Shadow Compositing

Gaffer has a special shading mode specifically designed for shots which have rendered geometry composited into a background plate. Almost always, you need to have the rendered geometry cast shadows "into" the plate, to make it appear as if it is truly part of the scene.

This turns out to be a tricky task. The method that is usually used is to place "stand-in" geometry into a scene. This is often just a large flat plane to stand in for a ground, or simplified geometry like a cube to stand in for a desk. The background image is then applied to this stand-in geometry by using LightWave's Front Projection mapping. When the shadow of your main 3D geometry is cast onto the standin geometry, the surface darkens and you get a shadow in your composite scene.

This sounds easy and effective, but in practice it's a tedious, difficult process. The background plate intensity changes because of your lighting, and a visible seam is visible wherever the background shines though. Shadow colors are hard to match. The luminosity and diffuse values of the standin need to be juggled to get the proper behavior, which is never exactly correct.

Gaffer has a special mode that simplifies this shadowing step enormously. You add and map the standin geometry as before, but there's no complex juggling of lighting and surface properties. The "Luminous Shadow Darkening" option in Gaffer simply finds the areas where shadows fall, and fills those areas with a shadow of a color and opacity you define.

The simplicity of this control cannot be overstressed. Before Gaffer, a full day of tweaking would produce a good, but not exact match of the background plate. With Gaffer, you can produce an exact match, with the shadow color and opacity you want, in less than a minute. All of Gaffer's other options, such as area light shadows and selective lighting are still available, which gives you even more control over the shadow appearance.

To the left is a simple example of how Gaffer can help with this kind of composite. The original plate at the top left shows an empty room. We want to insert a rendered toy car sitting on the floor, and have its shadow fall onto the floor. The middle image shows the effect after adding a front-projected plane to receive the shadow. Note how the intensity of the floor has changed, and how there's a seam along the back wall where the plane ends and we see the unshaded background image. The bottom image shows Gaffer in action. Note how even the color of the shadow matches the real room lighting.

Note: The above images also show before and after examples of Gaffer's Area Light Shadows, Selective Lighting and Lighting Intensities. See related sections for more details.