View Full Version : Green screen information help

08 August 2009, 05:19 PM
I was wondering if someone could give/tell me where I can get information about green screen.
I made one myself but I couldnīt find the green I needed so I used a green from a pool table, Iīve got some nice amator results but for an academic work I think itīs enough.
What I need is some information in how to use it correctly to prepare it to insert 3D.
Hope someone can help.
Thanks in advance

08 August 2009, 04:53 AM
While this DVD series is relatively expensive, Visual Effects for Directors is VERY cheap for what you learn from it.

With the current 30% discount, it's about $230, but it tells you quite a bit about green screens and how to make your own as well as how to integrate live action into 3D. It's an incredible set and I can't recommend it enough. The Master Course for High End Blocking and Staging is also VERY good.

08 August 2009, 03:29 PM
There are various technical terms for this ... an early trade-name was Chroma Keying.

Any video editing program will have the ability to do this. You simply specify a certain color that you want to remove. Now that we're in the computer-age, there are also a lot of "anti-aliasing" features in those filters.

The output from this process includes not only the RGB information but a fourth channel called Alpha, which controls transparency. (1=solid, 0.5=translucent ghost, 0=invisible.) This is how the green areas are subtracted: they get Alpha = 0.0

The various layers of 3D material also use Alpha to define "what is image and what is not," and this becomes the basis of the subsequent compositing processes. (3D outputs actually have many such "channels" of information in addition to RGB and Alpha.)

08 August 2009, 10:18 PM
I have some experience with green screening but I'm not sure what sort of advice you need.

If it's advice with the filming you need, I'd say this. Most people say that it is vitally important to light the screen evenly and have it free of creases, but in my experience you can get away with quite a lot of uneven light and quite a lot of creases. It doesn't even matter if you have some areas of the screen that are vastly brighter than others because you can key on luma values as well as just chroma. But I have found that having extremely dark shadows is not good, so it is essential to use lighting that will deal with this. You don't need to eliminate shadows completely - just make sure they aren't deep. I have used soft box lighting for this purpose. Point these lights at the screen - not at the subjects. The subjects should be lit for the environment in which they will be placed.

If it's advice with the postproduction stage, then it depends what editing software you use. I have learnt an eight-fold process: first, colour smoothing; then keying on chroma, saturation and luma; then edge thinning; then a 'matte choker' to thin the edges further; then softening of the edges; then cropping of the green screen footage (or using a garbage matte) to remove parts of the picture that are never covered by the subjects; then spill suppression (this is where you add magenta to the image to counter any reflected green light on the subjects); then colour correction of either the foreground or background layers or both.

08 August 2009, 01:10 AM
Thanks a lot.

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