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Contents 1 Raytracing in shading language 1.1 trace 1.2 gather 1.3 tranmission 1.4 indirectdiffuse 1.5 occlusion 1.6 raydepth 1.7 raylabel 1.8 rayinfo

[edit] Raytracing in shading language

For an object to be raytraced, it must be marked as raytraceable by setting the visibility attribute.

Attribute "visibility" "trace" 1

The following new shading language constructs and functions are available:

[edit] trace

color trace(point P,vector R);
float trace(point P,vector R);

The trace shadeop fires a single ray from P in direction R and returns either the color hit (the color version) or the percentage visibility (the float version). You can use the explicit prefix in situations which are ambiguous. Trace also takes these optional arguments:

uniform float bias	The self intersection bias.
uniform string label	A label for the ray

[edit] gather

gather(	string category,
    point P,
    vector dir,
    float angle,
    ...)
    statement
    [else statement]

This construct is very similar to PrMan 11's gather construct. It will shoot numSamples rays rooted at P distributed in a cone centered at dir making angle angles. For each ray that hits something, the first statement will be executed. Otherwise else statement will be executed. The following optional parameters can be provided:

uniform float bias	The self intersection bias.
uniform float maxdist	The maximum intersection distance
uniform string distribution	The sample distribution ("cosine" or "uniform")
uniform string label	The ray label
varying type shadertype:variablename	The query value (output)
varying float ray:length	Length of the ray (output)
varying vector ray:direction	The ray direction (output)

For example:

color totalColor = 0;
color hitColor;
float numHits = 0;
gather("irradiance",P,N,PI/2,256,"surface:Ci",hitColor) {
    totalColor += hitColor;
    numHits++;
}
totalColor /= numHits;

is equivalent to

totalColor = indirectdiffuse(P,N,256);

[edit] tranmission

color transmission(point P1,point P2,...);

This function gives the transmission between the two points. This function can be used to get raytraced shadows in light source shaders:

illuminate (from) {
    Cl = transmission(Ps,from) * intensity * lightcolor / (L . L);
}

[edit] indirectdiffuse

color indirectdiffuse(point P,vector N,float numSamples ...);

This function gives the average irradiance as seen be a hemisphere centered at P N. The irradiance is computed by raytracing. numSamples gives the number of rays to shoot. Depending on the irradiance attributes of the current primitive, this value can be cached. The following optional parameters can be provided:

uniform float minR	The minimum distance between samples.
uniform float maxR	The maximum distance between samples.
uniform float bias	Self intersection bias
varying float occlusion	The occlusion amount (output)
uniform float maxdist	The maximum intersection distance
uniform vector backgroundColor	This color will be used for rays that don't hit anything
uniform float maxBrightness	The ray brightness will be clamped by this number to avoid splotchy results
uniform string environmentmap	The incoming color for rays which don't hit anything will be estimated by sampling this map if specified
output varying vector environmentdir	The argument takes a variable, into which the average unoccluded environment direction will be placed
output varying color occlusion	The argument takes a variable, into which the value of occlusion will be placed

[edit] occlusion

float occlusion(point P,vector N,float numSamples ...);

This function computes the fraction of the hemisphere that is not occluded by another geometry. The interpretation of the parameters is the same with indirectdiffuse. The following parameters can be passed:

uniform float minR	The closest distance between samples.
uniform float maxR	The maximum distance between samples.
uniform float bias	Self intersection bias
varying color irradiance	The irradiance amount (output)
uniform float maxdist	The maximum intersection distance

[edit] raydepth

uniform float raydepth();

This function returns the depth of the current shading point. It returns 0 for the camera rays.

[edit] raylabel

uniform string raylabel();

This function returns the label of ray that caused the shading. You can assign labels to rays that are generated by the gather statement.

[edit] rayinfo

uniform float rayinfo(uniform string arg,varying <type> val);

The rayinfo shadeop returns information about a ray. The arg parameter specifies which piece of information to fetch. The value will be placed in val. If no value can be found, rayinfo returns 0. The table below shows what information can be obtained. Note it's that values are premoted to varying as each ray hitting the surface can have different values.

varying string label	The label of the ray
varying float depth	The depth of the ray
varying point origin	The origin of the ray (P where it was fired from)
varying vector direction	The direction of the ray (R when it was fired)
varying float length	The length of the ray (distance from P)