Overview

This section controls and fine-tunes various aspects of the global photon map. The Global photon map rollout appears only when the Photon map method is selected as a Primary engine or Secondary engine on the Global Illumination rollout .

For more details on how the Photon map engine calculates GI, please see the Global Photon Map page under Indirect Illumination.

UI Path

||Render Setup window|| > GI tab > Global photon map rollout
(When Photon map is set as a GI engine)

Parameters

Note that the building of the photon map is also controlled by the photon settings of individual lights in the scene. See the Light settings dialog for more information.

Bounces – Controls the number of light bounces approximated by the photon map. More bounces produce a more realistic result, but take more time and memory.

Max density – Allows you to limit the resolution (and thus the memory) of the photon map. Whenever V-Ray needs to store a new photon in the photon map, it will first look if there are any other photons within a distance specified by Max density. If there is already a suitable photon in the map, V-Ray will just add the energy of the new photon to the one in the map. Otherwise, V-Ray will store the new photon in the photon map. Using this options allows you to shoot many photons (and thus get smoother results) while keeping the size of the photon map manageable.

Store direct light – When enabled, V-Ray will store direct illumination in the photon map as well. This may speed up the irradiance map or brute force GI, when used as a primary engine, and there are lots of lights in the scene. When this is off, direct lighting will always be computed by tracing the necessary rays. This may slow things down if there are lots of lights in the scene.

Auto search distance – When enabled, V-Ray will try to compute a suitable distance within which to search for photons. Sometimes the computed distance is okay, in other cases it might be too big (which will slow down the rendering) or too small (which will produce a more noisy result).

Search dist – Only available when Auto search distance is off. It allows you to specify the photon search distance manually. Keep in mind that this value depends on the size of your scene. Lower values will speed up the rendering but may produce more noisy results. Larger values will slow down the rendering but may produce smoother results.

Max photons – Specifies how many photons will be taken into consideration when approximating the irradiance at the shaded point. More photons mean a smoother (and more blurry) result and may also slow down the rendering. Smaller values mean a more noisy result but will render faster. When this value is 0, V-Ray will use all the photons in the given search range.

Convert to irradiance map –Causes V-Ray to precompute the irradiance at the photon hit points stored in the photon map. This allows fewer photons to be used when interpolating the irradiance during rendering, while keeping the result relatively smooth. It is important to note that the resulting map stores irradiance, but is not the same as the irradiance cache used by V-Ray for primary diffuse bounces.

Interp. samples – Controls how many irradiance samples will be taken from the photon map once it is converted to an irradiance map. Larger values produce smoother results, but may be slower; smaller values produces more noisy results but rendering is faster.

Convex hull area estimate – When disabled, V-Ray will use a simplified algorithm for computing the area, covered by a number of photons (by only taking the distance to the farthest photon). This algorithm may cause corners to be darker. Using the convex hull area estimate avoids the dark corners problem, but is slower and not as robust.

Retrace threshold – When greater than 0.0, V-Ray will use brute force GI near corners, instead of the photon map, in order to obtain a more accurate result and avoid splotches in these areas. This may slow down the rendering. When this is 0.0 , the photon map will always be used, which will be faster, but may produce artifacts near corners or in places where objects are close to each other.

Retrace bounces – Controls how many bounces will be made when retracing corners. If Retrace threshold is 0.0, then this parameter is ignored. Typically this should be equal to the Bounces parameter.

Notes

• The photon map cannot simulate secondary illumination due to skylight. The photon map is mostly useful for interior scenes with artificial lighting or relatively small windows.

• The photon map works only with V-Ray materials. Standard materials will receive GI, but will not generate any photons.

• The photon map calculations cannot be distributed among several machines with the help of Distributed Rendering (DR).

• For more details on how the Photon map GI engine works, please see the Global Photon Map GI page under Indirect Illumination.