iray render elements are specified by Light Path Expressions.
A Light Path Expression (LPE) is a regular expession that describes specific light paths. In the Render Elements interface for the iray renderer, these expressions select only certain light contributions to construct a particular element.
To be complete, a Light Path Expression must begin with a light source event and end with an eye event—or the other way around.
E designates an eye event. In 3ds Max, this is the point of view of the active viewport (usually, for renderings, a camera view).
The light source event can specify the kind of light:
| Light type | Meaning |
|---|---|
| L | Any light |
| Lp | Point light |
| La | Area light |
| Le | Environment or background |
For example, the LPE that generates a Light: Area element is La.*E, where the asterisk (*) matches any number of Area light events. The LPE for the Light: Environment element is Le.*E.
The LPE that generates a Mtl: Self Illumination event is LE. In other words, it renders light sources but not interactions of light with objects.
Interaction events describe the interaction between a light ray and a material in the scene. In a Light Path Expression, an interaction event is enclosed in angle brackets, and contains two components: the type and the mode.
The event itself describes the interaction of a ray and an object. There are three types of interaction event:
| Event Type | Meaning |
|---|---|
| R | Reflection |
| T | Transmission |
| V | Volume interaction |
The mode describes the scattering that occurs. And there are three possible modes:
| Event Mode | Meaning |
|---|---|
| D | Diffuse |
| G | Glossy |
| S | Specular |
For example,
<RD>
... describes a simple diffuse reflection.
The "dot" (.) character is a wildcard that matches any type or mode.
<R.>
... describes all the reflections of the event.
You can abbreviate <R.> as R. This applies to any event that specifies only one type or one mode.
Be careful when you read abbreviated code: <RD> describes a single diffuse reflection, while RD describes a reflection event followed by a diffuse scattering event.
To match multiple events, you can enclose them in brackets ([]) to form a set.
[RT] combines a reflection and a transmission event. [DGS] combines all scattering modes. So <[RT][DGS]> is an event that includes reflection and transmission, with all kinds of scattering, excluding only volume interaction.
You can also exclude a type or mode from a set by preceding it with a caret (^).
<[^T].> excludes transmission from the event, and this is equivalent to <[RT][DGS]> or simply <[RT].>.
You can use a number of operators to combine sub-expressions. These are the operators allowed (A and B represent sub-expressions; m and n represent integer values):
| Operator | Meaning |
|---|---|
| AB | Matches first A, then B |
| A|B | Matches A or B |
| A? | Optionally matches A. A might not be present in the scene. |
| A* | Matches zero or more occurrences of A |
| A+ | Matches one or more occurrences of A |
| A{n} | Matches n consecutive occurrences of A |
| A{n,m} | Matches from n to m occurrences of A |
| A{n,} | Matches n or more occurrences of A |
Operators are evaluated according to this precedence:
?, *, +, {}
AB
|
You can use parentheses (()) to group sub-expressions and change the order of evaluation. Sub-expressions within parentheses are evaluated first.