How to create a material which lets light pass through unaffected?

Question

Ok, so im looking to make a material which;

• allows 100% of light to pass through unaffected. As if the surface was never there.
• allows normal textures to be applied as usual whilst looking as it should ( diffuse,normals,roughness, etc)

Blender 2.80, using cycles.

I made a mini demonstration which kinda shows what im trying to do. Say for example i want to have the light travel through the square hole like the cube on the right whilst also having the green square at there at the same time.

I hope this makes it easier to understand what im trying to do.

Im not even sure if such a thing can be done at all which is why im asking to see if its possible. I seen in one of blender guru's videos he managed to make glass which was able to do this but im not very knowledgeable with the node editor so i wasn't able to modify node setup to meet my requirements.

Thank You

Answer 1

If you want to set this up per material, you can use the Light Path node to make a material transparent to shadow rays: (Note the Geometry node is not relevant to this first example)

with this result...

...but as you can see from the shadow on the floor plane, this is transparent in both directions. If you want to fix that, you can wire in the Geometry node, so the the surface is transparent only to shadow rays to which it is backfacing .. (not forgetting the rays are fired from the shadowed surface towards the light)...

.. the multiply is being used to 'AND' the two booleans.. with this result.

You can go further, to make the material transparent only to specified lights.. see here.. or here.. but as you can see, that gets more involved.

Answer 2

Set the green object properties so that it is not included in shadow calculations.

If that is not what you want then can you link to the tutorial you have referred to so we can see what you're looking for?