Chroma Key - Just can't get it right!

T

Thomas Theron Cooper

Member
Hi,

This is a great forum! So much useful information! I need some advice. I've been trying to use chroma key in Sony Vegas 12 Pro but can't seem to get it right.

I'm shooting the talent against a green screen about 3/4/5 feet behind. The green screen is lit with two lights. The one on the left lights up the right part and vice versa. I've even had the screen appear wrinkle free, with no hot spots and yet when I try to remove it in Vegas I end up with a lot of spill. If I try to remove the spill by being more aggressive with the high and low threshold, parts of the talent seem to get cropped. Can the talent wear blue if the screen is green? I've been avoiding blue, green, black and white clothes on the talent. I tried tweaking the high and low threshold and then working with the HSL setting but even that gave me mediocre results. Vegas is such a widely used software that I know it has to be me going wrong somewhere. Please give me some tips. Thanks.
 
Okay, you mention low and high threshold, so you know about that. Do you also know you can change the Chroma Keyer colour to match your key colour? Click on the colour in the popup screen, then the eye dropper, then click some of your green (probably on some of your footage on the timeline, or the preview window without Chroma Keyer switched on). Even better, you can drag to select a range of green. I only touch low and high threshold after choosing my green (or whatever key colour I'm using). Other than that, Vegas does Chroma keying pretty well - most other problems are due to bad lighting, not Vegas.
 
If you do get to much spill I would consider moving the subject away from the chroma background and reduce exposure on the chroma background.
 
clang said:
..most other problems are due to bad lighting, not Vegas.
Click to expand...
That could be it. In some test footage, the talent is wearing a shade of blue. That may be the trouble I think. After using the 'show mask only' checkbox typically when I increase the high threshold everything starts going white up to a point. Then when I use the low threshold, the background starts going black. At some point, before everything in the background has been blacked out, what was white also starts turning black. This means the talent's image starts getting cropped. I attribute this to spill from incorrect lighting.

Osslund, I tried doing that. Unfortunately in that footage, the talent is wearing apparel that has is partially black in color. This makes it difficult. I don't have perfect footage yet I think.

Thanks.
 
You do a lighting for the background and one for the foreground so anything the subject wears shouldn't matter as long as it's not the same or about the same colour as the chroma background.
 
Your talent sounds way too close. This is a problem you might not be able to avoid due to space restrictions, I'm guessing? But yeah, further away you can get talent from screen, the better. Of course, the further away they are, the larger a background you need to key them against, given the same "size" shot. Of course the green doesn't have to fill the screen, it only has to surround the talent (you can garbage matte/mask out the rest). Lastly, I didn't see anyone mention the magic of minus green. This is a gel like any other lighting gel specifically geared toward killing green spill like you describe. You put it on a light like a hair light, edge light, call it what you will. Point is, it's a light (or several to kill spill on each side and top, for instance) that is pointing back toward the camera, hitting the talent, from near(ish) the screen, covered with this minus green gel. There are different strengths, and the one you use will depend on the amount of green spill you're getting. Too little and you won't negate it enough, too much and the light will look magenta ('cause the gel is magenta).

Also, there should be plugins for "spill killing" that help with that. I used one in a project recently, from Magic Bullet, inside FCP. Hopefully Vegas can utilize those plugins.

I've also heard (though have not tried) applying multiple instances of the chroma key plugin in your NLE. Each instance catches a specific range of green (unless you lit the screen perfectly, which is tough in low budget situations). together, they get it all.
 
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