Green screen without lighting?

[Matthew Raymond]

Not sure how practical this is, but I had an idea on the way home from work. It goeth thusly:

Step 1: Spray paint a wall with a retro-reflective paint.
Step 2: Light your subject normally a reasonable distance from the unlit, painted wall.
Step 3: Put a green piece of plastic over the camera flash LED on your phone.
Step 4: Turn the LED on, as if you were using it for a flash light.Step 5: Film the subject with the phone camera.

If you've sufficiently lit the subject, the green light from the LED should be too dim to be visible on the subject, but should reflect easily off the retro-reflective paint. Since there's only a tiny angle difference between the LED and the camera, you should get a strong green light from the background into the camera.

Here's a video about one brand of retro-reflective paint on YouTube that actually shows what it looks like on a phone camera with the flash turned on:

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

Accept third party cookies

 

[jespah]

I wonder (assuming this works) if it could be used to create a funky uniquely-shaped "green screen", where maybe you just paint something like, oh, I dunno, the silhouette of a horse or something.

 

[Matthew Raymond]

Checked the Internet for "retroreflective chromakey", and apparently there are professional kits to do this, but they're thousands of dollars. I'm going to see if I can find the retroreflective spray paint so I can try spray painting some cardboard and seeing if I can do that as a backdrop. Otherwise, I'll have to try and order the fabric online, but I'd prefer it if the paint works because you can use stencils as opposed to gluing down bits of cloth when you have to do complicated mattes.

 

[Professor Zoom]

Why don't you just buy the right kind of green paint? Paint the same wall you were going to paint with reflective spray.

 

[Maurice]

I think the real issues are

1. does it take less gear to do this Krypton-esque thing (meaning bouncing light off some Scotchlite/front projection/reflective material to make something glow) than a few cheap lights and a cloth or painted greenscreen, and...
2. are the results => than those?

The one advantage I could see is that you could change the key color from green to blue or whatever, in the event you needed to shoot different colors.

I'd also add that the video isn't super convincing because the guy never does the simple thing of spraying a smooth coat on a white card and showing us the difference tween the unpainted white and the reflective white.

 

[Matthew Raymond]

Why don't you just buy the right kind of green paint? Paint the same wall you were going to paint with reflective spray.

There are a number of reasons to use retroreflective paint and a light near the camera instead of a green surface:

1. You don't need lights for the keying screen (save the one the camera, which in this DIY case would just be the flash on your phone).
2. There's no spill over of green light, because the light is all coming from the direction of the camera, and its extremely faint compared to the lighting you're using for your subject. The light only appears bright where it hits the retroreflective surface, and only because it's being bounced back directly towards the source. (Now you do have to make sure it's just bright enough to show on the background while not so bright that you can see a green tint on the subject. That said, the tint would be even over the entire subject because it's coming from nearly the same location as the camera, so you can probably just adjust the color slightly after you've done the keying.)
3. Because there's no spill, you can put the subject much closer the the background.
4. And finally...

The one advantage I could see is that you could change the key color from green to blue or whatever, in the event you needed to shoot different colors.

Right. You're keying color is what every color of light you want to use. Heck, if you can get a mini projector close enough to the camera, you could do front projection.

I'd also add that the video isn't super convincing because the guy never does the simple thing of spraying a smooth coat on a white card and showing us the difference tween the unpainted white and the reflective white.

Well, the cloth would probably be better, since that's basically what professional outfits use, but it was easier for me to find the paint. (I found it on clearance for about $3 dollars a can, so I got three of them. Hopefully that doesn't mean they're going to stop making it.) I'm thinking I could create a cardboard set with simple geometry, paint it with grey primer, then the retroreflective paint, and see if I can basically fake a nice set using the simple one and green screen.

 

[Maurice]

I think you're perhaps misunderstanding. I'm not talking cloth v. card, I'm talking about testing how truly reflective the stuff is by testing it on a white card with one half painted and the other not, because if it's really truly retroreflective the paint side should look a lot more dazzling than the untreated white.

