[Radiance-general] front and back transmission in BSDF

[Greg Ward]

You can think of it that way.  In fact, the reversal of axes on front and back surfaces means that it is not necessary to transpose the data in the matrix.  This is the way Joe Klems designed the system, for easy multiplication of layer matrices.

Best,
-Greg

> From: Jia Hu <hujia06 at gmail.com>
> Date: October 16, 2012 8:27:17 AM PDT
> 
> Thank you very much for your detailed explanation. From WINDOW 6 manual,  Incoming and outgoing ray direction in XML is defined as this: https://dl.dropbox.com/u/15562376/angle.gif    So,  simply speaking (without considering RGB channels), in the formula E = VTDs, T ("back" in WINOW 6 convention) is actually transposed from the XML file then multiplied by solid angle for each direction? 
> 
> Thanks,
> 
> Jia
> 
> On Tue, Oct 16, 2012 at 12:47 AM, Greg Ward <gregoryjward at gmail.com> wrote:
> Hi Jia,
> 
> It is confusing, because WINDOW 6 defines the front of a window as the outward-looking face, whereas Radiance faces window surfaces inwards when they are used as light sources.  Thus, the notion of front and back get reversed between the WINDOW 6 XML file and Radiance.
> 
> In addition to this, there is the fact that Radiance generally follows rays backwards from their destination (the point of measurement) to their source.  Therefore, calculations in the interior of the space trace light rays backwards to the window surface, then use the WINDOW 6 "Transmission Back" data to compute light contributions from the other side.
> 
> One more thing that bears mentioning is that if only one of "Front Transmission" or "Back Transmission" is available in the XML file, the BSDF library knows how to apply reciprocity to estimate the opposite direction.  Thus, using only one of "+forward" or "+backward" in genBSDF still works for transmission.  For reflection, you need to compute the distribution for the side(s) in which you are interested.
> 
> To be absolutely clear, the "+forward" computes the WINDOW 6 "Front Transmission" and "Front Reflection" components, whereas the "+backward" option computes "Back Transmission" and "Back Reflection."  The back/front confusion is why I chose to call these options "forward" and "backward" rather than "front" and "back," which would be ambiguous.  Forward means rays entering the space from the outside, and backward means they are going the other way, which is generally what we are interested in with Radiance (hence the default "+backward" and "-forward" options.)
> 
> I hope this helps.
> -Greg
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