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Multiexcerpt
MultiExcerptNameSpecular Roughness

In some situations, increasing the Specular Roughness specular_roughness may help to reduce fireflies as it is effectively creating a larger, softer specular hi-light. Increasing the Specular Roughness of the Standard Surface specular_roughness of the standard_surface shader that is assigned to the boat reduces the number of fireflies. However, in this instance, there are still some fireflies evident when using this method.

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MultiExcerptNameglossy samples

Due to the almost mirror-like surface of the sea shader, we are getting extremely high Specular values reflected from the sun onto the Specular sea surface. These high Specular values are very difficult to sample and therefore, in this situation, increasing the number of Specular Samples specular_samples does not help to eliminate the fireflies.

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Multiexcerpt
MultiExcerptNamerayswitch

A better method is to assign a simpler shader for Specular and diffuse rays using a Rayswitch ray_switch shader.

  • Assign a RaySwitchray_switch shader to the boat. 
  • Connect a Standard Surfacestandard_surface shader to the Diffuse Reflection and Specular Reflection diffuse_reflection and specular_reflection rays of the RaySwitch ray_switch shader. Make sure that the Specular Weightspecular_weight is set to 0to 0. This will force specular and diffuse rays not to evaluate the specular component of the shader, which is the source of the fireflies.
  • Connect the boat's Standard Surface standard_surface shader (with Specular Weightspecular_weight) to the remaining attributes (camera, specular reflection, transmission, and shadow).

 

When we look at the AOVs we can see that the fireflies are not visible:

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Multiexcerpt
MultiExcerptNameClamp Sample Values
Another option is to clamp the output value of the pixels in the scene. This control will clamp pixel samples to the specified maximum value for all shaders in the scene.
The images below show the effect clamping has on the fireflies. A value of 1 eliminates the fireflies completely. However, it  it also removes any dynamic range from the render.  
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