 

[Matthew Raymond]

*** UPDATE ***

I painted a large piece of cardboard with grey primer, then coated it with a couple layers of retroreflective spray paint. I didn't have a good place to store it for drying, so I ended up drying it mostly on my patio. Thus, it was exposed to the elements a bit, and doesn't really reflect what someone can do in a proper work area. Here's the result:

Spoiler: Images

Well, it sort of works, but it's rather uneven. Honestly, I think it's easier to simply use retroreflective cloth, and I doubt it would be that much more expensive once you factor in all materials and labor. I was hoping paint would work, because then you could easily create environments using cardboard that your actors could move around in and interact with without being bathed in green, but I now see that it would be extremely difficult to get an even coat of retroreflective paint, especially if you're an amateur like myself. Perhaps fabric would be cut and applied to cardboard using spray adhesive?

I also tried putting green plastic over my phone's flash LED to use as a light source to reflect off a retroreflective surface, but I found two problems. First, my camera app would shut off the flash LED, even if I turned it on with a flashlight app. Second, the light was too dim and wasn't sufficiently green enough. It really makes more sense to create a cheap DIY light ring.

So I still think people can potentially do retroreflective chroma key for relatively little money, but not quite as cheap and easy as I'd hoped.

But at least I have a way to keep myself safe while I'm jogging at night.

 

[Sgt_G]

you could change the key color from green to blue or whatever, in the event you needed to shoot different colors

I went on a tour of a TV studio many years ago, invited by the weather guy. He showed us how the green-screen worked. They also had a blue panel to use in case he wore a green shirt/coat. They let us step in front of the camera / green-screen (not live, of course). We had green/brown/black BDU uniforms on, so the effect was ... interesting.

 

[Matthew Raymond]

It would be nice to have some kind of tool that can analyze video without a particular chroma key background and calculate the keying color for you. Then you could use something like a Phillips Hue light strip as a light ring and change the color to give you the best calculated key.

 

[Matthew Raymond]

UPDATE: Sorry to resurrect this old thread, but I wanted people to know that I've learned that one of my problems with the paint is that the cardboard too readily absorbs paint in general and that I should have applies a lacquer or gesso to the cardboard before painting. I'm going to try this again with a smaller piece of cardboard after applying gesso and see if the paint ends up more even.

 

[Maurice]

You should primer almost anything before painting it.

 

[Matthew Raymond]

I did two coats of primer. The problem was that the cardboard wasn't properly sealed, so it just absorbed the paint, causing surface deformation. Gesso will both seal the surface and allow a sand-able surface to make it perfectly smooth. In theory I could use a lacquer of some sort, but I like the option of sanding it smooth so that the retroreflective coat of paint is more even.

Thought: Black primer? Might work better for green screen windows into the blackness of space.

 

[Maurice]

Black primer under the reflective paint?

 

[Matthew Raymond]

Well, it's just a thought, but the idea is to use it for starship windows. That way, even if your chroma key is kinda spotty, the background that shows through is black, so it'd be like showing a star field on a monitor with a few pixels burnt out. Nobody would notice unless you insisted on putting a nebula in your composited background.

 

[Maurice]

You're needlessly complicating something that's fairly straightforward. Use a surface that takes paint well. Use WHITE primer to make sure your reflective paint has as much reflectivity as possible, not black which will be the inverse.

 

[Matthew Raymond]

You're needlessly complicating something that's fairly straightforward. Use a surface that takes paint well. Use WHITE primer to make sure your reflective paint has as much reflectivity as possible, not black which will be the inverse.

Then you have to worry about ambient light levels, which would be especially bad for windows into space. Besides, it's the tiny beads that give the retroreflective properties. The green actually SHOULDN'T show up on the primer, because it should be too faint to show on your talent. If you could see the green on the primer, the white paint would put a green wash on the back of your talent. FYI, the retroreflective backdrops that are actually used for green screen are a medium grey, which is the color I used as a primer in the first test.

 

[Maurice]

That's because Scotchlite material looks gray. Look at the Superman (1978) Krypton costumes without a flash hitting them.

 

[Matthew Raymond]

That's not really the same use case, and spill wasn't a issue because they wanted it to look like it was glowing. However, if you like, I can get some black and white paint and make three separate samples for my next test instead of one.

 

[spacecrafter3d]

Did you have any luck with this? I would like to try it myself.