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These individual GI points are called "Irradiance Cache Points" and are using during rendering (through interpolation and filtering) to produce a smooth, final result. The figures below show how Irradiance Caching works.


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During the irradiance cache computation pass, points are stored on surfaces visible by the camera. On flat areas, lighting changes slowly so few points are needed. Corners typically need more points.



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During rendering, final shaded pixels (white) are GI-lit using the previously constructed irradiance cache points. That is much faster than computing GI for each individual pixel.

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  • Irradiance Cache Points are computed during a separate rendering pass so interactive feedback isn't possible.
  • While a low number of rays shows up as grain in brute-force GI, a low number of rays shows up as "splotches" and flickering in animations with irradiance cache. These artifacts can be more visually distracting than grain.
  • There are several user parameters that control point spacing and the overall quality. Proper configuration of these is important for splotch-free and flicker-free results. Learning how to use these settings will involve some trial-and-error.
  • If the scene contains a lot of geometric detail (example: foliage covering a big part of the screen), too many irradiance cache points might have to be generated. This cancels the benefits of the irradiance cache algorithm and, even worse, places a burden on GPU memory resources because of the many points which will have to be stored. For these scenes, brute-force might be a better choice. Thankfully Redshift contains a "force-brute-force" per-object option so that such problematic objects can use brute-force instead, while everything else uses the irradiance cache.

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Mode/Filename

When "Mode" is set to "Rebuild (don't save)", Redshift will compute a new irradiance cache from scratch (for each frame) but will not save it to disk. The frame will be rendered to completion.

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Info
If you're getting irradiance cache flickering issues, we recommended rendering a few frames with and without reflections/refractions. You can globally enable/disable reflections/refractions in the Redshift optimizations Globals tab. Please ensure that you enable the "use separate points for secondary rays" option only if you are see flickering with reflections/refractions enabled and no flickering with reflection/refraction disabled!

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Scene uses a "Min Rate" of -3 and a "Max Rate" of -1. The max rate being -1 is the reason why there are lighting artifacts around the thin geometry

Increasing the max rate to 0 solves these issues. Similarly, if you get artifacts with a "Max Rate" of 0, you could increase it to 1.


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Using only 100 rays. Splotches visible on wall corners and between the wall blades. Irradiance cache computation time: 1.5 seconds.

Using 2000 rays. Splotches are improved a lot but the still some faint artifacts on the walls. Irradiance cache computation time: 2.4 seconds.


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The problem here is that the default adaptive threshold of 0.01 is preventing the system from using enough rays. Halving it to 0.005 uses more rays but helps produce a clean result. This is shown below.


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For a near-perfect frame we decrease "Adaptive error threshold" to 0.005. Irradiance cache computation time: 3.8 seconds.

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"Adaptive Amount" set to the default 0.85. Visible artifacts can be seen on the bottom half of the image

"Adaptive Amount" set to 0.5. Although there are still some faint issues, this is a big improvement over the 0.85 example. This comes at the cost of more initial rays, though.


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Brute-force rendering. Notice the well-defined shadows.

Irradiance caching using default values. The shadows are blurred to the point of being completely invisible.

Irradiance caching with blurring passes disabled, radius factor was left at 2.0, color threshold was reduced to 0.001. The shadows are now more visible.


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Default Values with Distance Threshold set to "Medium" (which is also the default)

Default Values With Distance Threshold set to "High". There is only a slight improvement near the base of the pillars.


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