Author Topic: [Max 2022] Excluding object from Direct Lighting / Backface Cull for lighting  (Read 281 times)

2021-06-08, 14:56:41

Fireside Dreams

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(edit: Apologies for creating a second post on this. I was logged out halfway modifying the original, and accidentally clicked 'new post' once logged back in)
Hi there

I've updated this post to hopefully make it a bit clearer in what I need help with.

I'm trying to render a scene where the roof is not directly visible to the camera, but is still used in calculations for indirect lighting etc. Effectively this illustrates what I'm after.



I want the light to pass through one side of the roof (like a one-way mirror, almost), directly hitting the interior, but for the indirect lighting to bounce off the other side of the roof.


For context, I run a game of DnD for some friends and thought to use corona to make battlemaps for the game. Exterior scenes are obviously fine to render, the issue lies with rendering interior scenes the way I'd like. For interior scenes, I'm looking for a pseudo-realistic effect. I want the interiors to be illuminated primarily by interior lights, but I also want to place a corona sun (or any light source) that will illuminate the whole scene from above at an angle, doing things such as causing walls / pillars etc to cast shadows.

I created a very basic scene to illustrate what I'm after. It's just a simple room with four pillars, and a bright blue roof (to help highlight the indirect lighting effect I'm after).



What I'm hoping to achieve is ultimately the effects of those two renders combined, without the need for compositing. In the top left, with no roof at all, the direct lighting causes the western wall and the four pillars to cast a shadow. In the other two pictures, the rays are reflected from the roof via the hidden light, giving the desired indirect lighting effect.

I have so far tried using a ray-switching material, and using a front-back opacity map for the roof (as it's a single-sided object, not solid), but I have been unsuccessful thus far.

Is it possible to create this 'cutaway' effect?