Chroma key – problem with shimmering in hair

[Ciaran O connor]

Proper lighting from head to toe is the first stage, which you seem to have sorted. Same for using a fast lens to make the chroma background more uniform.

On the post proudction side of things, don’t expect just one pass of Primatte or Keylight to work. First make a garbage mask based on exactly what you need, which gives the plug-ins less to worry about. Punching up the green values before keying often helps too and then, you often need to luma matte the footage to a black & white version of itself, and then use that for colour correction and alpha matte-ing in a second or third comp.

Here’s a good starting point project which should be of some help to you….

12389_greenscreenchromagoodness.aep.zip

[Steve Bentley]

I’m not sure how a “fast” lens will help with uneven lighting. Uneven is uneven and opening up your fstop (in other words: a faster lens) will make the dark spots brighter but will also make the bright spots even brighter and the lighting will still be uneven.
Color correcting the screen color (punching it up) before keying is also a no-no. Any time you color correct something you are throwing information away. Look at the the histogram of any image. Now color correct that image. Look at the histogram again. You will notice gaps where information, (color information) is now missing and the smooth transitions from one color to another are now steppy.
Back in the optical days we had to hit a specific color of screen to make the chemical process of the film work. But these days you can key off any color. The saturation of the color and the differences between the screen color and everything else are what’s important, not the actual color itself.

That shimmer you are seeing is digital noise. This is probably caused by the compression of the footage either in the camera or when it was compressed after the camera (to make it a manageable file size to send). It could also be noise from the camera’s chip. I will concede a faster lens can help the noise problem out a little: when a CCD doesn’t have enough light work with it injects voltage in to the chip and you get more noise – a gopro camera does a beautiful job in full sun but looks noisy noisy noisy at night. So a faster lens will let more light in. But in a green screen scenario you should have more than enough light to work with.

Always key with as uncompressed footage as you can. HD (in 8bit) is 186 mega bytes per second. If you are using H264 you are, at best, probably running around 20 mega bits per second. That’s a factor of about 70! That’s a huge amount of information that has been lost. And most of that lost info is going to be the subtle stuff that you need to separate the hair from the green screen.

But not all cameras shoot uncompressed (or have interchangeable lenses) sooo.
Key what you can and then compress the key a little further and use a dechatter effect to help out (if needed). The goal on this first layer is to create a smooth edges key (as in: doesn’t chatter over time). This won’t let the fine wispy hairs through but it will keep the edge from shimmering. It will look like a helmet head key from a B grade scifi movie.
Now on another layer do a rough garbage matte that just lets you see the edge of the hair but not the core of the person and overlaps the compressed key you did in the last step. Don’t use a key puller effect on this garbage matte layer but do use a key color suppressor, so you end up with colored hair tips on gray. Color correct this layer to lighten the gray and then overlay this layer on the rest of the comp with a “darker color” blend function. (or you can crush this layer even further to make a matte for the hair and pass an uncolorcorrected hair layer through this matte.)
You can also remove the color from this hair tip layer when you start shifting levels around. Using extreme curves or levels on an image with color creates all sorts of noise, where doing the same on a grayscale image does not. With subtle little things like hair tips, surprisingly, they don’t always need to be in color, or you can tint them after the fact. (when we used to do photo retouching by hand, small imperfections were always done with gray scale paints, never color. The eye better at discerning brightness than color so things like hair tips aren’t noticed if they are the right brightness but not in color.)

[Karen Pouye]

Thanks for the template Ciaran!
I probably do need to go back & do a few more tutorials on the keying process again as it’s been a while and it’s quite a complex process.
I must admit that I did try a few video tutorials, but they were extremely basic and it was a matter of them throwing in perfect footage and getting a fairly instant & perfect result.
Thanks for your help and suggestions.

[Karen Pouye]

Hi Steve,
Thanks for all your advice & suggestions.
I think that the fast lens only helped in providing more light without the use of gain, although my testing results were not always consistent.
I will certainly try the dechatter effect and the techniques you suggested. I actually have 2 shoots that I was thinking of reshooting because of the shimmering problem. Maybe I’ll give your ideas a try…and maybe avoid having to reshoot (particularly as one of my actors is not available).

[Karen Pouye]

Well I have spent days trying every conceivable technique possible to eliminate the shimmering hair in my footage (not referring to the samples here) and I have been unable to reduce it significantly at all!

I don’t know what has caused it, but I think it is just too bad to chroma key.

If anyone wants to prove me wrong, here is some sample footage (50MBs)
https://www.dropbox.com/sh/l53j8eh5oof7jko/AADdBqzuPibpX6jiiVxq8W_ia?dl=0

[Tero Ahlfors]

I didn’t finesse it but two Keylights for two different ranges and the advanced spill suppressor would be a pretty good start.

[Tero Ahlfors]

Also a lightwrap might do some good when you have the background ready.

[Karen Pouye]

Thanks Tero, I’ll give your suggestions a whirl. I tried just keying the top of the hair all on it’s own & I could not even get that happening. It just won’t stop moving even when I shrink the matte right back.

[Tero Ahlfors]

It’s also good to remember that sometimes half-ass is the proper amount of ass. If you have badly lit, compressed footage then you should make it look good overall instead of trying to fix some small part that viewers probably won’t care about.

[Steve Bentley]

Will the background be white as in your original sample? If not can you supply a frame of the BG? What it’s like determines much of what the key must accomplish.