Post Production

Fixing Problem Green Screen Shots – Poor Lighting Issues

Poor Lighting Issues

This has to be the number-one reason for bad green screen shots. Good lighting is the key to getting a great shot with which to composite. In most cases, the photographer either attempts to light the background but fails to light it evenly or doesn’t scope the shot through the camera and lights it too bright or unevenly.

In some cases, the shot is too washed out because the background wasn’t lit independently in the studio and the wrong color backdrop was used. Perhaps the studio used a green cloth that wasn’t intended for green screen or tried to cut corners by making a run to the hardware store for a couple gallons of paint. But that’s OK, because when they get your bill for dealing with all the compositing hassles, they may not make those mistakes again.

Case Study: Poor Lighting

Figure 16.1 shows an example of a close-up shot for a major commercial I was working on. It has all the elements of a crappy green screen lighting issue. I can’t tell exactly what the studio used for a backdrop, but judging from the lighting in most of the shots, it looks like the cheap muslin I tell people to stay away from. They used available ambient light, and there is motion blur on the actress’s face and hair—not to mention that the color processing is way off. Even for a preliminary production phase, it’s totally flat and overexposed. This is what happens when inexperienced photographers get their hands on a RED camera without understanding the workflow.

Figure 16.1: Original frame from the poorly shot green screen.

I first tried Keylight in After Effects to extract whatever green I could. It got the general area, but when I adjusted the matte it appeared that the footage was too noisy to extract the hair properly (see Figure 16.2). I’m surprised it managed to maintain some of the fine hair without looking like the actress was wearing a hair net.

Figure 16.2: The first pass with Keylight reveals noise on the model’s hair and face.

I then duplicated the original layer and eliminated the Keylight effect from the duped layer. Using the Pen Tool, I drew a mask around the hair where some of the green started to show through and feathered it about 25 pixels (see Figure 16.3 below). This mask had to be adjusted over time with keyframes.

Figure 16.3: Creating a roto mask to follow the original layer’s hair just inside the green spill.

I then selected the keyed layer and adjusted the Screen Balance to allow the colors to blend slightly between the keyed layer and the roto’d layer. I applied a lens blur on the keyed layer to match the original footage motion blur. The result was fairly seamless: without affecting the actress’s face and hair, I obtained a usable key (see Figures 16.4 below).

Figure: 16.4: Using the Lens Blur effect to blend the roto mask layer with the keyed original.

You must take great care to watch the roto mask as well as the amount of lens blur you apply over the timeline, to match the original footage as close as possible.

Remember, in After Effects, anything that can be keyframed can be changed over time.

Shot Too Dark

In some cases, if a shot is underexposed and the screen isn’t properly lit in the background, the result is a great deal of visible noise on both the green screen background and the actors, even in HD. The same is true if you use the wrong color green or blue for the background materials. If the shot is too noisy, there’s little you can do to fix it without it looking like it’s over-processed and filtered. But if the foreground is salvageable, then you can work the green screen background if you can pull a decent key. It may depend on the footage used as the background plate in the composite.

In the example shown in Figure 16.5, I purposely shot the scene dark and left the green screen lights off, with the green screen background illuminated only by the actors’ lights in the studio. The actors are about 15 feet from the background in this shot. (Honestly, I’ve received much, much worse footage from clients, but this is as bad as I could try to make it.)

Figure 16.5: A dark-lit green screen shot can produce a great deal of noise and make compositing difficult.

The first application of Keylight in After Effects produced an extremely noisy and uneven matte. When I tried to adjust the matte clipping to normalize the background and foreground matte and get rid of the noise, I clipped the edges too abruptly and created nasty artifacts around the edges, as shown in Figure 16.6.

Figure 16.6 Trying to adjust the matte to reduce the noise yields poor results.

The next step was to try to get a decent key around the actress by adjusting the matte settings. It wasn’t perfect, but it was enough to work with. Looking at Figure 16.7, it’s easy to see that the same settings for the girl won’t work on the guy in this shot. He has a dark ridge around his right side and needs to be processed separately, not to mention the noise in the background.

Figure 16.7 The Keylight matte has been adjusted enough for the actress.

I duplicated the original layer and drew a roto mask around the girl in the early frames of the clip, using keyframes down the timeline to track her movement. This allowed me to create a garbage matte to reduce the background noise, at the same time isolating the girl on her own layer (see Figure 16.8).

Figure 16.8 Using a roto mask around each actor in a scene and isolating them on their own layer

I then masked the duplicated layer around the guy in the clip and keyframed the roto mask along the timeline to track his movement. After he was isolated, I adjusted the Keylight for him separately and removed the bad edges (see Figure 16.9). In this case, the screen shrinkage adjustment setting in Keylight was all that was needed to remove a pixel of hard edge around him.

Figure 16.9 Creating a roto mask for the second actor, and applying the Keylight adjustments for his layer separately

The end result looks pretty good, even though the scene wasn’t lit properly for the intended background plate (see Figure 16.10). Again, this is where proper planning and setup could have made a simple keying job a matter of a couple clicks and letting it run.

Figure 16.10 The final composite is acceptable, and the shot is saved.

Excerpt from The Green Screen Handbook:Real-World Production Techniques, 2nd Edition by Jeff Foster © 2015 Taylor and Francis Group. All Rights Reserved.

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