreground is people in black suit. And you are right, i am a new combustion’s user, i use to work with flame and it is really not the same way to work.

thanks a lot for your answer 🙂 if you have any other idea … let me know

[Michael Pekic]

mail me a pic and a cws (or several) of what you have done so far.
i’ll see what i can do. btw., the concepts of doing those things in fusion and combution are the same.
[email protected]

cheers!
rayk

[Author]

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