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Key grading and keying advice needed [ANSWER]

Posted: Tue, 14th Nov 2006, 12:19pm

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petet2

Force: 2899 | Joined: 27th Dec 2005 | Posts: 1043

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Last night I was finally getting to use some green screen test footage I shot in the summer and was applying a key grade (as the green screen had a big shadow of the subject across the middle). I think I understand what i am doing but would be grateful for some advice.

1. Am I right in thinking that the key grade is purely about making the green screen background as uniform as possible and so I shouldn't worry about the effect on the subject in the foreground? (it ended up almost completely silhouetted).

2. When keying out the green screen background can I apply two keys - one for the main green screen and one for the darker green of the shadow of the subject?

3. I filmed outside using a green curtain and half way through the shot the sun came out completely changing the shade of the green screen. As a result half way through my composite shot the green screen suddenly becomes visible. Would I be better dealing with this by key framing the original key settings or by adding a second key?

Thanks for any advice. I have to say that the green screen footage I am using is dreadful on reflection (in my defence it was filmed in a hurry with a reluctant subject one blisteringly hot afternoon in August) but the results that Comp Lab Pro has acheived to rescue it are quite amazing.
Posted: Tue, 14th Nov 2006, 12:40pm

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Jazzmanian

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Without seeing some of the original footage, or at least some stills, it's hard to say for sure, but...

petet2 wrote:

Last night I was finally getting to use some green screen test footage I shot in the summer and was applying a key grade (as the green screen had a big shadow of the subject across the middle). I think I understand what i am doing but would be grateful for some advice.
I've been finding out the hard way over the last six months that lighting is the make or break thing in green screen shooting. In this thread you'll find a picture by NickD which will show you the optimum way to set up your key light, fill light, back light and background light for optimum results. It really does make a difference and the close you can get to that situation, the easier life will be in post production. CLab can do some amazing things to clean up mistakes, as is shown in the tutorial that comes in the manual, but the old "garbage in - garbage out" rule still applies to some degree. The shadow can probably be overcome in CLab to some extent, but it's not a good thing.

petet2 wrote:

1. Am I right in thinking that the key grade is purely about making the green screen background as uniform as possible and so I shouldn't worry about the effect on the subject in the foreground? (it ended up almost completely silhouetted).
You need both. Your subject should be well lit so you have the best possible finished key footage to work with when you're ready to composite it into your film. But at the same time, you need to put just as much attention into lighting your greenscreen properly for the best results. Again, see NickD's sketch on that subject.

petet2 wrote:

2. When keying out the green screen background can I apply two keys - one for the main green screen and one for the darker green of the shadow of the subject?
\

I'll have to leave that one to wiser heads. I've never tried it. It *does* sound like something you could do if you used two layers of the same footage and a garbage mat to pick out the shadowed area, but if there's a lot of motion going on that's going to be a nightmare to block.

petet2 wrote:

3. I filmed outside using a green curtain and half way through the shot the sun came out completely changing the shade of the green screen. As a result half way through my composite shot the green screen suddenly becomes visible. Would I be better dealing with this by key framing the original key settings or by adding a second key?
You probably should have just started the shoot over when the sun came out of that was at all possible. (The sun is a pretty good light source if you can keep things constant and use a fill light to make sure you kill the shadows on the screen.) If it's a sudden change from clouds to sun, you also might try using the cutting tool to snip your film into two segments and key them differently according to the light conditions?
Posted: Tue, 14th Nov 2006, 2:49pm

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petet2

Force: 2899 | Joined: 27th Dec 2005 | Posts: 1043

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Thanks for your help. This was very much some test footage just so I could have a play with green screening - I had been using Comp Lab Pro with 3D rendered models prior to that. I have learnt a lot since then about shooting techniques so will ensure that future footage is of a much higher standard.

My point 1 was relating to a comment in the video tutorial on here about key grading (which I watched very late last night so apologies if I am misquoting). The tutorial shows a key grade being applied and the narrator observes that the subject looks washed out but that this does not matter because the key grade only applies to the keyed background area and will not affect the appearance of the subject in the final composite. Is this correct?
Posted: Tue, 14th Nov 2006, 3:17pm

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Simon K Jones

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Indeed, key grading won't affect the final output (other than with regard to any actual keying you apply, obviously). So while your actor looks silhouetted, he'll be perfectly normal by the time it's done.

This can be really useful when working with poor footage, as you can completely nuke it to pieces in order to pull something off it, and still get a good result. In theory, anyway. smile

Jazz's advice is all excellent, so I won't repeat that.

With regards to using multiple key filters, yep, that's fine. You can apply as many as you want, and different types too. So you could use a colour difference key to get rid of most of it, then use a hue key to tidy something up, perhaps.

Multiple colour difference filters don't often work too well, due to the way they work - on a specific chroma green. But different hue keys, or luminosity keys etc, each set to slightly different colours, can work really well.
Posted: Tue, 14th Nov 2006, 5:20pm

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petet2

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That's excellent, thanks.