Green Screen Subject Doesn't Match the Background

After the time-consuming process of pulling a good key is finally complete, placing your subject in the background can sometimes seem like an afterthought. But without proper planning, the way your subject is lit or framed can really make it tough. In this segment we talk about
positioning your subject in the background, color correcting your subject to match the background, creating a lightwrap to blend your subject into the background more naturally, and how to deal with mismatched lighting.
Using these techniques will really help immerse your subject into your scene, and make your green screen work more believable.

Blending your subject and foreground in a convincing way is crucial to selling your green screen scene. The first step is to make sure that your subject is properly positioned in your background. Let's take a look at an example in after effects.
In this scene our actor clearly needs to be scaled down and repositioned. Now, I like the position of our actor, but I'd like to reduce the headroom. When I raise the shot, you can see I've reached the bottom of my background. In some cases, such as this one, you may be able to extend your background to compensate. First I'll create a solid and sample the color of the snowy ground. Next, I'll create a mask on my solid by selecting the rectangle tool. Double clicking the rectangle tool while selected on my solid will draw a mask the exact size of the layer. Now I'll drag down the top of the mask near the edge of my scene. Hitting "f" on the keyboard while selected on my layer will bring up the feather properties for the mask. We'll uncheck constrain proportions, and add a nice feather to blend the solid with our background. Dragging down the bottom of the mask will make sure the feather doesn't effect the bottom of our solid. You may need to play with the settings to get it right. This positioning looks pretty good.
One of the most time-consuming and important steps when you're blending your subject into your background is matching the tonality and color of your shots. Let's go back into after effects and take a look at the same scene.
This background shot has a pale, purplish look to it, while our subject is warm and bright. In this case, we'll start by using the curves effect, in the color correction menu. I'll use the dropdown box to select blue, and raise the shadow up to try and match our scene. Now that we've got the color matched a little better, we'll need to pull out some of the color in our actor using the hue/saturation effect in the color correction menu. We'll drop down the saturation level until it's closer to our background... and this looks a lot better. As a final step we can apply a tint to our clip to really dial our look in. You'll find this effect in the color correction menu as well. I'll select one of the dark shadows in the trees to make sure my tint is in the right color range. And then lower the amount to tint parameter, until it looks good. Unchecking our effects can really show you what a big difference this makes in selling the scene.
When you shoot a subject standing in a scene, the surrounding light naturally wraps around the edges of your subject. You can achieve this lightwrap effect in after effects, and other editing programs to help put the finishing touch on blending your subject into your scene. Let's start in after effects.
In this composition, we've got our keyed subject over our background. The first step to creating a lightwrap is to make two extra copies of your keyed clip. Select your keyed layer and hit control d on the keyboard to duplicate your layer. Hit control d a second time to create the second copy. I'll close up my effects here, and then go into the effects, blur and sharpen menu, and apply the gaussian blur to the layer. The amount of blur will control how far your light wrap will extend into your subject. Now select the clip just below your newly blurred clip, and use the dropdown box under track matte and select alpha inverted matte. If you don't see this column, right click near source or layer name, go into the columns menu and select modes. Now select the two new clips and hit control-c to pre-compose the two shots. I'll title this lighwrap and hit okay. Now select your background layer and press control-d to duplicate it. Move the duplicate copy just underneath your precomposed lightwrap. Now go into the effects, blur and sharpen menu and apply the Gaussian blur to the background. We'll turn up the blur a little bit to create a nice effect. Now we use the track matte dropdown box to select alpha matte. If I hit the solo switches on these two layer, you can see this created a blurred version of the background that wraps around the edges of your subject, also known as a lightwrap. Adjusting the opacity of your blurred background layer will adjust the intensity of the lightwrap. Turning the blurred background layer on and off can show you the effect nicely.
You can also create a lightwrap Premiere Pro CS5, most editing programs should have similar features.
Start with your keyed clip over your background. Make a copy of your keyed subject, and open a new sequence with the same settings and call it lightwrap. Paste your keyed footage onto the new timeline, and then paste a second instance of the keyed footage above the original. Now go into the effects folder under blur and sharpen, and apply the gaussian blur to the top layer, be sure to check repeat edge pixels, and increase the blur amount. Adjusting the amount of blur will change how far the lighwrap extends into the subject. Now go into the effects folder, under keying, and apply the track matte key to the first layer. Check the reverse box, then use the dropdown box within the effect and choose the layer above. . Now let's go back into the original sequence. Copy your original background layer and place it above your keyed footage. Now apply a Gaussian blur to the new background layer, and increase the blur parameter. In your project window, drag the lightwrap sequence into the timeline and place it above your blurred background. Now select the blurred background layer, and apply the track matte effect. Use the dropdown box within the effect to select the lightwrap layer. Adjusting the opacity of your blurred background layer can reduce the intensity of your lightwrap. Turning the background layer on and off can show you the difference it makes.
Another common problem in green screen work is mismatched lighting. This occurs when your subject is lit with shadows on the wrong side, or with too much or too little intensity to match your scene.
In this scene you can see that the shadows on the road are falling in the opposite direction of the shadows on our subject. Sometimes, you might get lucky and be able to flip your subject or background horizontally. In this case, we'll use the "s" on the keyboard to drop down the scale parameter, uncheck constrain proportions, and change the x value to negative 100. A little repositioning of our subject, and our shadows match the scene. If the shadows in your scene are too intense or weak, you can try using the shadow/highlight effect. By default, auto amounts is checked, and you can adjust the amount by tweaking the blend with original parameter. Or you can uncheck the by default box, and adjust a host of parameters yourself.
Nobody ever said green screen work was easy, and without proper planning, making it work is definitely a challenge. But using the techniques we've shown can help you achieve a good result, even when the footage isn't ideal. In our next segment, we talk about what to do when there's camera movement in your green screen shot. Being able to track your shot and match it with your background can help save otherwise unusable footage. It's one more way to fix tough green screen problems.