'seen by GI' question

[alienesque]

I'm working on a big architectural scene with several glass objecs I want to exclude from the GI calculation, to lower rendertimes. I could place compositing tags with 'seen by GI' unchecked on all of them, but that would take me a lot of time. Does it have the same effect when I just go to my glass material settings and put Generate & Receive GI to 0%? As wel for renderspeed as for visual effect? Testing would take up too much time, therefore my question

you could just select all the glass objects and then add the tag to them all at once..

[mogh]

Attention !

This is a very serious question and it rumbled through the forums a year ago.

If you uncheck the "Seen by GI" in the Compositing Tag the object is only not taken into account for the GI "Shadow Bounce" calculation .... (as i understand it with my humbled knowledge) but still all calculations will be done son merely no speed improvements will occur!

If you disable GI in the respective Material it will not be included in the calculation as you suspect it n the first place.

So disabling the Material is the right way to go for transparent materials ...

regards mogh

[andrasn]

Yes, the SEEN BY GI is a bit misleading...
Mogh is right, you have to disable GI in the material level so it will not take that object into account.
Disabling SEEN BY GI means that the object will not cast shadows of the GI calculation, only casts shadows from direct lights. It's great to avoid those ugly blotches e.g. if a glass or a semitransparent object is very close (1-2 cms) to a wall. The tipical situation is a staircase with glass handrails just in front of the slab....

Andras

[rikke]

...so I've been doing it the wrong way for all these years! I always thought this was the right way to go. Thanks Mogh & Andrasn, that's some very usefull information.
Another question: for exteriors, this is my first GI attempt. First tests look fine, but all windows are pitch black, no reflections. Although I'm using the same method for creating these: planes and a sphere with a hdri material assigned to it, for reflections. It always worked for me with traditional lights, but not in this case. I'm using double-glazed windows, making them single-glazed helps a bit but not the way I want it. Any thougths?

[andrasn]

I'm quiet busy at the moment and no time to make tests.... but I'm not sure if HDRI works or not if both GI is disabled in material levels. Try to activate it.... or make test with just RECEIVE GI on.
Anyway, lots of times I don't disable GI for glasses because the timegain is not so much.

You mentioned you make your glasses with a plane? It's much better to give them thickness so fresnel reflections are really better.

An other possible error can be flipped normals on the glass. Revise it!


Cheers,
Andras

[rikke]

I did some testing by placing simple cubes with the same glas material in the scene, reflections still work that way. Only, if I test it on my buildings' windows it goes black. Frustrating, I hope I can avoid needing to use an extra GI bounce or putting lights inside

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