Why do these shadow catcher planes seem to let light pass through making new direct shadows to appear?

Question

I'm solving a VFX problem: virtually stage an indoor scene.

My workflow:

1. Create a room, carve out windows, assign material and use HDRI to light the scene.
2. Add the objects on the floor and walls.
3. Make all objects created in step 1 "Shadow Catcher"
4. Render the scene.

Problem: While the objects seem to have been lit with global illumination, they also seem to cast direct shadows as if the HDRI was lighting the scene without the walls in the way. Here's the rendered image:

To compare, I disabled shadow catcher planes and rendered the whole scene. The dark shadows are no where to be found and the shadows look consistent with the scene. (pls ignore the blotchy look on the wall; too few samples)

Given the popularity of shadow catchers, this problem might be due to my limited understanding how they need to be used. Please advise.

Answer 1

Shadow catcher is most of the time used to blend CGI Objects into real world scenes, you could also use the shadow catcher to render an object with transparent but with the shadow below. These are not the only ways to use the shadow catcher, but some examples. Blender Manual