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Luma Key or Chroma Key Shoot for White Background Compositing?

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Paul CarpenterLuma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 9:36:21 am

Hi everyone,

This is my first post here and I'm also pretty new to the wonderful world of AE. I hope you someone can help!

I am about to shoot for a vfx sequence to be made in After Effects CS5.5. It will include sportspeople performing on-the-spot moves to camera. Footage is to be composited into a pure white 3D space along with some vector art assets and other effects. The result will be a clean and minimal virtual 3d set with moving cameras.

I have been working towards a green screen shoot for this task but have no experience with full-length chroma key, which worries me. I have however, done full length luma key with static subjects and that worked a treat.

For this project,some of my performers will be hopping around on BMX bikes, dancing and doing acrobatics. This obviously adds a lot of potential motion issues and particularly chroma reflection issues.

I have started to wonder, given that my virtual set will be white with no discernable 'floor', would I be better of shooting for a luma key rather than a chroma key?

I wonder if chroma key and it's complications would be asking for trouble, given the end composite will be fairly plain and there is no motion tracking to be done. Also, my limited lighting resources may mean an evenly-lit full length greenscreen may be difficult to achieve, whereas a white screen may be more forgiving. I guess at this stage, my judgement is limited by the fact that I don't really know the limits of my capabilities with After Effects.

It is imperative I achieve a professional end result, so my choice must be a well-informed decision, hence this post!

Thanks for taking the time to read and I hope you can help put my mind at rest!

Paul


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Tudor "Ted" JelescuRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 5:24:34 pm

I think that you will not find any shortcuts fr this one- either you spend money in production or in post. In production on lights and a large studio with a infinity white cyc (cyclorama), or in post on many hours of roto work.

Tudor "Ted" Jelescu
Senior VFX Artist


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Paul CarpenterRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 7:48:37 pm

Thanks for the reply. Oh dear, that does not sound promising!

I was hoping to shoot against a long polar white paper roll, keyframe a feathered mask, apply a luma key and rely on any remaining thin white edges to blend with the white bg of my virtual set.


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Kevin CampRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 8:19:18 pm

[Paul Carpenter] "apply a luma key and rely on any remaining thin white edges to blend with the white bg of my virtual set"

the issue that you'd probably run into is if/when you need the subject over something else; you mentioned that there would be some other graphic elements... at that point, any holes in the matte or white edges on the subject may become very problematic, so that's the concern with straight luma keying.

if you can work around something like that, then you may be ok.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Paul CarpenterRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 8:59:15 pm

Thanks for your reply. I guess I could patch any internal white holes (caused by blown highlights) by placing a white solid behind the problem area of the footage? I see what you mean about the edging issue though. Do you think the luma key will leave thick edges or very thin ones? Could I sort the edge with a feather?


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Kevin CampRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 14, 2012 at 9:16:02 pm

personally, i'd plan on chroma keying. chroma keying give you more information to produce a key that luma keying.

the only real advantage that luma keying is giving you is not needing to despill.

if you think you can specifically (or strategically) place background the elements so to not show the fringing that could be very apparent, then you might be ok... otherwise you could be in for a fair amount of roto work.

i would try testing this before planning goes too far, so you can see they kinds of issues you may need to overcome.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Paul CarpenterRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 15, 2012 at 8:03:05 am

Thanks again.

I'm running tests to the best of my ability, although I do not have studio space here.

I see what you mean about the luma key leaving a fringe which is an issue when overlapping. However, I waved an aluminium ladder around to simulate a chromed BMX performing stunts and the CHROMA key test couldn't cope at all. I've since read that crews often use anti-shine sprays and the like on a 'proper' set. I guess this indicates the scale of this problem with metal surfaces and chroma screen?

I'm edging towards a luma shoot again now, with a view to compensating for the white fringe by avoiding high-contrast overlaps in the composite.

What I cannot test right now, is how well an AE luma key handles very dynamic motion such as a bike jumping off a half pipe ramp. Does anyone have experience applying a luma key to this type of footage?

I would also like to know if I can improve a luma key by shooting fast motion at our standard shutter speed of 1/50, thus introducting some edge blur, or if it would be better at a higher frame rate of, say 1/100 for sharper edges?

Many thanks again. This help is invaluable to me!


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Spey FisherRe: Luma Key or Chroma Key Shoot for White Background Compositing?
by on May 24, 2012 at 7:47:43 am

Testing the ladder is a great start-- I'd also test how the wheel spokes will key


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