shadows and normals

[meeotch]

So here's a weird one.  While going thorough the H15 Rendering Masterclass, I noticed some weird artifacts in the example scene renders.  Eventually tracked it down to the background geo, which was just a box with most of the faces removed.  Check out the attached hip file...

If you render the mantra node (which is set to all defaults, I believe), everything looks fine.  Run the render in the Render View, for reasons I'll get to in a minute.  Shadows from the grid and the ball are casting on the box.  Now go into the box object, and turn off Cusp Normals on the Facet node.  (Or alternatively, put the render flag on the Blast node.)  Now the shadows are totally wrong!

o.k., so at first I was thinking - maybe it has something to do with the fact that, in PBR, everything is driven by BSDF's, and lighting effects come "free" with the global physical simulation of light.  Maybe it's the fact that the normals on the box are smoothly interpolated by default, so that along the edges, you get a normal which isn't actually perpendicular to the sides of the box - and presumably, the normals are affecting the BSDF.  (Problems with this theory below.)  So, o.k. - switch to Micropolygon rendering.  Shadows are still wrong - wtf?  But wait - now turn off Preview mode, and in fact Micropolygon rendering displays the correct shadows.  Again, wtf?

So, a decent theory - but here's my problem with it:  no matter which direction the BSDF was "pointing" (based on the normal), it's not going to change the set of overall directions on the surface that can "see" the light.  The geometry hasn't moved, so the shadow should be falling in the same place.  So can someone explain what I'm missing?  (Also, what's up with Preview mode, that causes Micropoly to give PBR-like results?)

  

shadows_and_normals.hip

Edited July 20, 2016 by meeotch