[Radiance-general] ambient include / exclude options (-ae, -ai, -aE, -aI)

[Greg Ward]

Hi Lars,

Your analysis is correct.  The interior surfaces in your test box do not participate in the ambient calculation when you exclude them.  This is not the same result you would get if they were completely "black."  It only means that the -av value is substituted for an indirect calculation at that point.  The direct calculation proceeds at all diffuse surfaces, so your initial sampling from your test point using -I+ will see a bright ceiling.

Cheers,
-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: May 12, 2013 7:42:17 AM EDT
> 
> Hi all,
> 
> I had a closer look at the behaviour of the ambient exlude option -ae,
> which is one of several tweaks for detail geometry in large scenes.
> 
> I set up a very simple test scene. A box 1 x 1 x 1 is indirectly lid by
> a surface below the ceiling, pointing up. The walls of the box are
> assigned a material "copy", which inherits its properties from a
> lambertian plastic material "original". A sensor records illuminance
> below that light source, which means that it should receive only light
> reflected back from the box surfaces. I run rtrace four times:
> 
> a) direct only:
> 
> rtrace -ov -I+ -h -ab X -av 0 0 0
> 
> This should give a zero reading, as the luminous (upper) surface of the
> light source is not directly visible.
> 
> b) direct+indirect:
> 
> rtrace -ov -I+ -h -ab 3 -av 0 0 0 test.oct
> 
> This should give a reading of the light reflected by several bounces
> from the ceiling and walls of the box.
> 
> c) direct+indirect with -ae original:
> 
> rtrace -ov -I+ -h -ab 3 -av 0 0 0 -ae original test.oct
> 
> This should give the same result as b), as excluding the original
> material definition, which is not applied to any surface, should not
> affect the ambient calulation.
> 
> d) direct+indirect with -ae copy:
> 
> rtrace -ov -I+ -h -ab 3 -av 0 0 0 -ae copy test.oct
> 
> This should give a zero reading, just as case a), as the light can be
> received only by the diffuse walls, which have their modifier excluded
> from the ambient calculation.
> 
> Initial expectations:
> a) = d) = 0
> b) = c) < 0
> 
> The results are:
> a) 0.000000e+00	0.000000e+00	0.000000e+00
> b) 3.035680e-03	3.035680e-03	3.035680e-03
> c) 3.203512e-03	3.203512e-03	3.203512e-03
> d) 2.749251e-03	2.749251e-03	2.749251e-03
> 
> The difference between c) and b) is due to the default sampling
> algorithm, so switching to -u- leads to equal values:
> 
> a) 0.000000e+00	0.000000e+00	0.000000e+00
> b) 3.159767e-03	3.159767e-03	3.159767e-03
> c) 3.159767e-03	3.159767e-03	3.159767e-03
> d) 2.845761e-03	2.845761e-03	2.845761e-03
> 
> Still, d) is obviously non-zero. Why? So is that 2.85 the result of the
> diffuse-direct calculation, which is not affected by the -ae option?
> That would mean that even though specularity is zero, rays reflected
> from the (invisible) test surface introduced by the -I+ switch towards
> the ceiling receive a value from there, which results from the direct
> calculation at the ceiling. Setting all -ab 1 leads to an identical
> result for a), b), c) and d):
> 
> b1) 2.845761e-03	2.845761e-03	2.845761e-03
> 
> Now, the one ambient bounce is "used up" by the reflection from the test
> surface introduced by -I+, rays send out from there only return values
> from the direct calculation at the next intersection point.
> 
> I hope my conclusions are correct. If they are, it may useful to share
> this for others looking into using the ambient include / exclude options.
> 
> Cheers, Lars.