Home of Professional Cinematography since 1996

Alexa & Blue Screen

 

Does anyone have any sort of for guidelines on how to shoot the blue screen on Alexa?

Shooting on Alexa Using "chroma blue" blue screens

Any help would be greatly appreciated as our experience is more with film than with digital acquisition. For example one would have asked for green screen to be shot two stops under in the olden days.

Thanks

Abel Milanés | Compositing Supervisor METHOD STUDIOS Vancouver || 50 West 2nd Ave | Vancouver BC


 

Is there a reason for blue rather than green? Bayer sensors have twice as many green photo sites as blue, so theoretically green should give you a better, higher resolution edge to you key. I have spoken to one VFX supe who still prefers blue, but that conversation was specifically about EPIC. I could see the EPIC's increased resolution being an advantage in that case so, it's hard to say. As usual, test both if possible.

Jason Goodman CEO 21st Century 3D http://www.21c3d.com


 

Hi Abel,

I shot some blue screen elements with the Alexa without any issues-we had a girl painted green(I guess she was meant to be an alien, remember the old Star Trek episodes?) Since she was painted green we did everything blue screen, and I can't recall any noise issues but then again I wasn't involved in post-

Good luck with your shoot

Best,

John Babl DP Miami


 

I heard that wen shooting Greens Screens on digital is a good practice to over-expose a bit... at least with RED One. Don't know the science behind it. Anyone that can confirm if is the same for Blue Screens and Alexa?

Abel Milanés Compositing Supervisor Vancouver, BC, Canada


 

I DIT'd VFX unit on Alexa/Codex Studio feature 2 months ago which we shot on Blue Screen. DP & Gaffer lit the screen with HMI's to about 50IRE on the waveform than shot it. Most of our actors and elements were wearing green camo and shooting.

I would treat it like any other chroma key situation. I would recommend using HMI's over tungsten if you can for noise reasons to give the compositors alittle cleaner image to work with.

best,

- Dane Brehm ICGDIT Oakland, CA


 

> I heard that when shooting Greens Screens on digital is a good practice to over-expose a bit...

 

Err, that's the kind of thing I hate here, " I heard that..."

NO!

Personal experience, lets not let gossip be the defining information.

Ive shot a fair amount of green screen and ive NEVER found overexposure to help.

It will cause edges to bleed and the key colour to spill and bleed.

Colour difference is all that matters, mid grey in the green channel, nothing in the red and blue.

Noise can be a problem but then you test and establish what the REAL speed of the camera is, whatever camera you use.

The fair amount was British understatement, ive shot shit loads of GS, entire movies and commercials with the budgets of movies.

Dont overexpose your screen!

-- Cheers

Geoff Boyle FBKS Cinematographer EU Based


 

> Dont overexpose your screen!

What Geoff said.

In **general**, using the same temperature lights as the key source:

Blue lit to key.

Quality green (like from Composite Components) lit 1/2 to 3/4 stop under key.

The foamy greenish-yellow crap that rolls off most grip trucks, lit to key.

Given a choice, I prefer blue. Nicer flesh tones, and easier to reproduce blond hair properly, especially if it comes from a bottle.

--Bob

Bob Kertesz BlueScreen LLC Hollywood, California


 

Light the subject to however it matches your plates and light the bs to the same stop. Separation is the best, get some distance btwn subject and screen to minimize contamination. As long as the screen is pretty even you shouldn't have any trouble.

Nick Hoffman dpnyc


 

Geoff screamed:

Dont overexpose your screen!

Absolutely agree, especially on Alexa. If anything you should underexpose it, and I would suggest by 2/3-to-1 stop. Not only will this help protect from bleed or spill, but the way that ARRI maps and matrices the sensor is different from other manufacturers. As you get higher on exposure on most video cameras (particularly Sony), the color saturation increases. But on Alexa at a certain high exposure the colors desaturate toward white. This is to more closely emulate a film look. So too much exposure could actually lead to poorer color fidelity on an Alexa.

BTW, this is from firsthand experiential knowledge, not just theory or conversation.

Mitch Gross Applications Specialist AbelCine NY


 

I've found it easier to deal with blonde hair and fair skin with blue rather than green, but the main thing, use a color not being used in the foreground. Green is far more light-efficient than blue, and as you note is more simpatico with Bayer CFA chips and component video. Back in 601 standard-def days uncompressed was a big deal because we had so few pixels, but no-one cares about that anymore.

Tim Sassoon SFD Venice, CA


 

> But on Alexa at a certain high exposure the colors desaturate toward white. This is to more closely emulate a film look. So too much exposure could actually lead to poorer color fidelity on an Alexa.

I've done the testing and this is true: beyond about 35-40% Alexa colors do not saturate, they just get brighter. As 40% is middle gray in Rec 709 it makes sense to expose it there or slightly below for Alexa.

