Visible item with ZERO lighting influence on scene?

[MrFurious]

In most of my interior archviz renders I have a single poly item in the background (outside a window for example) which has an emissive texture applied to it, usually a sky or trees. The sole purpose for this item is to be visible, and cast reflections over the entire scene. Problem is this polygon effects the diffuse lighting in the scene do some degree ie: it's diffuse shadows (not diffuse emission) effect the brightness and colour cast. I've tried moving this item as far away in the distance and away from the scene as possible, but it always effects the overall sun and/or environment lighting to some degree.

My emission texture has these flags:
- Visible on Diffuse [OFF]
- Visible on Specular [ON]
- Transparent Emission [OFF]
- Cast Shadows [OFF]

Is it at all possible to have an object in my scene which is only visible to camera and does not effect the lighting or diffuse shadows in any way?

[funk]

Dino, also try putting this geometry in its own mesh item, activate live geometry and disable shadow visibility

[MrFurious]

Dino, also try putting this geometry in its own mesh item, activate live geometry and disable shadow visibility

Thanks Funk, this does remove the direct shadows, but not diffuse shadowing and there's some colour bleed as well.

Box A

Box B

Clearly there's diffuse colour bouncing, but also some diffuse shadowing to the right of the box... subtle but it's there.

- the red box has zero emission, basic diffuse material everything zero'd except diffuse colour and opacity
- live geometry enabled
- shadow visibility off

[MrFurious]

This is probably a better everyday use example. Simple room with a few objects in it and a single poly mesh item outside the window:

Scene

Sun &/or environment lighting setup to my liking, backdrop item hidden:

Add the backdrop mesh item.
- Shadow visibility OFF
- Material set to Emission
- Visible on Diffuse [OFF]
- Visible on Specular [ON] (although no items in the scene have specularity)
- Transparent Emission [OFF]
- Cast Shadows [OFF]

Scene is now noticeably darker, and adjusting the exposure doesn't really correct this as the light-falloff also changes ie: the highlights aren't as smooth and evenly distributed and shadowy areas are more contrasty. In some scenes I notice a colour cast coming from the backdrop item into the room. Sometimes I'll want to have multiple (single poly) backdrop items and layer them ie: sky, treeline, single trees, cityscape and it's gets a little worse the more I add.

Workaround for my current project: Rendered the image with no backdrop items and with 'Alpha Channel' enabled in octane kernel. Add my backdrop layers in photoshop. Why doing this sucks: backdrop reflections are not rendered into the scene.

Surely there is a way to add a visible mesh item which does not effect the lighting solution in any way?

[funk]

Dino, if you dont mind using some OSL, you can do this with raytype

Here is an example which makes the background poly visible to camera and reflections rays only.

The osl plugs into the material's opacity channel. It sets opacity to 1 for camera and reflection rays, but 0 for all other rays.

Here is what it looks like. My room material (mat2) has no reflections here.

[MrFurious]

Funk....


this
is
freaking
awesome.

never delved into OSL and I should have a while back. Having some fun with your scene. Thank you!

p.s. can even use the multiply node with an alpha --> opacity for cutouts..... so nice.

[funk]

Yeah Ive been experimenting with OSL and it's really cool.