Light cache

General

Light caching (sometimes also called light mapping) is a technique for approximating the global illumination in a scene. This method was developed originally by Chaos Group specifically for the V-Ray renderer. It is very similar to photon mapping, but without many of its limitations.

The light cache is built by tracing many many eye paths from the camera. Each of the bounces in the path stores the illumination from the rest of the path into a 3d structure, very similar to the photon map. On the other hand, in a sense, it is the exact opposite of the photon map, which traces paths from the lights, and stores the accumulated energy from the beginning of the path into the photon map.

Although very simple, the light-caching approach has many advantages over the photon map:

It is easier to set up. We only have the camera to trace rays from, as opposed to the photon map, which must process each light in the scene and usually requires separate setup for each light.
The light-caching approach works efficiently with any lights - including skylight, self-illuminated objects, non-physical lights, photometric lights etc. In contrast, the photon map is limited in the lighting effects it can reproduce - for example, the photon map cannot reproduce the illumination from skylight or from standard omni lights without inverse-square falloff.
The light cache produces correct results in corners and around small objects. The photon map, on the other hand, relies on tricky density estimation schemes, which often produce wrong results in these cases, either darkening or brightening those areas.
In many cases the light cache can be visualized directly for very fast and smooth previews of the lighting in the scene.

Even with these advantages, light caching is similar in speed to the photon map and can produce approximations to the global lighting in a scene very quickly. In addition, the light cache can be used successfully for adding GI effects to animations.

Of course, the light cache has some limitations:

Like the irradiance map, it is view-dependent and is generated for a particular position of the camera.
Like the photon map, the light cache is not adaptive. The illumination is computed at a fixed resolution, which is determined by the user.
The light cache does not work very well with bump maps.

Parameters

Calculation parameters

These parameters affect the calculation phase of the light cache; they do not affect the final rendering.

Subdivs

This determines how many paths are traced from the camera. The actual number of paths is the square of the subdivs (the default 1000 subdivs mean that 1 000 000 paths will be traced from the camera).

Sample size

This determines the spacing of the samples in the light cache. Smaller numbers mean that the samples will be closer to each other, the light cache will preserve sharp details in lighting, but it will be more noisy and will take more memory. Larger numbers will smooth out the light cache but will loose detail. This value can be either in world units or relative to the image size, depending on light cache Scale mode.

Scale

This parameter determines the units of the Sample size and the Filter size

Screen	The 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 will be smaller, and samples that are far away will be 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.
World	The sizes are fixed in world units everywhere. This can affect the quality of the samples - samples that are close to the camera will be sampled more often and will apear smoother, while samples that are far away will be noisier. This value might work better for fly-through animations, since it will force constant sample density everywhere.

Passes

The light cache is computed in several passes, which are then combined into the final light cache. Each pass is rendered in a separate thread independently of the other passes. This ensures that the light cache is consistent across computers with different number of CPUs. In general, a light cache computed with smaller number of passes may be less noisy than a light cache computed with more passes, for the same number of samples; however small number of passes cannot be distributed effectively across several threads. For single-processor non-hyperthreading machines, the number of passes can be set to 1 for best results.

Store

With this option, the light cache will also store and interpolate direct light. This can be useful for scenes with many lights and irradiance map or direct GI method for the primary diffuse bounces, since direct lighting will be computed from the light cache, instead of sampling each and every light. Note that only the diffuse illumination produced by the scene lights will be stored. If you want to use the light cache directly for approximating the GI while keeping the direct lighting sharp, uncheck this option.

Show

Turning this option on will show the paths that are traced. This does not affect the calculation of the light cache and is provided only as a feedback to the user. This option is ignored when rendering to fields - in that case, the calculation phase is never displayed.

Adapt

When this option is on, V-Ray will store additional information about the incoming light for each light cache sample, and try to put more samples into the directions from which more light coming. This may help tp reduce the noise in the light cache, particularly in the case of caustics.

Reconstruction parameters

These parameters control how the light cache is used in the final rendering, after is has been calculated.

Pre-filter

When this is turned on, the samples in the light cache are filtered before rendering. Note that this is different from the normal light cache filtering (see below) which happens during rendering. Prefiltering is performed by examining each sample in turn, and modifying it so that it represents the average of the given number of nearby samples. More prefilter samples mean a more blurry and less noisy light cache. Prefiltering is computed once after a new light cache is computed or loaded from disk.

Filter

This determines the type of render-time filter for the light cache. The filter determines how irradiance is interpolated from the samples in the light cache.

None	No filtering is performed. The nearest sample to the shaded point is taken as the irradiance value. This is the fastest option, but it may produce artifacts near corners, if the light cache is noisy. You can use pre-filtering (see above) to decrease that noise. This option works best if the light cache is used for secondary bounces only or for testing purposes.
Nearest	This filter looks up the nearest samples to the shading point and averages their value. This filter is not suitable for direct visualization of the light cache, but is useful if you use the light cache for secondary bounces. A property of this filter is that is adapts to the sample density of the light cache and is computed for a nearly constant time. The Interpolation samples parameter determines how many of the nearest samples to look up from the light cache.
Fixed	This filter looks up and averages all samples from the light cache that fall within a certain distance from the shaded point. This filter produces smooth results and is suitable for direct visualization of the light cache (when it is used as the primary GI engine). The size of the filter is determined by the Filter size parameter. Larger values blur the light cache and smooth out noise. Typical values for the Filter size are 2-6 times larger than the Sample size. Note that Filter size uses the same scale as the Sample size and its meaning depends on the Scale parameter.

Gloss

If this option is on, the light cache will be 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.

Notes

Do not set the Adaptive amount in the DMC sampler rollup to 0.0 when using the light cache, as this will cause excessive render times.
Do not apply perfectly white or very close to white materials to a majority of the objects in the scene, as this will cause excessive render times. This is because the amount of reflected light in the scene will decrease very gradually and the light cache will have to trace longer paths. Also avoid materials that have one of their RGB components set to maximum (255) or above.
If you want to use the light cache for animation, you should choose a large enough value for the Filter size in order to remove the flickering in the GI.