MOLTEN PICTURES LLC

A Day in Heaven
The green screen from hell
status - nightmare

For anyone who doesn't know, greenscreens are used a lot in CG. The basic idea is that the greenscreen material in the scene can be identified and extracted leaving a transparency matte. It's not as easy as that though. Setting up a good greenscreen means flat even lighting so that it is bright and a computer program can easily figure out which pixels need to be taken away. You can't get it too bright though because the green will reflect back onto your actors (thats called spill), and it can't be too dark either because then the program you are using can't figure out what's going on.

One way you can pull a matte from green screen footage is to sample the color or chroma values of the green in your footage. Good greenscreen material has a very narrow color band when it is lit right so you don't need a lot of samples. If the greenscreen is badly lit then you need to sample more colors and that might mean that other parts of the shot may be affected.

For A Day in Heaven we have a couple green screen shots. The ones for the car flyby aren't so bad. All the shots in heaven are a nightmare, and I will attempt to show you why. Sometimes your production schedule is so tight that mistakes might happen on the set, which is what happened here, but taking an extra hour on the set to get your greenscreen right could save you several days of post production work. If you ever have to do this, or give your FX person some greenscreen footage maybe I can help you save a little bit of time and money.

Here is the greenscreen plate, Carmen passes through the gate, and I need to replace the background with a heaven graphic. The camera isn't moving but there is a lot of movement in the scene.

Here are some things I need to point out.

1. There are a lot of shadows casting on the greenscreen. When it gets this dark I can't just key it out anymore without getting a lot of the rest of the image.

2. This is good! If the entire background looked like this I would be done already.

3. Why is there a lightstand in my shot! Seriously WTF? Not only is it not green but it's got shadows and bright reflections all over it. The angel's wing passes right in front of it. This is a big problem for me because I am either going to have to cover it up or rotoscope the wings and bars. To rotoscope the wing means tracing the outline of the wing (which is very complicated) for every frame the wing is passing in front of the stand, that's between 60-70 frames for this shot.

4. Wrinkles on greenscreens are bad m'kay, don't do this please. This means that I can't easily sample the green here.

5. The greenscreen fabric didn't quite make it to the floor here. Yeah, that's all I have to say.

6. A heavenly water bottle, this probably shouldn't be here.

line

And here is what the final shot looks like:

You can see a layer breakdown of this here (5.6 MB)
and the final version of the shot here (2 MB)

What I ended up doing was knocking out as much of the green as I could, making a roto-mask of the bars and gate, and croping out the problem corner. Then I droped in my background and applied the heavenly glow to everything. It's still not as nice and clean as I would like it to be, but I've spent two days getting this worked out and I just can't devote any more time to it without risking a missed deadline.
line

10-11-06 UPDATE!
Watch before and after comparison movie (3.2MB)
Finished the last of these nasty shots. The third shot in this series was a total bummer, made worse than the others because carmen walks too close to the greenscreen and casts a bad shadow on it. Finally got it worked out by dividing the frame into segments so that I could isolate the many different shades of green in the background. This last shot required a lot of roto. It took me way too long but I finally got them done.

Here is a before and after comparison of the last shot where Carmen walks back through the gate. I am using DFX+ to do my compositing so I included a shot of what my flow looks like. On the left you can see how I had to break the frame into five regions so I could fine tune the matte.

line

Let me finish this off with a note about compression. Most video cameras use some kind of compression when recording. The problem is that most of these compression schemes are designed to appeal to the human eye, not someone who needs to pull a matte from greenscreen. DV is especially bad and Sony's HDV isn't very good either. If you can get your hands on a camera that supports no compression of the luminance and chroma information it means a much better matte. There aren't many that do this, and it wouldn't save me on this shot, but if you have to do a lot of this kind of stuff and want the best quality with the least effort you might want to seriously consider finding one. Some HD cameras that can do this are the Canon XLH1 and JVC HD-100.

For an in depth look at why you want a camera with very little compression look at comparison of the DV and Beta formats on this page :
http://www.nattress.com/Chroma_Investigation/chromasampling.htm

INDEX