Rayfront User Manual

Trans Material Types

The trans, trans2, transfunc and transdata materials are for translucent surfaces. Transmitted and diffusely reflected light is modified by the material color. A pattern modifying the trans material types will affect the material color.

Trans

Trans is the simplest transparent/translucent material type.

Color:
The color values will modify the diffusely reflected and all transmitted light. Values lower than 0.01 or higher than 0.9 are unusual.

[0.0 0.0 0.0]

black (full absorption)

[1.0 1.0 1.0]

white (no absorption)

Specularity:
The fraction of incident light that is immediately reflected in mirror like fashion. Values greater than 0.1 are unusual.

0.0

matte

0.07

satin finish

Roughness:
Roughness is specified as the rms slope of surface facets. A value of 0 corresponds to a perfectly smooth surface, and a value of 1 would be a very rough surface. Roughness values above 0.2 are unusual. The roughness affects only the specular components.

0.0

polished

0.2

low gloss

Transmissivity:
The fraction of penetrating light that travels all the way through the material.

0.0

opaque

1.0

transparent

Transmissive Specularity:
The fraction of transmitted light that is not diffusely scattered.

0.0

diffuse

1.0

clear

The parameters of the trans material are not what they seem to be at first glance. The simulation takes the following steps when encountering a surface with a trans modifier:

1. From the total amount of incident light the specular reflection is seperated first. Consequently, specular highlights will be in the color of the light source.
2. The rest of the light is colored, ie. multiplied with the rgb values of the material color for each band. You can think of the difference to the previous value as absorptance.
3. Now the Transmission factor divides the remaining amount of colored light between the two sides of the surface.
4. The remainder is reflected diffusely.
5. The Transmissive Specularity seperates the transmitted part that travels through the material in a straight path.
6. This leaves us with a diffuse part, which is scattered across the hemisphere.
7. Both the reflected and transmitted specular light is then modulated by the roughness, which may widen up the rays to a certain degree.

Trans2

Trans2 is similar to trans, but with anisotropic roughness. This means that specular highlights on the surface will appear elliptical rather than round.

Roughness u:
Roughness v:
The roughness is specified seperately along the direction of the anisotropy (u) and along the direction on the surface perpendicular to it (v). Both values can't be smaller than 0.001. The highlight will be narrower in the direction of the smaller roughness value.


The direction of the vector u is defined on the Procedural page of the dialog.

Function File:
This pop-up list offers all function files known to the system for selection. If a file is active, then the variables in that file can be selected for one or several of the vector coordinates. Otherwise, the coordinates must be entered as expressions.
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.

Direction Vector X/Y/Z:
The coordinates of the direction vector orienting the anisotropy can either be selected from the variables in a 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.

 
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.

Transfunc

The transfunc material type is for translucent surfaces with arbitrary bidirectional reflectance and transmittance functions (BRTDFs) and has the same basic properties as trans, with the exception of roughness. Roughness isn't required here, since the BRTDF specification is much more powerful.

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.

BRTD Function:
An arbitrary bidirectional transmittance and reflectance distribution function. The function takes four arguments, the x, y and z direction towards the incident light and the solid angle subtended by the source (which is supplied to faciliate averaging, but is often ignored). Radiance will also accept functions with less parameters, as in the example image. The pop-up list will offer all formally valid functions from the function file for selection.
The selected function should integrate to 1 over each projected hemisphere.

Real Arguments
Rayfront will check the selected function file to decide how many real arguments the material definition should supply. If this number is greater than the mandatory arguments specifying the basic properties, then the needed number of entry fields in the list is made available. Supplying those arguments is optional, but failing to do so may result in runtime errors when the selected function relies on a specific argument which is not set.

 
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.

Transdata

The transdata material is used for arbitrary BRTDFs that are most conveniently given as interpolated data and type has the same basic properties as trans, with the exception of roughness. Roughness isn't required here, since the BRTDF specification is much more powerful.

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.

BRTD Function:
An arbitrary bidirectional transmittance and reflectance distribution function. The function takes four arguments, the x, y and z direction towards the incident light and the solid angle subtended by the source (which is supplied to faciliate averaging, but is often ignored). Radiance will also accept functions with less parameters, as in the example image. The pop-up list will offer all formally valid functions from the function file for selection.
The selected function should integrate to 1 over each projected hemisphere.

Data File:
The file containing the reflectance data for interpolation. Rayfront currently limits valid data to three dimensions.
Import...
This button invokes a file open dialog, and the selected data file will then be imported into the project.

Index Functions:
One or several functions (depending on the number of dimensions in the data file) to index into the data. The functions take four arguments each, the x, y, and z coordinates of the direction to the incident light and the solid angle subtended by the light source. Functions with less arguments are also valid.

Real Arguments
Rayfront will check the selected function file to decide how many real arguments the material definition should supply. If this number is greater than the mandatory arguments specifying the basic properties, then the needed number of entry fields in the list is made available. Supplying those arguments is optional, but failing to do so may result in runtime errors when the selected function relies on a specific argument which is not set.

 
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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