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Green Screen Test Wedges

Mark Weingartner wrote :

class="Body" >Every single post house on every single job picks different densities from the one's >they or someone else picked on the previous job...After scanning test wedges that >are only labelled with slate letters and not with what their nornal exposure is...

Hi Mark: I'm curious about the test wedges. Do you shoot them of only the Green screen material? Or do you have som sort of standard object you put infront of them, for a test composite? Also in your experience, or anyone who has a bunch of greenscreen experience, Are there compensations to be made to the greenscreen exposure based on the foreground object? I.E. Someone wearing a light colored dress, as opposed to a dark (red) dress, or Mid town jsacket opposed to a Black leather Jacket? Also What about skin tone? I'm just curious if you have to make adjustments, or is it the case that once you and the post house pick the Greenscreen exposure, then how the foregropund element is exposed (or what it consists of) is not an issue.

Thanks

class="Body" Steven Gladstone Cinematographer - Gladstone Films

class="Body" Cinematography Mailing List - East Coast List Administrator

class="Body" Better off Broadcast (B.O.B.) New York, U.S.A.


class="Body" >Also in your experience, or anyone who has a bunch of greenscreen experience, Are >there compensations to be made to the greenscreen exposure based on the >foreground object?

I treat the greenscreen as a separate element yet one related to the foreground. First off, I light a greenscreen for what it is and what it's saturation is supposed to be regardless of the talent. It is the greenscreen that must be cut and I want to give the people who do that a greenscreen that is true to the color saturation it is supposed to be. I remember a few years back doing a full hardwall cyc screen for a dream sequence. Someone suggested I use regular green paint that I have matched to the greenscreen color because it was cheaper. I told them there is a reason why that paint is a certain color and that color is critical to proper cuts. Sure I could find a range of colors that could work, but I'm trying to make everyone's life easy and a screen to me is it's own element that should be treated with as much respect as your talents exposure.

As for talent, I light them not necessarily related to the green screen but related to the setting they are going to be in when that element is added in the background. So in my example, it was a night exterior, the moon in the sky, a sense of a foggy night on the screen. I turned off the screen in my head once I had it lit properly and lit the talent for the scene she was going to be placed in. I gave her the blue backlight I wanted to represent the moon, softly lit her from one side depicting a soft night source, etc.

Actually many times I then turn the screen off and light my talent in limbo. I think in many ways the screen can be a detriment to lighting your talent properly and shutting it off after getting it right for me is the difference between lighting my talent properly for the scene they are going to be in in the composite and making it look like a badly lit element within the composite.

What she had on, whether it was light or dark had little to do with the screen for me in this case other than her exposure range was determined once I found the optimum range for the screen, then everything was bracketed within that exposure range. Of course, I've watch a hundred guys treat screens a hundred different ways so do what works best for you. This method has been foolproof for me.

Walter Graff


class="Body" >Actually many times I then turn the screen off and light my talent in limbo.

Excellent advice.

The eye gets easily fooled by screen bounce, and it often appears as if there is enough back/fill/top/side light, when in fact what you are seeing is screen bounce, almost all of which will be removed when the composite is made. You are then left with a much higher key to fill than you intended, and dark edges to boot. Turning off the screen and laying black on the floor (lighting your talent in limbo, as Walter said) will help prevent that sort of thing.

class="Body" Bob Kertesz BlueScreen LLC

http://www.bluescreen.com


class="Body" >Do you shoot them of only the Green screen material? Or do you have some sort of >standard object you put  infront of them, for a test composite?

This depends on what we are testing. My default test includes a foreground humanoid with hair, often teased out., sometimes pieces of set dressing that are particularly spill-susceptible (brushed aluminum or stainless if it is part of the set, for instance) and tracking marker samples, if applicable.

I generally light the foreground "normally" and wedge different blue/green screen materials and exposures.

As Walter and Bob have mentioned, the screen is a separate element and what is important is the neg density of the screen, not its ratio to the foreground...though a good test should also have some underlit foreground if you are likely to have that on the set so that the post house can be reminded of the issue of spill suppresion.

I do not endorse the practice of back or edge-lighting with color complementaries - minimize your spill with blacks teased in to the edge of your matte line on the screen and let the spill suppression software do its work..

I heartily endorse Walter's suggestion of lighting with the screen off...after all, once the matte is pulled, that is supposed to be what the foreground element is going to look like, so that is what you should be lighting for.

It doesn't hurt to roll a few reference frames of your foreground with the screen off so the postfolk can see what you were aiming for.

class="Body" >Are there compensations to be made to the greenscreen exposure based on the >foreground object?

Generally not.

There are other factors , including, but not limited to the prevailing luminance levels of the background plate with respect to the foreground, which may affect the post house's choice of screen densities. Sometimes they will want to skew the screen exposure one way or the other.

Mark "available as a process supervisor for shoots, weddings, and bar mitzvahs" Weingartner

class="Body" VFX, Photography & Lighting for Motion Pictures & Television

class="Body" LA based


Mark Weingartner said :

class="Body" >I do not endorse the practice of back or edge-lighting with color complementaries - >minimize your spill with blacks teased in to the edge of your matte line on the screen >and let the spill suppression software do its work..

