Light Cache Settings

This page provides information on the Light Cache rollout in the Render Settings.

Overview

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This rollout is accessible when Light cache is selected as the Engine for Primary bounces or Secondary bounces in the GI rollout. See the Light Cache GI topic for more information on how this option works.

 

Light Cache Parameters

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

Preset – Offers default Light cache settings for either Still or Animation rendering. 

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 will be traced from the camera). For more information, see The Subdivs Parameter example below. 

Sample size – 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 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. For more information, see The Sample Size Parameter example below.

Show calc. phase – Enabling this option 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.

Store direct light – When this option is enabled, 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.

World Scale – Determines the unit scale for the Sample size and Filter size parameters.

When enabled, sizes are 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.

For more information, see The Scale Parameter example below.

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 will produce too large an error. This is especially obvious with 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.

Mode

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Mode – Determines the rendering mode of the light cache:

Single frame – Computes a new light cache for each frame of an animation. 

 

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Example: The Subdivs Parameter

The Subdivs parameter controls the number of rays that are shot into the scene, and the "noise" quality of the light cache samples.

Here is a scene rendered with different settings for the Subdivs parameter, all other settings are the same.

As we add more samples, the noise is reduced, but the render times increase.

 

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Example: The Sample Size Parameter

The Sample size parameter controls the size of the individual light cache samples. Smaller values produce a more detailed lighting solution, but are noisier and take more RAM. Larger values produce less detail, but take less RAM and may be faster to calculate.

Here is a scene rendered with different values for the Sample size parameter. All other values are the same.

Note the difference in the noise level between the samples.

 

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Example: The Scale Parameter

The World Scale option determines whether the Sample size and Filter size parameters are in screen space, or world space.

In the table below the World Scale checkbox is not enabled, meaning that the Screen scale is used, and the scene was rendered with different image and world sizes. The Sample size was constant (the default value of 0.02) for all images.

As you can see, we always get the same number of light cache samples for all cases, regardless of resolution or scene size - in fact, when scaled to the same size, the images look identical. This is why the Screen Scale is applicable to a large variety of scenes.

In the table below, the World Scale mode was used. Again, the scene is rendered at three different resolutions and scales, but with the same light cache parameters. The sample size was set to be 1/6th of the sphere radius in the scene at normal scale.

You can notice that the number of samples again does not depend on the image resolution, but it does depend on the scene size.
Also notice how the samples near the camera appear larger because of the perspective - in comparison to the Screen scale mode, which compensates for this effect and tries to make the samples with more or less equal size on the image.

 

 

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Example: Retrace Threshold

 

The first set of images shows how the Use retrace threshold option alongside the Retrace threshold value can be used to reduce light leaks due to the light cache interpolation. The scene is an interior with parts of the exterior visible. For the GI settings, Brute Force is used for primary bounces (it is selected as the Primary engine in the GI tab of the Render Settings) and Light cache is the Secondary engine. 

 

The bright light cache samples from the exterior blend with the darker samples from the interior, causing light leaks when the light cache is calculated. The Use retrace threshold option (with the default Retrace threshold value of 2.0) successfully resolves the problem at the expense of increased calculation time for the light cache. When disabled, corners in the scene produce light leaks and inaccurate shadows. This is visible in the shelving unit and on the ceiling's adjacent wall. When enabled, light leaks are successfully eliminated at the expense of longer calculation time.

 

The second scene in this example shows how the Use retrace threshold option can be used to improve the appearance of glossy reflections and refractions with the Use for glossy rays option. In this case, V-Ray dynamically decides whether to use the light cache or not, based on the glossiness of the surface and the distance from it.

Without retracing, the light cache samples are clearly visible in the glossy reflections and refractions.

In this third example, when Use retrace threshold is turned on, the Retrace threshold value determines whether the brute force will be used instead of light cache near corners or objects that are close together. When the Retrace threshold is greater than 0, only brute force is used, which helps give a far more accurate result, although this may slow down rendering. However, if the Retrace threshold value is 0.0, then only the light cache will be used and thus, retracing is disabled. This will be faster but less accurate and more prone to noise and artifacts from light leakage.

 

Notes

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