visualizing BSDFs with BSDFviewer and rmtxop -fc

asked 2015-08-25 19:52:02 -0600

stephenwaz
117 ●3

updated 2015-11-08 12:43:47 -0600

I know the question about BSDF direction has been asked and answered all over the radiance web, but I'm still confused and have a new question about the various ways of viewing BSDF data and how they make sense of the forward/backward issue:

When I make an image of a BSDF.xml file that has data for both forward and backward directions, which matrix does it return? Matching pixels to patches on the BSDFviewer, it matches "Visible Transmission Front" in distribution but the values are not exactly the same. What confuses me about that is when using rmtxop in the 3-phase method, I assume it is using the back transmission/reflection data. Any explanations?

Thanks

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answered 2015-08-25 22:12:41 -0600

GregWard

1203 ●6 ●5 http://www.anyhere.com/gward

updated 2015-08-25 22:13:43 -0600

Hi Stephen,

The rmtxop -fc outputs an HDR image of the "Transmission Front" matrix if it is present. Otherwise, it flips the "Transmission Back" matrix around appropriately for the Klems coordinate system to compute the forward matrix assuming reciprocity.

In either case, the values will not match those shown in Andy's BSDFviewer because rmtxop multiplies each matrix entry by the projected solid angle corresponding to the incident Klems patch to derive values needed for a flux transfer calculation. Even a constant (Lambertian) BTDF will not end up constant in the output due to this flux conversion.

Hope this helps! -Greg