Is this because the back/edge lighting doesn't match the lighting scheme desired for the element, or for other reasons? Thanks.

James Wallace DP IA669 Vancouver


class="Body" >Is this because the back/edge lighting doesn't match the lighting scheme desired >for the element, or for other reasons? Thanks.

Primarily yes, you should light the foreground as appropriate for the plate it will be comped to. Additionally, Ultimatte and other systems for extracting chrominance mattes have come a long way since the late 70's, and do not need the "help"...especially when the "help" may create odd colored edges to foreground elements that then have to be treated separately.

Likewise, spill suppression has come a long way with regard to "dialing out the green/blue" from the foreground element.

Above all, talk with the post house. This is as important politically as it is technically. If they have been consulted, they can't point the finger at you without its pointing at them a bit too:-) If they recommend a complementary edge light, you might want to find out a bit more about their pedigree and experience.

Mark Weingartner


Mark Weingartner wrote :

class="Body" >Primarily yes, you should light the foreground as appropriate for the plate it will be >comped to.

Absolutely!!! If you light the edges in a complimentary color the compositor will not only have to dial out the green spill, but they will have to dial out the odd color as well.

The method I prefer of dialing out the green spill goes like this:

There is a great tool which can selectively color correct based on hue.

Using this tool, I'll suck the saturation out of the green in the image, preserving the luminance, then I'll tint this same area to a color matching the skin, or other edge color of the foreground object.

If it's a particularly nasty composite where someone might have long blond hair in a red jacket with alot of motion blur, I'll use rough soft edged garbage mattes to selectively tint areas of what was the greenscreen to match the various colors in the foreground plate (hair, skin, jacket) etc... This can be done fairly quickly.

This lets you pull a very nice soft matte with alot of detail, and not really worry about if it is perfectly laying in with the edge because if the matte is a little loose, any background showing through is the same color as the foreground object you are laying in. This is a particularly useful method for preserving motion blur and retaining every single hair which might otherwise be clipped off by other methods of suppression.

So specifically lighting edges is not necessary, but controlling spill will save you a fair bit of time in post. It can be fixed, but you would rather spend that time (and money) on making that shot better, rather than just trying to make it work.

The only time lighting the foreground object with a complimentary color will work is in a motion control model shoot where the matte pass can be shot separately from the beauty pass. In this case you are aiming for a silhouette of the model against the greenscreen, and the beauty is shot against black.

One of the things which have caused me the biggest headache is seams in the greenscreen behind hair. Please, please please, be nice to your compositor, and if you have hair blowing in the wind, make sure that it doesn't cross any seams in the green screen - even if you have to fly in a separate screen behind their head.

I know it sounds logical, but you would be surprised at the number of shots I've run across that have this problem. Removing the line through the loose hair is extremely difficult and will eat up alot of time (and your budget). (Of course I know that no one on this list would ever do something like that!) :)

If you have to have a seam, try to keep it below the neck. It's far easier to paint-fix clothing than hair. The same applies for the face.

Thanks,

Rachel Dunn DoP/VFX Los Angeles


I don't believe it's an all out 'don't use it' situation. If your trying to create a back light just to rim someone's body, I say don't do it, but if you are using it for an effect that works within the context of the green screen, then no one should have to be dialing anything. Take the moon example I mentioned. I used a low intensity blue light that spilled across the top of the persons hair without rimming their figure. IT worked just fine. I think as I said it's about two different things:

1. The green screen is it's own element. It has little to do with the talent

2. The talents lights should not affect the talent in a fashion that is going to cause problems in cutting the key. That doesn't mean you can't light people in the scene they are going to be inserted in. It just means you have to remember the edge of the person is the area that is where the cut needs to be clean so adding some kind of light that is going to affect that line might be detrimental.

Walter Graff NYC


class="Body" Quoth Walter :

class="Body" >...If your trying to create a back light just to rim someone's body, I say don't do it, but >if you are using it for an effect that works within the context of the green screen, then >no one should have to be dialing anything.

Thanks for the clarification...I was not trying to suggest that you avoid using color complementaries in blue or greenscreen work, only that you not use them with the mistaken impression that you are somehow "helping" the key. If the scene calls for those colors, by all means use them... just don't use 'em if you don't want to see them in the composite...then you are just adding work for the compositor.

Again, if you are trying to pull difficult live keys with a late 70's or early 80's chroma key switcher, you may want to use a hint of complementary hue... ...my advice is predicated on "modern" equipment and techniques.

Mark Weingartner

LA


In the Example you site, you lit the foreground so that it would be properly integrated with the inserted background. So putting a rim on the person is probably exactly what you would do if you were shooting a night exterior. That's definitely the way to go.

I thought that the other comment was talking about was using a complimentary color rim light just to get a better edge, regardless of the integration problems it might cause. Not a good thing.

-Rachel DoP/VfX Los Angeles


 

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