Fix needed for links and images

This is a core final-quality integrator in RenderMan. It implements the forward path tracing algorithm, which excels in outdoor, highly specular scenes. The simplicity of the algorithm generally makes it easy to use and to implement. Shortcomings may include slow convergence speeds, especially for scenes with significant caustic illumination (see the clampLuminance and allowCaustics parameters, below, for more information).

For direct illumination, PxrPathTracer looks at the values of the numLightSamples and numBxdfSamples parameters in order to determine the number of light vs. Bxdf direct illumination samples, and the resulting direct illumination contributions are combined using multiple importance sampling.

For indirect illumination, PxrPathTracer, the sampleMode parameter controls the strategy for how indirect ray counts are apportioned between diffuse, glossy/specular, subsurface, and refraction/transmission Bxdf lobes.

When the sampleMode parameter is set to a value of "bxdf" (the default value), then the number of indirect illumination rays spawned at each camera hit point is controlled by the value of the numIndirectSamples parameter, and the implementation of the Bxdf at each camera hit point controls the probability of generating indirect rays that are sampled from the diffuse vs. glossy vs. specular lobes of the Bxdf (or, in other words, the Bxdf controls how many indirect diffuse vs. glossy vs. specular rays are spawned at each of the camera hit points).

When the sampleMode parameter is set to a value of "manual", then the value of the numIndirectSamples parameter is ignored and the user is responsible for manually controlling the total number of indirect rays per lobe of the Bxdf. Specifically, when sampleMode is set to "manual" then the numDiffuseSamples parameter explicitly controls the number of indirect diffuse rays that should be spawned at each camera hit point, the numSpecularSamples parameter controls the number of indirect glossy/specular rays that should be created at each camera hit point, the value of the numSubsurfaceSamples parameter is used to determine the number subsurface rays that are launched from each camera hit point, and the value of the numRefractionSamples parameter is equal to the number of refraction/transmission rays that are created at each camera hit point.

This integrator controls the maximum ray depth as follows:

• The maxPathLength parameter is an absolute upper bound on the maximum ray depth for this integrator.
• The "trace:maxdiffusedepth" and "trace:maxspeculardepth" attributes control the maximum ray depths for indirect diffuse rays and indirect specular/glossy rays, respectively.

Single scattering of volumes is supported in this integrator: any path through participating media will perform volume integration of the incoming light along that path. No new paths will be spawned from a volume (therefore no multiple scattering will occur for volumes).