Setting Up Scenes for pwCatch
When you use pwCatch on an object, that object will be used to capture the shadows/reflections of other objects, and/or the backdrop image. The surfaces using pwCatch become invisible so that only those captured elements can be seen.
By reconstructing the basic shape of the objects in the photo using pwCatch-surfaces, you can integrate other 3D objects into the photo.
Let's look at some examples.
In this image, the red object has been set to "Use Backdrop" and the yellow objects are set to "Use Backdrop" and "Catch Shadows". Note that none of the boxes, which all use pwCatch, are visible in the render, however the shadows that would fall on them are, and they pick up the backdrop image to overlay it on the hippo.
A common mistake might be to apply pwCatch to the hippo, thinking that its shadows would then show up on the photo. Not so! You want to capture the shadows where they might fall, not on the hippo itself, so you create boxes and planes (or other shapes) to pick up the shadow.
Another example:
In this image, the green demon is set to "Invisible, but casts shadows". As you can see, the demon doesn't appear in the render, but the demon sends out the message to the lights that it is able to cast shadows, and the normal objects in the scene take the shadow just like they would if the demon used the default shader.
One more example:
In this image, the red objects are set to "Catch Shadows" and "Use Backdrop", and the blue objects "Catch Shadows", "Catch Reflections" and "Use Backdrop". The blue object picks up the dolphin's reflection and shadow without showing a big blue box, and becomes a good stand in for the water in the photo. The red objects mimic the structure of the basin and wall from the photo, and hide themselves while boxing in the dolphin and taking its shadows.