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Clarisse 4.0 User Guide

Physically-based renders can exhibit high contrast pixels that seem to be scattered randomly over the image. These bright spots are commonly called fireflies which are rendering artifacts resulting from paths that gives a poor estimation of the indirect illumination. For forward path tracers like Clarisse, these difficult paths are generally paths ending near light sources or caustics.

 

Example of fireflies in a render

Example of fireflies in a render

 

It's important to distinguish fireflies from general noise since they require a very large and impractical number of samples to be eliminated. More importantly, setting very large numbers of samples doesn't always solve the issue and worse, this can introduce new fireflies. Another important thing to consider is that these paths are likely to exhibit high contrast noise when the number of bounces increases.

 

Cornell box rendered with 1 bounce of indirect illumination

Cornell box rendered with 1 bounce of indirect illumination

Fireflies and high contrast noise visible with 10 bounces of indirect illumination

Fireflies and high contrast noise visible with 10 bounces of indirect illumination

 

The approach widely used by renderers to eliminate fireflies is clamping the energy of all indirect paths on an arbitrary maximum value specified by the user. This biased method is simple and effectively reduces the number of samples needed to eliminate these artifacts. However, not only aggressive clamping introduces obvious diffuse, reflection and transmission inconsistencies between direct and indirect illumination but also, it doesn't guarantee the elimination of fireflies.

 

Clamping the energy to the extreme value of 1 doesn't eliminate fireflies in this render

Clamping the energy to the extreme value of 1 doesn't eliminate fireflies in this render

 

Clarisse path tracer is able to detect and eliminate paths that are likely to produce fireflies. Instead of clamping indirect illumination based on an arbitrary maximum value, the user can control the threshold of the firefly detection using single attribute called Fireflies Filtering.

 

Clamping disabled and Fireflies Filtering set to 25%. Note that all fireflies have been removed.

Clamping disabled and Fireflies Filtering set to 25%. Note that all fireflies have been removed.

 

Compared to Sample Clamping, Fireflies Filtering dramatically reduces noise coming from indirect illumination when setting values beyond the default 25%. Another thing to take into consideration is that Fireflies Filtering is consistent with Light Path Expressions.

 

64 spp no clamping

64 spp no clamping

64 spp no clamping and fireflies filtering set to 25% (default)

64 spp no clamping and fireflies filtering set to 25% (default)

64 spp and Fireflies Filtering is set to 50%. All rendering aretacts are gone.

64 spp and Fireflies Filtering is set to 50%. All rendering aretacts are gone.

For reference, here is the same 64spp render with a Sample Clamping set to 10 (default).

For reference, here is the same 64spp render with a Sample Clamping set to 10 (default).

64spp with a super aggressive Sample Clamping value of 1. The render exhibits more noise than the render with Fireflies Filtering set to 50%

64spp with a super aggressive Sample Clamping value of 1. The render exhibits more noise than the render with Fireflies Filtering set to 50%

64spp and Fireflies Filtering set to 100%. The render exhibits even less indirect illumination noise

64spp and Fireflies Filtering set to 100%. The render exhibits even less indirect illumination noise

 

It is important to note that high values of Fireflies Filtering can introduce a potential loss of indirect illumination energy since filtering these noisy samples also reduces their contributions from the final integration. However, unlike clamping this loss of energy is reduced when the number of samples increases and gets closer to the unbiased result.

 

In the next example, the image is rendered with 16spp with an aggressive Fireflies Filtering value of 100%.

 

16spp Fireflies Filtering 100%

16spp Fireflies Filtering 100%

 

As you can see the image exhibits very little noise despite its low number of samples. However, it also exhibits a slight loss of indirect illumination energy when compared to the ground truth reference. Fortunately, as we will see this bias is reduced when the amount of samples is increased.

64spp Fireflies Filtering 100%

64spp Fireflies Filtering 100%

128spp Fireflies Filtering 100%

128spp Fireflies Filtering 100%

256spp Fireflies Filtering 100%

256spp Fireflies Filtering 100%

512spp Fireflies Filtering 100%

512spp Fireflies Filtering 100%

 

As you can see, rendering the image from 128spp to 512spp doesn't recover any missing energy so results are predictable with a somewhat small number of samples even with such aggressive value.

 

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