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Lahiru Waidyasekera

White Chroma Key lighting for post compositing

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Hello,

 

I need to do a chroma key shoot which will be composited in post. The studio has a white background. Currently my plan is to use CTO gels to light the subject and set the white balance to the white background which will be lit with CTB gels. Then set the exposure of the white background to 100%.

 

Will this work in post?

 

Cheers.

 

 

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No.

 

First of all, if you white balance to a background lit with blue gels, your foreground lit white will go orange and if lit with orange gels will get super orange. Second, if you expose your blue background at 100% then it's not a blue background, it's a white background (with orange people in front of it) and you maybe could pull a luminance key but not a chroma key, which by definition means you need a highly saturated colored background, not a white background.

 

I think you meant to say you'd white balance to the orange lit foreground which would make the blue lit background go bluer.

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Even using color balance tricks and gels you will have a hard time getting it to key cleanly since it wont have enough saturation to really distinguish its self from skin tones. You might be able to try lighting with really saturated gels but it would probably be better to re-paint or purchase some light weight green screen fabric and rig it over the stage.

 

Keying, even in the best of situations is always more of an art than science and even the smallest issues with your background will make your life miserable.

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Spill control is a serious issue in this type of shoots. As suggested the lightweight greenscreen fabric could be much better option.

 

Another thing is that you will need tons of light to get highly saturated color to the background, the strongly coloured gels may eat up from 75 to 90% of the light and lowering the screen exposure leads to even more spill problems

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Meaning that every stray light will easily lower the background saturation or make spots there making keying very difficult. Much more demanding to light than a real green or blue background

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If you don't have any white wardrobe or props, you could consider staying with the white, ideally lighting it well above key and using a Luma key instead?

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If you don't have any white wardrobe or props, you could consider staying with the white, ideally lighting it well above key and using a Luma key instead?

 

or hard backlights, reflections or spill. totally doable but one needs to underexpose the foreground a bit and avoid any reflections on the edges of the subject like hairlights etc

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Lahiru,

 

I produced a tutorial showing the Cineo TruColor HS2 with green remote phosphor panels. This might work to create a saturated green backdrop on a white wall. You just need to keep the key lights off the back wall, which means you need lots of flags or distance. Typically I would make the green backdrop around 40-50 IRE, depending on how much spill you need to handle.

 

It's also possible to use just about any remote phosphor LED light if you take off the phosphor panel and just use the blue LEDs for blue screen chroma key. These type of LED fixtures will create a very saturated blue color.

 

Using tungsten lights for this may create a lot of difficulties.

 

https://vimeo.com/173182075

 

Stuart

---------------------------

illuma.blogspot.com

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Lahiru,

 

Also, Kino-Flo makes chroma green fluorescent bulbs. You can use those to light a white cyc and get a green backdrop. I'm not sure how saturated it will be since I've never tested these.

 

Stuart

---------------------------

illuma.blogspot.com

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You can get greenscreen fabric fairly cheaply which will make your life during production and especially during post-production easier. Light those so that they are around 50%, depending on if you shoot log etc. Also, (clean) rimlights will help the keying - if they work with the light in the background. If you see spill in the talent's hair eg., try using violet gel on those rimlights as it helps to get rid of it.

 

@Stuart Allman

 

I was looking into them a while ago and they seem to be ideal to light an already green / blue background, but won't probably be enough for a white one.

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Maximilian,

 

In the video you can see a bit of green spill from the lights onto a gray wall. It was green enough to pull a good key. Using a green fabric made it a bit more saturated/better.

 

The real dilemma is trying to get a white wall dark enough to make it appear chroma green instead of pale green. The wall will also easily pick up the talent lighting, which could prove problematic. It's not a slam dunk by far.

 

Stuart Allman

---------------------------

illuma.blogspot.com

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Since lights like Kino green screen tubes have such a narrow wavelength output, they can turn a white wall to saturated green if you expose it correctly -- the problem is that a white wall will pick up any stray lighting in the room and thus wash-out the color saturation, so it's not easy to separate the lights on the white walls from the lights on the talent.

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Hello,

 

I need to do a chroma key shoot which will be composited in post. The studio has a white background. Currently my plan is to use CTO gels to light the subject and set the white balance to the white background which will be lit with CTB gels. Then set the exposure of the white background to 100%.

 

Will this work in post?

 

Cheers.

 

 

I'm reading the responses hear and most of them i agree w/. Some i don't. One problem that i would like to see you avoid is the bounce that's going to come off that screen if it's light to hotly. Really, to ameliorate the situation to a great degree you need to not light any part of the screen that's beyond your frame line and to keep the screen away from the subject. Any color bounce that hits the subject at even an oblique angle will be problematic. If you can use a narrow band light source as someone suggested here, you will have advanced your cause a long way. That should put you close to perfection.

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negative fill or white bounce helps for the spill problems. you can just cover all the screen areas which are not used with black molton or use solids/floppy flags/hanging pieces of molton to flag the unused screen area to reduce spill.

If shooting fixed camera shots or otherwise pretty static action I tend flag very close to the subject edge, leaving just enough room for the action and then taking the flags away with bezier mask afterwards and keying the edges normally

6727806765_231db777eb_b.jpg

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for example, reserve couple of (at least two) T-bars on combos for some for example 3m x 4m pieces of black molton and then you can use those behind the subject to flag the unused screen areas on sides. very quick and easy to assemble and move and if having good enough distance you can still light the screen easily

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Great photo. Thanks. Look at the top surface of the desk. The green screen is reflecting very clearly from that surface. It appears to be coming from a point on the screen that is well within the frame. This i a potential problem w/ no clear solution except to kill that reflection. Somebody out there probably has a sneaky way of fixing this. I can't think of one.

