This light source can be used to add some global illumination effects.
There are two techniques for computing global illumination. Irradiance computation will sample the hemisphere over the point being shaded to compute the incoming “ambient” light. This operation is typically quite expensive but can be sped up using irradiance caching (see the mantra output driver)
Photon maps are another mechanism to allow evaluation of the indirect light paths to the surface. These can be used to simulate caustics or more complex irradiance effects. It is possible to specify up to two photon maps.
Caustic map
This map contains photons which have been bounced off a specular or transparent surface.
Global map
This map contains photons which have been bounced off any diffuse surfaces.
If you want both global illumination and caustics, you can specify both photon maps.
When just evaluating ambient occlusion, the irradiance of other surfaces will not be evaluated. This speeds up computation but provides a simpler lighting model (i.e. no color bleeding) When computing irradiance (or ambient occlusion), it is possible to specify what the color of rays which don’t hit objects is. This is done by using an environment map or by specifying a constant background color. When specifying an environment map, the map color can be tinted. As well, the space of the environment map may be specified by specifying a null object.
See how to create an environment/reflection map.
Parameters ¶
Irradiance ¶
Global Tint
An overall multiplier on the global illumination. This affects all aspects of illumination (i.e. environment, background, photon maps, irradiance, etc.).
Irradiance Style
No Irradiance
Disable irradiance calculations entirely. Lighting will be computed solely from the specified photon map files.
Ambient Occlusion
Lighting will be computed from the environment map or background color for rays that miss geometry.
Transparent Ambient Occlusion
Lighting will be computed from the environment map or background color for rays that miss geometry. Rather than treating any hit surface as fully opaque, Transparent Ambient Occlusion will attenuate the environment color by the opacity of hit surfaces resulting in a more accurate result in scenes using transparency. There is a small additional performance penalty to using this mode as opposed to plain Ambient Occlusion since shaders need to be executed on hit surfaces.
Full Irradiance
The surface color (Cf) will be computed on the hit surface by executing the surface shader. This is the most computationally intensive mode, as the hit surface must execute in full - including texture lookups and lighting. When using full irradiance, it is often beneficial to specify a global photon map that includes direct lighting - as this will prevent hit surfaces from executing all light shaders.
Sampling Quality
The number of rays to send out for irradiance/ambient occlusion.
Adaptive sampling
Turns on an automatic optimization that will reduce the number of samples when there is little variation in occlusion above the sample point. This can improve performance at the expense of some possible flickering or additional noise. Adaptive sampling will only take effect with more than 64 samples.
Maximum distance
Maximum distance from the sample point to consider geometry for irradiance/occlusion.
Environment map
If irradiance/occlusion rays miss all objects in the scene, lookup the color from this environment map rather than returning the background color. The environment map should most likely be a .rat file which was processed by the stand-alone tool isixpack.
Environment tint
Tint the environment map by this color.
Null object
This parameter refers to a null object in the scene. The null object is used to specify the orientation of the environment map (only rotations matter). If no object is specified, then the transform of the light this shader is bound to will be used to orient the environment.
Background color
If irradiance/occlusion rays miss all objects in the scene, use this color. If there is an environment map specified, then this parameter is ignored.
Photon Maps ¶
Global Map
For secondary irradiance (or primary irradiance if no ray-traced irradiance is used), this photon map will be used to estimate irradiance.
Global Sampling
How many photons to use for irradiance evaluation. When the photon map is prefiltered, normally this should be set to a small value (default 1).
Global photon map stores direct lighting
Indicate that the photon map stores direct lighting. When evaluating irradiance, mantra will ignore any lights other than the GI light shader.
Prefiltered Global photon map
Indicates that the global photon map is prefiltered. Prefiltered photon maps require a much lower number of samples, since lighting has already been averaged during prefiltering.
Caustic Map
Specifies a map containing “specular” photons. These photons are typically the results of transmission or reflection.
Caustic Sampling
How many photons to consider when evaluating caustics.
Prefiltered Caustic photon map
Indicates that the caustic photon map is prefiltered. Prefiltered photon maps require a much lower number of samples, since lighting has already been averaged during prefiltering.