[Radiance-general] front and back transmission in BSDF

[Greg Ward]

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

> From: Jia Hu <hujia06 at gmail.com>
> Date: October 15, 2012 8:49:42 PM PDT
> 
> Hello:
> 
> What is the definition of "front" and "back" transmission in the BSDF xml file? I  test using BSDF files of Window 6 and genBSDF,  and find that it uses "Transmission Back" to calculate radiance. "genBSDF +forward -backward test.rad > test.xml" gave zero illuminance.  So "back"  denotes from outside to inside (towards inside the room)?  that is, the incident ray falls on the outside window surface?  
> 
> However, the three-phase daylight coefficient method  tutorial mentions that Radiance uses "front transmission" data. In addition, in source code: bsdf.c, line 1343:
>  
>                     # line 1343
> 			if (!strcasecmp(ezxml_txt(ezxml_child(wdb,
> 					"WavelengthDataDirection")),
> 					"Transmission Front"))
> 				break;
> 
> It looks using "transmission front". 
> 
> Which (front or back) transmission does Radiance use?
> 
> Thanks,
> 
> Jia