It was the same on film: you could get a bright blue or a saturated blue, but not both at the same time. Exposing a blue screen at middle gray was the sweet spot for both exposure and saturation.

With other cameras I tend to light the screens brighter as long as there's some distance between the subject and background to minimize spill. I also supersaturate the green or blue. Depending on what I'm shooting, and the camera I'm using, I've lit green screen as bright as 70% and gotten perfect keys. (I haven't shot blue screen for quite some time.) The more color difference between the screen and the foreground the happier everyone is, and I adjust that depending on what I can get away with. This works well with Sony, Canon and Panasonic cameras that saturate colors right up to 65-70%. (Before someone snipes at me, I'll clarify that you clearly can't do this if the person is standing on the screen.)

The one thing that's not negotiable is you should rate Alexa at ISO 400 for this. At 800 the camera is noisy enough to result in complaints from post, but at 400 it's very clean and post houses love it.

The first VFX spot I shot on Alexa was rated at 400, and another local DP rated a project at 800 that went to the same post house. I got the call asking what I was doing that made their keys so much easier to pull than his.

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

Art Adams | DP San Francisco Bay Area


 

> With other cameras I tend to light the screens brighter as long as there's > some distance between the subject and background to minimize spill. I also > supersaturate the green or blue. Depending on what I'm shooting, and the > camera I'm using,

Another way to achieve this saturation is with kino greenscreen/bluescreen bulbs or with color controllable LED lights like the Arri LC-7. They will help saturate the screen without adding too much spill.

I recently did a last minute greenscreen interview shoot in an office and managed to light a cloth greenscreen with a single LC-7 LED fixture (there was some falloff in the edges but this was a head and shoulders shot so it was off frame). Keying the subject with a couple of kinos 4x4s and some bounce. The LED light saturated the greenscreen cloth very nicely while keeping spill to a minimum. Ideally I would have liked to have 2 LC-7s but it wasn't in the cards for this particular shoot.

My biggest issue was flagging the kinos off the greenscreen ,as they could easily overpower the LED light, but for working in a cramped office without AC being able avoid any tungsten lights was a godsend.

Of course it all depends on the scope of the items you are lighting, your keylight stop, ect, but if its something small I would consider the LED route.

Daniel Colmenares Director of Photography Los Angeles, CA


 

I use to saturate colors, lighting the screen with the same color, whenever is possible, if its just a background with no floor.And at IRE 30, that will keep the spill out.If I have a vectorscope , I try to put the color on the exact spot of its vector.

All the best !

2013/4/18 milanesa <abelmilanes@yahoo.com>

> Thank you for the quick response guys. We are going to do some tests in a > few weeks. Will let you know how it goes.

-- Henrique Leiner Diretor de Fotografia Rua de Santanna 555 Casa Forte Recife 52060 460 Pernambuco Brasil


 

Separation is key, but saturation with coloured tubes allows you to shoot a very cool, power efficient studio, personally I like to underexpose slightly with colour tubes.....but I keep plenty of negative black cutters behind the subject to keep down side reflection.

Ed Mash Currently shooting green screen!!!!! UK DoP


 

Lately I've been encountering the same problem from various VFX Supervisors from 2 different reputable VFX vendors here in LA. There must be something to this, although my experience is it has to do more with screen dyes and lighting than the camera itself.

One VFX Super knew to underexpose the bluescreen, lit with saturated 420nm kinos. So much under that even for me it seemed too low and "in the mud". It was in the 25-30 IRE area, but it was pure. But that's what he said he wanted to reduce FG spill - he also didn't want me to use the superblue kinos, but I insisted, knowing I'd get a better screen with them. And his Compositors had no issues with top notch spill reduction, as I'd seen them remove it from glossy leather in the foreground once, and not have it look "gray" and fake. Still he wanted to go really low.

And I do have to admit, with the blue kinos there is some "UV" to the eye that looks magenta (doesn't show on camera), and maybe some mixing of colors if you backlight soft & warm in that setups, that it can get weird on the edges, but that's why you need to keep the blue screen far away and down in exposure.

They see us start lighting and see blue kinos and freak out, before we've even set any levels, like all they've been on lately is a student film.

On 2 other gigs now I've run into EXPERIENCED vfx supers wanting bright screens, even on the blue ones. Obviously a GS is more luminance on its own, so for me red flags go up when I hear that anyone wants it bright blue. Just last week I asked "its saturation and unique color, not so much luminance, right?" He said its also luminance. He also didn't want me to use 420nm blue tubes but use tungsten. In this case, I did as he asked and got a crappy "blue-gray screen". Not the one that we did last year for him which he referenced, and I had to remind him it was a Composite Component screen I'd insisted upon.

Few of them ever give proactive input about the screens we use. So first VFX shot we rent from the studio and get an old, tired, "blue-gray-screen".

