Green screen edges are animating

Al Min wrote on 10/31/2009, 11:47 AM
I shot a two person interview with the couple sitting on a brown couch against a green screen. I'm using the secondary colour corrector with a bit of chroma blur. The edge of top of the couch animates, as does part of the subjects - a few pixels leaping up and down. What causes this and is there a cure? I must admit I didn't light the screen and there is a patch of darkness between the people's heads. Any advice would be much appreciated.

Comments

Coursedesign wrote on 10/31/2009, 12:38 PM
You need to try chroma smoothing, search this forum for past posts.

Most consumer and prosumer cameras throw away 75% of the color information, i.e. 3 out of 4 color pixels. This leads to the blockiness you observed.
LReavis wrote on 10/31/2009, 2:30 PM
Sometimes I, too, have had trouble with edges animating. As I recall, I fixed that partly by putting Smart Smoother first in the chain of effects - before Chromakey/Secondary color corrector, etc. Try it, setting SS to the lowest value that seems to help fix the problem. I also use chroma blue - again from memory - I believe I used the chroma blue in the free AAV Colorlab plugin. I've been using V .95, but they now have V 1.0 posted, in both 64-bit and 32-bit versions.
farss wrote on 10/31/2009, 5:15 PM
What did you shooting it with i.e. which camera and in which mode.

Assuming the camera was well locked off then there's no need to CK the static elements such as the couch. Use a mask to handle that. Then you concentrate on using the CK on the bits that'd be a pain to mask i.e. the parts that move.

Line twitter, noise, aliasing, chroma sampling and interlaced mpeg-2, any of those could be causing your problem. To get a fair shot at getting a good key you need plenty of light to avoid the camera using gain. Shooting progressive helps. If possible turn down the Detail setting in your camera.

Bob.

Al Min wrote on 10/31/2009, 11:58 PM
Bob - I used a Sony HDR TG1 camera. I set the white balance manually. My main problem I think is that I relied on the subject lighting to handle the green screen as well. This worked at home, so I thought I'd be OK. Also, I did not allow for a 2 foot gap (as I've read you should allow) between the green screen and the couch. Once again, the close gap worked at home, so I took a chance. Can you help me out with a tutorial on using a mask - do you mean a bezier? I've done a search but cant find a step by step anywhere

Larry - by Smart Smoother, do you mean the one for Virtual Dub? I've never used VDub. Is it stand alone or does it work within Vegas?

Thanks for the advice guys. I'm just a learner trying to do his best.
farss wrote on 11/1/2009, 1:53 AM
"do you mean a bezier?"

Yes. I think how to create a mask is well covered elsewhere. How to use it in this situation can be a bit confusing as there's a number of ways to do it.
One way. Draw the mask. Get it right on the edges that don't move but leave enough room around the parts that do move so they never get cutoff by the mask. This is what's know as a garbage matte.
Now use the CK FX, I oftenly find that not sampling the key color and instead just using the green or blue screen preset work better.
I suspect the reason using a preset works better is when you sample a badly lit screen you end up with too large a 'window' of chroma values.

From my experience you need more than 2 feet between screen and subject. Having one set of lights for the screen and one set for the subject is a big help as well. In the end of course it depends how pixel perfect you want the outcome.

Bob.