### Ray Tracing in Shading Language

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

The following new shading language constructs and functions are available:

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 float 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);

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);
}

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 closest distance between samples.
uniform float maxR | The maximum distance between samples.
uniform float bias | Raytracing 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

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 | Raytracing bias
varying color irradiance | The irradiance amount (output)
uniform float maxdist | The maximum intersection distance

uniform float raydepth();

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

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.