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[Max] I need help! / [Max 2022] Excluding object from Direct Lighting / Backface Cull for lighting

« on: 2021-06-08, 14:56:41 »

(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?

2

[Max] I need help! / [Max 2022] Excluding object from Direct Lighting / Backface Cull for lighting

« on: 2021-05-28, 17:59:27 »

(edit: apologies for double-posting, I was logged out halfway through modifying the first post, and accidentally clicked 'new post')
Hi there

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.

However I'm getting issues with the lighting, where the floor / walls / anything under the roof isn't receiving direct lighting, making the scene render black (although the correct objects DO shop up in the alpha channel.

I've constructed a demonstrative scene to highlight the issue.

I've constructed a green 'room' and a blue 'roof'. Direct and Indirect Lighting passes shown



As you can see, the shadows fall as they should in direct lighting, and the green room has a slight blue halo around the centre from where the 'roof' resides.

However so far all attempts I've tried at rendering the green room, without rendering the blue roof but keeping its indirect lighting, have failed.

So far I have tried:

• FrontBack Opacity Switch on single side: Results in the image below. White for front face, black for rear face. In direct lighting the shadow is still rendered as if the opacity wasn't set to 0. Interestingly, if both sides are set to low opacity (second image, 0.5 for both sides) then the direct lighting changes accordingly

• Untick 'visible to camera' in Object Settings: Roof turns invisible, however it casts a shadow in direct light as if it were visible; indirect light does work
• Exclude from Corona Camera: Effectively removes it from the scene. Indirect lighting doesn't reflect the roof at all, as it's hidden

[li]Ray Switcher: Material hooked up to all nodes except direct: Functionally the same as unticking visible to camera. Indirect lighting is correct, but the invisible geometry still casts a shadow in direct lighting
[/li][/list]

I'm at my wit's end trying to figure this one out - any help would be greatly appreciated, cheers!