2nd time I bring in my swatch from the Erlands and we order the right spandex, and even without changing the lighting (or doing a day/ext screen in natural light) and you get instant improvement in the saturation of the screens. Real "color snap" and difference in a unique keying chroma. Yes the screen costs more, but its worth it if the shot doesn't have to be piped to China to have 10 Compositors roto it there even at $ 3/hr.

I think many VFX Supers are reacting to badly shot bluescreens (and in some cases by their own recommendation) then sitting at the workstation wondering why the edges don't snap in to a good matte. They may come to all sorts of conclusions as to how to avoid whatever edge problem they see, some of them smart conclusions, but I tend to agree that when you put the sampling cursor onto the screen, that 1023 blue, and in the teens or single digits with R & G is the way to go.

Its interesting to hear that GS is better due to more green photo-sites... but unless you record ArriRaw and do your own processing specific to get a good matte, I'm not sure you're gaining quite as much since all those extra green photosites are processing into a 1920x1080 pixel array for ProRes 4444. In other words, the math in the camera would already "decide" how green that pixel in the ProRes will be. If that logic held I'd imagine there's be many other problems with the final image that how well the sensor showed a green-screen vs blue. Still I admit its an interesting factor to consider if green spill isn't a bigger issue.

mdp

Mark Doering-Powell LA based DP


 

One question... and please understand I am the VFX guy... not the On-Set one.

When you messure the camera signal, in what colorspace is that signal coming? Will the Waveform be showing the blue signal in Scene Linear? Rec-709?

We will be getting AlexaLogC DPX footage. VFX work (keying) will be done within a Linear Light workflow.

Abel Milanés Compositing Supervisor Vancouver, BC, Canada


 

In digital terms, the software really doesn't care what the value is, so there's a lot of value in keeping it down to reduce spill. But for the screen, more important that it be pure and even. The narrower the color/value notch, the less other stuff gets pulled in, especially with translucencies like smoke. However, with log encoded images, I'd want to plop it somewhere in the middle to get the most code value separation. Being either at the top or bottom of the scale could bit-starve the screen image.

Tim Sassoon SFD Venice, CA


 

> I think many VFX Supers are reacting to badly shot bluescreens (and in some cases by their own recommendation) then sitting at the workstation wondering why the edges don't snap in to a good matte.

I see this a lot in people from LA, especially producers. They find one thing that worked once on a shoot and they repeat it ad nauseum on every other shoot because they know it worked once. Or, if something failed on a shoot, they never ever ever allow anyone to repeat this again. They don't really care about the particular circumstances, they just know that doing that one thing once backfired so no one working for them can ever be allowed to do it.

Rather than go out and learn how a blue screen is properly shot and make sure the DPs they are working with understand their requirements, they instead make blanket statements about super green tubes without realizing that if they actually go to the set and understand what's going on they can instead say, "Sure, use super green tubes if you can but watch very carefully for spill. When in doubt, give me even and a little dark vs. bright with lots of spill."

What they're really trying to do is take decisions out of your hands, because the more decisions they make the fewer they have to trust you with.

> Its interesting to hear that GS is better due to more green photo-sites... but unless you record ArriRaw and do your own processing specific to get a good matte, I'm not sure you're gaining quite as much since all those extra green photosites are processing into a 1920x1080 pixel array for ProRes 4444.

Right, that makes sense to me. With Alexa you're downsampling 3.5K to HD, so I'd think the reduced number of blue photo sites compared to green wouldn't be much of an issue.

I do always shoot blue screens in ProRes4444. Why? Because. So far no one has complained.

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

Art Adams | DP San Francisco Bay Area


 

Go to Arri's website and look for:

Alexa Visual Effects FAQ Workflow Guideline

You'll get a lot of questions answered there. Heck, I got some questions answered there that I hadn't asked yet.

There's much in there that confirmed what I had intuited, such as how green screen works well under both daylight and tungsten, whereas blue screen works better in daylight but will be very noisy in tungsten. (That's because manufacturers typically have to boost the blue gain by +6db before the camera will even white balance under tungsten light, because there's very little blue there. Green works fine in either daylight or tungsten: if you think of white balance as a seesaw, with green being the pivot point and blue and red falling on either side, then regardless of whether blue is deficient in tungsten light or plentiful in daylight, green is always present in significant amounts.)

They also discuss using lower ISOs like 400 or 200, which is no surprise as we used to switch to fine-grain stocks in film for VFX work for the same reason. Noise and grain amounts in normal shooting can be much higher than they can when you're trying to detect edges in VFX work, when you want a very clean signal for very fine edges.

They even show waveform and vectorscope images of what to look for in the ideal green screen. (You'll never see images this perfect again, so enjoy them while possible.)

Lots of good stuff in there.

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

Art Adams | DP San Francisco Bay Area