7.18.2017

A Few Thoughts About Shooting Green Screen.



West Texas Rest Stop. 

I'm resistant to shooting "off the cuff" green screen for clients. Like anyone not directly in our business clients tend to have a simplified view of the technique required to do it well. We use green screens in order to easily drop out the green background behind a subject and replace the background with a different image. Compositing is so easy in still photography now that one rarely even needs to bother with a green (or blue) screen but video comes at you at 30 frames a second and it would be more than a little time consuming to go into each frame and do selections, etc. so green screen is still standard if you want to layer in a different background behind a person or object in video. 

Like everything there is a right way to shoot green screen and a wrong way. The wrong way is to set up a green background without lighting and hope that available light and luck will get you a clean enough background to composite. It kinda works but requires a lot of post production masking to deal with variations in tone and color that make automated background drop outs tough.

I've done half-assed green screen in the past with reasonable results but I'm shooting a big video project tomorrow for a larger ad agency. It's mostly green screen and I wanted to understand the best way to do the work and what kinds of things to watch out for. Nobody likes having to make excuses
after the fact... The limitations for tomorrow's assignment are: We are working in their offices so we get the space they give us. We have one hour to set up before talent walks onto the set and we start. I'm doing the project with one assistant so there's a limit to just how much gear we can get from the parking garage to the freight elevator and then  down the hallways.

In my favor is that they don't need full length shots of the talent, including feet. The agency also understands that we can only reasonably set up a green screen that's 9 feet wide without having to bring in cinema rails and tons more light. They work with green screen video a lot and know how to mask the edges of the frames...

Here's the stuff I learned from the American Society of Cinematographers website and other sources:

1. Make sure your green background is stretched tight and is wrinkle free. Variations and creases can cause shadowing and enough tonal variation to push a creased area out of boundaries for quick compositing. 

2. Put your background far enough behind the subject that you will be able to light the background and the subject separately. The further apart the subject and background are the less light from the green background will kick forward and muddle the edges of your subject. Especially applicable if your subject is a physician in a white lab coat. That's an easy target for "edge color contamination." 

3. The lighting, from edge-to-edge and from corner-to-corner, has to be as even as you can make it. A good target is to keep the lighting consistent to within a quarter to a third of a stop everywhere. This will make dropping out that background so, so, so much easier. 

4. When lighting the foreground subject make sure you don't create shadows that fall on the background (this should not be possible if your background is lit at a high enough level to match the foreground.

5. Don't overexpose the background. In video it is suggested that when reading the background with a waveform monitor that the background level sit at 50%. 60% at the outside. But don't under expose either. Using an incident light meter both the subject and the background should be in the same ballpark. If using a waveform monitor to set overall exposure you set caucasian skin tone to about 70% and read the background at 50%. I prefer to meter and then double check on the monitor. 

6. Make sure your talent isn't wearing anything in the same color family as the background. No green if the background is green. No blue if the background is blue. The simple reason is that whatever they are wearing in the same basic color zone will disappear in the dropout because the application is looking for a color range.

Here are a few tips I've learned that also help get good, quick results:

1. Use a subtle backlight with a filter in an opposite color to kill contamination on the light edges of your subject. Our subject tomorrow will be in a white lab coat. I'll backlight him with a weak magenta backlight. The magenta will help cancel out any green reflected light that contaminates the edges of the lab coat. 

2.  Keep the subject in sharp focus. Soft edges and out of focus edges don't select well. Figure out how much depth of field you need to keep the main object sharp and you'll be happier in the edge. Too soft = a confusing and unconvincing dropout. 

3. Use a camera that gives you a true 10 bit file as your output. The more precise the color gradations the better the application can be at discriminating between your subject and the background. A good reason to use something like an Atomos Flame with a Panasonic fz2500 instead of depending on the internal 8 bit file. 

4. Use a camera with 4:2:2 color instead of 4:2:0 color for the same reason as point #3. More precision in color is always better. 

5. Charge your client more because you'll need to haul in more stuff than you would in a typical shoot. The background will require at least 4 lights, with stands, you'll need the gear with which to hang your background and you'll need a set of lights on the subject as well. 

Since this project will be heavily composited and post processed, with lots of stuff going on in Adobe After Effects, we made sure that the client would take the files from us and do the post production. We'll take along the SSD card reader so we can shoot and then deliver ProRes 422 files in 10 bit, 4:2:2 directly to their production department before Ben and I leave the building. We'll all be reviewing as we go just to be sure. 

And that's what I know now about green screen and the way to make it work without too much sweat. 








4 comments:

Fred said...

Kirk,
It sounds interesting and a fair amount of work to get it right. Here's hoping you don't learn any expensive lessons in the middle of this shoot.
I hope it is fun. I anticipate that you and Ben will do your usual very competent job.
Fred

Mohammad Shafik said...

Thanks for your valuable advice. I'm using green screen for video with good success except for one scenario, where I hope you have enough knowledge to help me.

One of my talents moves a lot with his hands and moves quite quickly too. This is a nightmare when using green screen. The edges of his hands and fingers always turn out green during their motion. It's as if he has green webbed fingers. I checked the single frames frame by frame, and the hand when moving fast is blurred enough at 25 fps that some of it is normal skin color, and the remaining part is green.

I searched a lot on Google but never found a solution to this issue. Good lighting and good back light solves part of the issue. But once he starts waving it around, it's no use. Aggressive use of keying can get rid of that color, but it chops half of his hand.

I hope you can enlighten me. :-(

Paul said...

Great tips, lighting of background and subject is critical to avoid colour contamination. From experience removing green colour cast from blond hair is no fun - I freely admit I hate green screen photography 😊

Greg Heins said...

First off, we don't do a lot of green screen here (no regrets about that!) but with that caveat, we've gone to seamless paper for the green. Maybe it's because we all cut our teeth on carting around and setting up seamless for stills, but we find it faster to set up paper and there are no worries about wrinkles. We use 9-ft. rolls if it's just a head-and-shoulders, 12-ft. if it's more waist up. If we're on location, we have the paper shipped right to the client so it's waiting for us, and we leave it there when we're done. (We recently had B&H ship a roll to Japan for us; no problem.)
There is indeed no such thing as too even lighting for the background. I love the idea of a weak magenta edge light for the lab coat attire; that's brilliant.
As for the gesture problem, the only thing I can suggest if to give the talent a heavy book to hold in each hand; that should keep their hands down. (You can tell them not to use their hands and they solemnly agree but two minutes in and the hands are flying all over the place.)
As always, thanks.