I haven't used Brazil in a production setting yet, but have been teaching it for three or more semesters to architecture students.  Still, I find I have this relatively newbie question:

What is the value of explicitly controlling Photon Maps?  I seem to get good results just working with Global Illumination and the Render Cache (as necessary).  I am able to control the # of bounces of light and do all the energy tweaks, etc. under the GI controls in the Luma Server.  What does the Photon Map add?

I just re-watched Brian's great video tutorial about 'three ways of lighting an interior' (here: http://brazil.mcneel.com/video/three-methods-for-interior) and while he describes how to activate and control the Photon Maps a little, he doesn't say WHY - I don't see anything in his results that would be different from just using the sun with careful GI control.  What am I missing?  Could somebody point me to an example of a rendering that *needed* Photon map control?

I guess behind this is an ignorance on my part of what Photons are.  The Brazil for Rhino help has good stuff on how to control them, but not on how they really work (at least, I haven't found this yet).

Any links, anecdotes, or advice would be much appreciated!

Thanks, George

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You'd have to crank the QMC settings sky-high to get a comparable result, if it's at all possible, on an interior lit mostly indirectly--which isn't exactly the case in that scene with the large window--without photons. And the caustic photons can do stuff QMC just can't.

Thanks for your reply, Jim!  Caustics, I basically understand - that's a specific effect.  But it sounds like you are saying that the main point of GI photons is to increase the overall quality of the rendering, without sacrificing the huge amount of time that very high levels of QMC would take.

I will test them out in an interior scene, as you suggest - thanks.  I want to find or create an example where the photons really make a clear difference.

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