RPC Shadow Theory
When working with planar RPCs (non 3D Plus RPCs) you may notice that occasionally your shadows look very thin, almost non existent, while at other times the shadows render nicely. This tutorial will discuss the theory behind using Ray-Traced shadows.
As light rays pass through the RPC's plane, shadows are created from the alpha mask. The RPC always faces the camera, and because of this the position of the light source in relationship to the camera dictates the way that the shadow looks.
As the light approaches being perpendicular (at a 90 degree angle) to the camera the RPCs shadow will become thinner.
The following image will show what kinds of shadows will be created at particular angles for the light source in relationship to the camera.
As a general rule, try to keep the light source from being perpendicular with the rendering camera to obtain the best shadows.
Here is an example of a rendering with the light source at 85° in relationship to the camera.
Here is an example of a rendering with the light source at 45° in relationship to the camera.
Here is an example of a rendering with the light source at 0 degrees in relationship to the camera. At 180 degrees the shadow would be the same, but in front of Olivia.
You may have noticed that, depending on the position of the light source, it is also necessary to turn on Cast Reflections to make the RPC Shadow appear. If the RPC is between the light source and the camera, you need to turn on cast reflections to create a plane that faces the other direction so that there is something to cast shadows with.