green/blue screen and shutter speed

wahrez

wahrez

Well-known member
Hello

I have a couple of questions. I'm shooting a scene for keying where a hand will come in to contact with the keying colour. I've read somewhere that blue is a better choice if the subject is in contact with the fabric...is this true? Does anyone have any other tips for this scenario.

Also with a test on greenscreen when the hand moves across the shot some green remains around the fingers due to motion blur...is it more typical to shoot at a high speed to have a cleaner image to solve this problem?

Thanks
 
While I'm not an expert, it would make sense to me that keying would be easiest at a very high shutter speed. Any motion blur you want can be (relatively) easily added in post.
 
If noone has a deffinate answer...we'll find out tomorrow!!! - i'll post the results!
 
HBlack said:
While I'm not an expert, it would make sense to me that keying would be easiest at a very high shutter speed. Any motion blur you want can be (relatively) easily added in post.
Click to expand...

Yep. You will get a cleaner key with a higher shutterspeed. It reduces motion blur. You will want to test it out first and make sure it will not ruin the look you are going for. Perhaps just increasing a little will help keep the footage looking not so different while adding the slight advantage.

As far as green vs blue, it really depends on what you are keying. Sometimes green is better, sometimes old school blue. As far as skin tones? I usually prefer blue with skin because the slight ambient hue that is cast upon the skin is easier dealt with when using blue. Slight green tints on skin look nasty and are a bit more complicated to correct.

my personal opinion on green vs blue.
 
You can shoot with a fast shutter if you're working with primitive keying software. Then adding blur afterward is pretty simple. If you're using something nicer, like Ultimatte, you'll be able to pull the blue out and make the blurred object semi-transparent. You can shoot at any shutter speed and the software will compensate.
 
i always shoot highest shutter speed my lighting will allow and add motion blur in post. much easier to key that way. as far as green vs. blue goes, i have had better luck with blue when shooting highly compressed video such as dv or hdv. i like green better when on the hvx. but really both work and it's really which one exists the least in your subject's clothing. neither really exist in skin tones.
 
Blue nor green are very apparent in skin tones, but going back to my skin tones comment earlier: I've found when using large lights, lots of ambient light is bounced back from the keying screens (blue or green) causing a slight hue to be cast upon the actors. A blueish cast is preferable to me vs the sickly green. Plus it is easier to correct. Just trying to clarify.
 
If you are getting a green, or blue bounce back on your subject, then fix the problem at the source.
Place you subject as far from the backdrop as is possible and keep the screen within a stop of your subject, either way.
That also allows you to light the subject without effecting the screen.

Work smarter, not harder. I know this isn't always possible for space, size of screen, amount of gear reasons, but when it is possible it is a much better way.

Good luck.
 
I think it really depends on the format your using, as well as the quality of the key you use.

As for the green VS blue, by far, green is better for just about every digital camera I've ever seen. I recommend shooting green screen under daylight balanced light. The green channel is always cleanest, under daylight or tungsten lighting. I recommend day light simply because it helps clean up the blue channel more, and that never hurts even if you are keying out green.
 
Blue channels are usually the noisest. I usually light my screen about one to two stops brighter than subject.

You can backlight your subject with a minusgreen or a yellow gel against a green or blue screen respectively, if done properly you can cancel out spill for situations where you're forced to put the talent close to the screen. In most situations, I find that a backlight is desirable anyway.
 
Has anyone considered using a black screen when the subject has to come in contact with the screen. Black doesn't cast the ugly hue of blue or green, and it's pretty easy to key out. I understand if the actor is wearing black, or a dark color, but if you can make sure they are lit well and try and keep as much light off the black as possible, it works pretty good.
 
Black works great if you're going to matte in a dark background anyway, you can't use aggressive key/fill ratios, and if your actor has black hair or dark skin, it's simply not an option. When luma keying, usually you use a white limbo instead of black.
Using a luma key with a black screen is similar to double exposure, like the old "invisible man" movies.
 
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