pwCatch

When using pwCatch, the surface's Diffuse Color is ignored except for preview reasons.

You can change the Diffuse Color to whatever you like without it affecting your final render.

Bump and Displacement work identically with pwCatch as they do with the default shader.

You can use Bump and Displacement to simulate the different surfaces in your image that you're trying to cast shadows/reflections onto. For instance, if you had a photo of a grassy lawn that you wanted to cast shadows onto, you could use Displacement to have the shadow be choppy like the grass. If you were trying to catch a reflection on a photo of water, you could use displacement to create ripples.

Opacity works similarly with pwCatch as it does with the default shader.

You can limit the shadows/reflections being caught using Opacity. Use an Opacity Map to cut shapes out of the surface or lower the Opacity to fade the reflection/shadow.