keying an under-lit green screen

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dv416

Well-known member
hey all

in my experience, green screens have to be bright and saturated neon green to make for the easiest time keying. i am working on a scene right now with a very dim or washed out green screens.

when i set the screen color to match the green in the shot, keylight takes away lots of blacks and alpha from the foreground image, and adds TONS of grain. the shots with washed out green screens usually just produce grain. yes, i have 'clipped' black and white, screen gained, etc. but the results are not great. obviously, the solution would be to light the screen better, but too late. right now i am experimenting with auto-traced garbage masks and just spill suppressing the leftovers to black, but im wondering how a green screen veteran might handle this sort of problem.

also, i feel like i HAVE seen behind the scenes footage for big budget features that show a washed out or dimly lit green screen on set, yet the final product in the film is obviously perfect. i wonder how they did it...

do you guys have any advice?
 
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The most important thing is that your screen is evenly lit.

What kind of footage are you trying to key? DV? HDV? DSLR? All of these formats suck for keying. You can get decent results, but that makes the lighting all that more important.

As for the big budget features, it's simple. They were lit properly and they are either shooting on film or another high quality format.
 
It's easy after the fact to talk about how a green screen scene should be shot. But the VFX job is take what you have and make it work. Grumble and moan, sure, but you rarely get to reshoot it.

In general you have to stop thinking about loading one copy of the footage and applying one copy of Keylight, setting some controls and getting a perfect matte. That isn't how it works. You most likely need many, many copies of the footage, each one with it's own masks and image processing. The idea is to build up the matte with layers. Your end goal is a clean usable matte. How you get there is a combination of image processing (to make the dark green screen more usable) and a host of tools to turn that into a grayscale image.

If the green screen is really pure black, then your going to need to luminance key those areas (maybe). Or perhaps you may need to hand draw the mask with the pen tools. Don't discount rendering the matte to frames and then painting on them in Photoshop. Sometimes splitting the channels apart can help you isolate what you need. Lots of tricks here but almost all are knowing what you are heading towards and making every thing you do help get you closer to it.

Usually you should be able to create a core matte, which is the area with no edges without much trouble. Then your focus is on creating the edge matte, which you do by processing the too dark image and masking it. Then you combine the two mattes (or ten mattes) to make one final matte image that you then use on the original footage. Then you start working on spill suppression and other issues.

Sure, from time to time some wiz-bang plugin might help along the way. But you'll find that most of these magic bullets misfire a lot. Stick to the simple basics and things usually will go better.
 
msorrels said:
... In general you have to stop thinking about loading one copy of the footage and applying one copy of Keylight, setting some controls and getting a perfect matte. That isn't how it works. ...
Click to expand...

But that's exactly what I seem to be getting and I don't even have to breath hard. Yes, I used to do the multiple composite keys, but that was way back in ancient times (the 90's) when technology was a mere shadow of what it is now. These days I can pretty much just huck a green sheet in my living room and clickety boo, instant keys.

easy_green.jpg


I posted this example in another thread recently and the shoot really was a no brainer. Click here to see the clip. The hanging material wasn't even smooth and had wrinkles and folds in it and the lighting wasn't perfect either. For me, if I'm getting a decent matte around strands of hair from a single click, I think that's good enough. I'm sure if I spent time fussing a lot I could get much better results, but I'm already keying at the sub-pixel level. How much finer do you need? The director was happy and we even wrapped early enough to pop down to the pub for a pint!
 
Did someone say hanging green sheets...

Ok, so the Wife had a need to try to mock up a project idea for more interactivity on one of her artworks. Here's a 'sample' of 'no budget, on the cheap, quick and dirty'...


Frame of the original setup. Shot with a Sony SR11, in a hallway of an office building using Target green bedsheets as the keying material...

attachment.php


Here is the AE Keylight keyed out sheets.

attachment.php


Here is the garbage matted away remainder...

attachment.php


Here is the composited (approx shot...) result.

attachment.php


The point being, is if one does have pro material available well of course take advantage of it... but if one does not... there are DIY things one can do, and get
reasonable results for the intended goal...
 
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hey dv416, put a curve on it before keylight, then another curve after
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0.5 screen pre-blur might help any rough edges
 
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msorrels said:
Usually you should be able to create a core matte, which is the area with no edges without much trouble. Then your focus is on creating the edge matte, which you do by processing the too dark image and masking it. Then you combine the two mattes (or ten mattes) to make one final matte image that you then use on the original footage. Then you start working on spill suppression and other issues.
Click to expand...

thanks for posting! all i've really been able to find online are keylight tutorials - and keylight does NOT like the green screen in this case. i'm having some luck with auto-tracing the semi-keyed rendering from keylight, but i would like to learn more the traditional 'core / edge matte' workflow that you describe above. there is a complete void of information on this approach online, so can you elaborate a bit? i'm a DIY-er to the core, so I'll put in the time to learn it, I just need a few resources! how did you learn this?
 
If Keylight doesn't like your footage, you need to change your footage until it does. Curves, levels, etc. All those color correction tools can be used to make your screen more usable by Keylight.

As for how to do more complex keying I'm not sure I had any one source of information. I'm also not convinced I'm that good at it, yet. I did pick up a lot from the tutorial course that came with Digital Fusion DFX+. But I'm not sure that's even available anymore. Creative Cow has a good keying DVD made by the Video Copilot guys. If you can afford it a semester at FxPhd was very eye opening on solving real keying problems.

Rather than look for After Effects and Keylight tutorials you'd be better off looking for Nuke/Shake/Fusion tutorials. They tend to be slightly more advanced than how to tweak simple settings in Keylight.

3D World magazine had a really good piece with a complex keying problem in Issue 134. The footage to which is actually available online here(at least for now):
http://www.3dworldmag.com/page/3dworld?entry=tutorial_files_for_3d_world14
It's using Nuke but the greenscreen footage can be used in anything. It's interesting because at first glance you think "no problem" then you find even the all powerful Keylight has issues.

While it's ancient this video is actually pretty good as a basic course: http://vimeo.com/8772577

I'd recommend all the tutorials on Keylight at the Foundry website as well. Great stuff.
 
dv416 said:
thanks for posting! all i've really been able to find online are keylight tutorials - and keylight does NOT like the green screen in this case
Click to expand...

Can you post some examples of the 'green', and 'underlit'... It could help us understand why things are not working for you...
 
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