Hack 71. Create a Shadow for a Green Screen Image

Compositing someone or something into a scene is an accomplishment. But to make it believable, you might need to add a shadow.

A green screen image is one where an image is captured with a background consisting of only one color, green. You can also acquire a blue screen image, where the background is blue, or just about any other color you can imagine. The essential ingredient is that your image is in front of a single, solid-colored background, and that your subject not have the same color as your background on it. It also helps if you are able to light and set the scene with compositing in mind [Hack #22].

When you composite a scene using a green screen, you might discover there is no shadow being cast from your subject. If you discover the scene you are attempting to composite needs a shadow, because it just doesn't look right, there is an easy solution to your problem. Even better than that, the solution can be found within the problem itself.

This process works well for a shot of a single moving subject, such as a woman walking across a beach. It can also work just as well for more than one subject; however, the more movement there is between the subjects, such as dancing in a circle, the less successful this process will be, because your shadow won't cast from one subject onto the other.

6.12.1. Creating a Copy of Your Image

The first step in creating your shadow effect is to duplicate your video image. To do so, simply create another video layer on your timeline and drop a copy of your green screen video onto it. You will want to apply the same exact effect you used to initially composite your scene [Hack #70]. When prepared correctly, you should have two layers of video, with the same effects applied.

The best approach to duplicating a green-screened image is to copy-and-paste it into the timeline. After applying the necessary effects to key out the green screen on the original image, copy the entire clip, including the effects, and paste it to another (lower) layer on your timeline. This might require shuffling your footage onto newly created layers in your timeline.

6.12.2. Layering Video to Prepare for Shadow Creation

In order for the effect to appear correctly, you will want your original image on a layer above the shadow image's layer. If you do not do stack the layers correctly, the shadow will appear in front of your subject. Figure 6-40 shows the difference between having the shadow layer above the original layer and below the original layer.

Figure 6-40. The subtle but distinct difference in layering

6.12.3. Create the Shell of the Shadow

Now that the image layers have been prepared, you need to remove the color from the image. By removing the color, you should end up with a black representation of your original image. To accomplish this:

Avid

Open the Effect Editor for the clip and choose Show Alpha.

With Avid, by changing the key to a moving matte, via the Show Alpha option, any additional effects you apply are independent of any other layers. After creating the moving matte, apply a paint effect to the clip, to nest the key effect:

Tools Effects Palette Image Paint Effect

Then, drag a matte key on top of the paint effect, to provide the shadow:

Tools Effects Palette Key Matte Key

Final Cut

Effects Video Filters Image Control Brightness and Contrast

Premiere

Effects Video Effects Adjust Brightness and Contrast

6.12.4. Shear the Shadow Off of the Subject

Once you have created the basic shadow element, you need to shear it away from your subject. If you don't shear the shadow image, your audience will never see it because it will always be behind your original image. To shear your image, you need to distort it:

Avid

Tools Effects Palette Blend 3D Warp

Final Cut

Effects Video Filters Perspective Basic 3D (or use the Distort Tool)

Premiere

Effects Video Effects Distort Corner Pin

Getting your shadow to fit the scene takes time and patience.

There are no specific rules I can provide to shear your shadow realistically because every shot will require different settings. If there are other shadows in your background, use them as a reference point to help evaluate how well your settings match the scene.

6.12.5. Add Subtlety to the Shadow

Depending on your personal preference, as well as any preexisting shadows in your scene, you will possibly want to blend your new shadow. Blending the shadow is easy to do: just change its opacity. In order to add a little more realism, you can add a Blur effect to your shadow in order to soften it.

If you want to adjust any of the parameters on your effect, simply edit the effects you've applied until you obtain acceptable results. Again, if there are any preexisting shadows in your scene, you should attempt to match your self-created shadow as much as possible. Figure 6-41 shows a series of images from a completed scene with and without a shadow applied.

Figure 6-41. A completed scene with (top) and without (bottom) shadow applied

6.12.6. Hacking the Hack

There will usually be final, small details that you need to pay attention to. You will more than likely have to perform some minor adjustments to your subject image, including changing brightness and contrast to match your background. You might also have to do some color correction in order to have your original image blend in correctly.

When you've finished compositing your shadow, try toggling your shadow layer on and off, merely to remind yourself that you've created a shadow that follows your subjects…just like the real thing.