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

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Example: The Sample Size Parameter


The 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.

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Note the difference in the noise level between the samples.

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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

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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.

 

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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.

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When enabled, light leaks are successfully eliminated at the expense of longer calculation time.

 

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Close Up

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. 

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

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 If a higher Retrace threshold value is used, while light leakage and artifacts disappear, brute force would be used for a larger area and, as a result, the render may appear more noisy. Therefore the default value of 2.0 is sufficient in most cases.

 

 

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Notice how the light leaks disappear around the window frame and shadows in certain areas appear darker.
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As a result only the light cache has been used, which was faster, but produces artifacts near corners or in places where objects are close to each other.

 

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Notes

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  • 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.
  • There is no difference between saved light caches computed for primary bounces (direct visualization) and for secondary bounces. You can safely use light caches computed in one of these modes for the other.
  • Similar to the photon map, you can get "light leaks" with the light cache around very thin surfaces with substantially different illumination on both sides. Sometimes it is possible to reduce this effect by assigning different GI Surface ID's to the objects on both sides of the thin surface (see the VRayObjectProperties); the effect can also be reduced by decreasing the Sample size and/or the filtering.

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