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What do you mean by "no clear solution"? Here are some typical solutions:

https://www.videomaker.com/courses/729/fixing-unwanted-green-and-spill-to-green-screen

https://www.digitalanarchy.com/demos/chroma_color.html

https://helpx.adobe.com/after-effects/how-to/fix-green-screen-footage.html

http://help.thefoundry.co.uk/nuke/content/comp_environment/keying_with_primatte/spill_removal_method1.html

 

Green screen spill, while not desirable, is common enough that vfx compositors deal with it regularly, especially if shooting something like a car on a greenscreen stage.

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As I said, the problem isn't getting a white cyc wall to go green by using narrow wavelength sources, the problem is not polluting the green with ambient light, washing out the chroma, because a white wall picks up everything. You'd have to completely flag all lights off of the white wall except for the green lamps and then flag all green lighting off of the subject. This requires some distance between the wall and the subject, and of course you couldn't have the subject framed head to toe on a white cyc.

 

The issue of spill and reflections on semi-shiny surfaces would happen no matter which approach you took to create the green screen. You'd probably dull spray whatever you were allowed to but at some point, you need a compositor to deal with it in post as well, so hopefully you used a good codec to record the image on...

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we used polarizer with the table shots and just manually masked the most problematic parts afterwards.

 

It is also possible to reflect white or colored surface from the most problematic shiny spots to replace the green reflection with other type of reflection (like white kapaboard etc) but it may take too much time to do it for multiple shots like in this case so one just needs to decide the amount of work done on set VS the amount saved for post production.

 

Every surface is a "shiny" surface and with the reflection angles normally encountered in greenscreen work SOME spill is inevitable. one needs to try to replace it with something else like reflecting something white from the same angle (white reflection, backlight, etc)

The problem with for example backlights trying to kill spill is that they have to be quite wide so that they can entirely replace the green's reflection on surfaces like hair, shiny tables, etc.

 

The edges are entirely possible to correct afterwards manually, even by rotoscoping single hairs to separate them from the background if keying is not possible or practical. but that takes enormous amount of time and no one has that ;)

(it is possible to do these effects even entirely without greenscreen if you have good enough compositors but they will go absolutely crazy for the amount of work so not recommended :lol: it can be used however if you need lots of background for 3d matchmoving and your subjects are very smooth and simple edged so that rotoscoping is easy, like in this Ruairi Robinson short https://vimeo.com/4644064 . making of: https://www.youtube.com/watch?v=1ylHndX8nFw )

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What do you mean by "no clear solution"? Here are some typical solutions:

https://www.videomaker.com/courses/729/fixing-unwanted-green-and-spill-to-green-screen

https://www.digitalanarchy.com/demos/chroma_color.html

https://helpx.adobe.com/after-effects/how-to/fix-green-screen-footage.html

http://help.thefoundry.co.uk/nuke/content/comp_environment/keying_with_primatte/spill_removal_method1.html

 

Green screen spill, while not desirable, is common enough that vfx compositors deal with it regularly, especially if shooting something like a car on a greenscreen stage.

Oooh. Someones got his panties in a bunch. The category is "Lighting", not "Digital Gizmos". NO CLEAR LIGHTING SOLOUTION. GOT IT?

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absolutely clean greenscreen shooting is only necessary if you do high end live broadcast where you can't mask anything afterwards or fix with rotoscope and spill removal.

 

in any other case one just makes it as good as possible on set with available resources and everything else is fixed in post. cleaner original material is preferred if it is cheaper solution than doing more post work to fix the glitches.

 

as I mentioned earlier, every surface is somewhat reflective and if the light angle is very close to 90° from the surface normal (like the green background immediately behind the subject's edge) it is plain impossible to prevent reflection whether the material is hair, skin, concrete, wood, metal, glass... you can, however, try to add another more desirable reflection over the green edge reflection to mask it enough that you don't need that heavy post work and the edge is cleaner, especially with bad codecs and blurry edge.

....this is why everyone is recommending using heavy edge lights to reduce spill. masking the green reflection by adding more desired and brighter white reflection over it. that, however, easily leads to a boring result when every greenscreen shot in a movie is lit in the same way: simplistic lighting with all the edges heavily backlit whether it is motivated or not.

 

I personally prefer less clean greenscreen stuff with much more creative control over lighting and fixing the possible spill issues in post if they can't be resolved on set with flagging/negative fill/white bounce/reflecting something else from the surface. it just needs better compositor with more advanced tools and more time, a simple "Premiere Pro chroma key" won't work for that kind of material

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we used polarizer with the table shots and just manually masked the most problematic parts afterwards.

 

 

Glad to hear from a sane person. I like your version of reality. I did Chroma-Key when it was a pretty wild animal; ie. Magenta backlights, blue screens, green screens, directors putting the talent right up near the screens. You name it, we had no magic boxes to fix our problems. Yet we prevailed. The whole thing requires thinking on your feet. Thanks for your input.

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I'm sorry Sandy for not understanding your comment and then incorrectly correcting you.

No...I apologize for getting nuts. I respect your knowledge and will try to keep my ears open and my mouth shut from now on. Thank your for your response.

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Oooh. Someones got his panties in a bunch. The category is "Lighting", not "Digital Gizmos". NO CLEAR LIGHTING SOLOUTION. GOT IT?

Sandy, as you're only 17 posts into your membership of this forum, perhaps you could try a little harder to not insult long-standing and incredibly helpful members like David Mullen.

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