GI Tab

Global Illumination (or GI) is the illumination in a scene that effectively comes from reflected (or bounced) light as opposed to coming directly from a light source. This enables more naturalistic and accurate lighting solutions.

The indirect illumination controls in V-Ray are divided into two large sections: primary diffuse bounces and secondary diffuse bounces.

On – Enables/disables indirect illumination.

GI Engine – Specifies the method to be used for secondary diffuse bounces.

Brute force – Selects the brute force method (direct computation) for primary diffuse bounces. See the  Brute Force section for more information on functionality.
Light Cache – Selects the Light Cache method as the primary GI engine. See the  Light Cache section for more information on functionality. 

GI caustics represent light that has gone through one diffuse, and one or several specular reflections (or refractions). GI caustics can be generated by skylight, or self-illuminated objects, for example. However, caustics caused by direct lights cannot be simulated in this way. You must use the separate Caustics section to control direct light caustics. Note that GI caustics are usually hard to sample and may introduce noise in the GI solution.

Reflective Caustics – Allows indirect light to be reflected from specular objects (mirrors etc). Note that this is not the same as Caustics, which represent direct light going through specular surfaces.

Refractive Caustics – Allows indirect lighting to pass through transparent objects (glass etc). Note that this is not the same as Caustics, which represent direct light going through transparent objects. You need refractive GI caustics to get skylight through windows, for example.

These parameters affect the calculation phase of the Light Cache; they do not affect the final rendering until the Light Cache is used.

Subdivs – Determines how many paths are traced from the camera. The actual number of paths is the square of the subdivs (the default 500 subdivs mean that 250 000 paths are traced from the camera). 

Sample Size – Determines the spacing of the samples in the Light Cache. Smaller numbers mean that the samples are closer to each other, the Light Cache preserves sharp details in lighting, but it is more noisy and takes more memory. Larger numbers smooth out the Light Cache but lose detail. This value can be either in world units or relative to the image size, depending on whether or not the World Scale option is checked or not. 

Show Calc. Phase – When enabled, shows the paths that are traced. This does not affect the calculation of the Light Cache and is provided only as feedback to the user. This option is ignored when rendering to fields - in that case, the calculation phase is never displayed.

Store Direct Light – When this option is enabled, the Light Cache also stores and interpolates direct light. Note that only the diffuse illumination produced by the scene lights is stored. If you want to use the Light Cache directly for approximating the GI, while keeping the direct lighting sharp, clear this checkbox.

World Scale – Determines the unit scale for the Sample Size parameter. When enabled, the size is fixed in world units everywhere. This can affect the quality of the samples. For example, samples that are close to the camera are sampled more often and appear smoother, while samples that are far away are noisier. This option is often the best choice for fly-through animations, since it forces a constant sample density everywhere.

When disabled, units are fractions of the final image (a value of 1.0 means the samples will be as large as the whole image). Samples that are closer to the camera are smaller, and samples that are far away are larger. Note that the units do not depend on the image resolution. This value is best suited for stills or animations where the Light Cache needs to be computed at each frame.

Use for Glossy Rays – When this option is enabled, the Light Cache is used to compute lighting for glossy rays as well, in addition to normal GI rays. This can speed up rendering of scenes with glossy reflections quite a lot. When you use this option, it is recommended to also enable the Use Retrace Threshold option, which prevents Light Cache from being visible in very glossy surfaces.

Mode – Determines the rendering mode of the Light Cache:

Single frame – Computes a new Light Cache for each frame of an animation.
From file – In this mode, the Light Cache loads from a file. The Light Cache file does not include the prefiltering of the Light Cache; prefiltering is performed after the Light Cache is loaded, so that you can adjust it without the need to recompute the Light Cache.

Light Cache File – Specifies the file name to load the Light Cache from, when Mode is set to From file.

Auto Save – When enabled, the Light Cache is automatically written to the specified file. Note that the Light Cache is written as soon as it is calculated, rather than at the actual end of the rendering. 

Auto Save File – Specifies the file name to automatically save the Light Cache to, when Auto Save is enabled.

Depth – Determines the length of the light paths to be traced. It is important to note that even though in Light Cache the depth is limited, due to the recursive nature of this method, there are longer paths in its end result. 

Use Retrace Threshold – When enabled, this option and its corresponding Retrace Threshold value improve the precision of global illumination in cases where the Light Cache produces too large an error. This is especially obvious with the Use for glossy rays option, or near corners where light leaks might be possible because of the Light Cache interpolation. For glossy reflections and refractions, V-Ray dynamically decides whether to use the Light Cache or not, based on the surface glossiness and the distance from it, so that the errors due to the Light Cache are minimized. Note that enabling this option can increase render time.

Retrace Threshold – Specifies the retrace threshold value.

Path Guiding (Experimental) – Uses a path guiding method (Intel® Open Path Guiding Library) to optimize the sampling quality. Path guiding is useful for scenes where GI or glossy reflections are hard to sample, such as interior scenes or scenes with a lot of occlusion. It is supported by V-Ray classic Material, Environment Fog with enabled GI, and Volume Grid with enabled GI. When using this option, note that the Subdivs should be at least 1000, and for some scenes - up to 3000. This option is available with V-Ray CPU only.

Depth – Controls the number of light bounces that are computed when Brute Force is used as the secondary bounces engine.