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BRTDfunc gives the maximum flexibility over surface reflectance and transmittance, providing for spectrally-dependent specular rays and reflectance and transmittance distribution functions.
Care must be taken when using this material type to produce a physically valid reflection model. The reflectance function should be bidirectional, and under no circumstances should the sum of reflected diffuse, transmitted diffuse, reflected specular, transmitted specular and the integrated directional diffuse component be greater than one.
![[dialog]](images/brtdbasicdef.gif)
Front Reflectance Color:
The color values will modify the diffuse reflection
on the front side of the surface.
Back Reflectance Color:
The color values will modify the diffuse reflection
on the back side of the surface.
Transmittance Color:
The color values will modify the diffuse transmittance
of the surface. Transmittance must be equal for both
sides by physical law.
A pattern will modify the above three color settings, and will be available through the special variables CrP, CgP, and CbP.
![[dialog]](images/brtdprocdef.gif)
Function File:
This pop-up list offers all function files known to the
system for selection.
Edit...
Start the text editor that was configured in the project
preferences to edit the function file.
New...
Create a new function file, and start the text editor
to edit it.
Reflection Color Red/Green/Blue:
Those three variables specify the color coefficients for
the ideal (specular) reflection of the surface.
Transmission Color Red/Green/Blue:
Those three variables specify the color coefficients for
the ideal (specular) transmission of the surface.
The reflection and transmission values can either be selected from the variables in the function file as selected above, or they may be entered as expressions. An expression can contain numeric values, operators and all functions and variables known to the material. If an expression is not valid, then the entry field will have a red background, and the message bar at the bottom of the dialog will display a diagnostic message.
BRTD Color Functions:
Those three functions from the function file compute
the color coefficients for the directional diffuse
part of reflection and transmission. They take four
arguments, the direction to the incident light and
it's solid angle.
The Transformation page of the dialog can be used to transform the vector, possibly to bring it in alignment with the geometry of the surface the material modifies.
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