This article will show you how to deal with scenes having too much noise or fireflies. Most of the time this happens when the scene setup is wrong. Here are the most important sources of noise:

  1. Colors in the Diffuse Channel that exceed safe zones.
  2. Point Lights having soft-radius enabled may intersect with geometry.
  3. Light going through Transparent or Translucent surfaces.
  4. Mesh lights having a high amount of polygons.
  5. Lights near brightreflective surfaces.
  6. Tips for Debugging scenes.

Let’s see them one by one and try finding the best ways to fix them.

1. Colors in the Diffuse Channel that exceed safe zones.

This limitation only has to do with the Diffuse Channel of the Basic Layer.
Pure white or values higher than the ones inside the safe zone can be used to the rest of the channels (reflectance, transmittance, etc)

If you take a better look at image 1a, you will notice two dashed lines. The lower dashed line defines the highest reflectance (86%) that can be found in everyday products or paints. It is safe to always have this limit in mind when creating materials. Any value that is in between the two dashed lines is for special cases like snow for example.

Values above the upper one may give unwanted results like low-contrast images and higher render times. In Image 1b a value of 255,255,255 has been given to the Diffuse Channel leading to infinite light bouncing due to the unrealistic brightness of the surface.

Both images have been rendered with Unbiased TR1 for 10min.

Image 1c was able to reach a higher amount of samples just by moving the color to a value of 220,220,220 (1st dashed line).

Thea for SketchUp material presets automatically adjust color values and image brightness.

2. Point Lights having soft-radius enabled may intersect with geometry.

Point lights in Thea (Omni, Spot, IES and Projector) have a Soft-Radius parameter that controls the softness of shadows. Soft-Radius controls the size of an invisible virtual sphere positioned in the light’s source. Noise can be introduced when this virtual sphere intersects with geometry in the scene as we see it in the example below.

To eliminate noise, always take note of the value used in soft-radius and have it in mind when placing the light.

Thea for SketchUp by default moves point lights 1 inch away from the point of creation. Read the article here.

3. Light going through Transparent or Translucent surfaces.

Whenever possible, you should avoid trying to light a scene using lights behind translucent surfaces. You should always try to make things as simple as possible. In the following example, we have simplified the light fixture resulting in lower render times.

On the 1st image, we have 1 rectangular light above each white translucent surface that lights the whole scene. We used Presto MC to render the scene at 1024 samples but the final image had noise.

In the 2nd image, we changed 3 things:

  1. Removed the emitters behind translucent surfaces.
  2. Converted the translucent material to an emitter one.
  3. Used a radial gradient map to the emitter so that it looks like it is being lightened from the back side.

When dealing with flat and simple transparent surfaces, like windows, you have two options:

  • Use a Thin Film layer that does not take Refraction into consideration, so there will be no indirect caustics in the scene.
  • Use a Glossy layer and force the geometry to not cast shadows (uncheck Shadow Caster option in Object Properties)

Image 1 uses a Thin Film layer.
Image 2 uses a Glossy layer with the Shadow Caster option enabled. The reason you see noise in the rendered image is due to the fact that we now have Indirect Caustics lighting the interior that are hard to solve.
Image 3 uses a Glossy layer with Shadow Caster disabled. We do not have Indirect Caustics because the transparent surface is not being taken into account.

4. Mesh lights having a high amount of polygons.

Mesh lights having a high amount of polygons will need more time for the noise to converge. In the following example, we rendered the same scene with Presto MC at 2048 samples.

As you may notice, both images reached the same amount of samples and they look nearly the same. What is most important here is the fact that the 1st image used high-poly mesh lights and needed 16m24s to complete while the 2nd one finished in 14m34s just by reducing the polycount of the mesh lights.

Considering that this is a simple scene, in complex scenes, using simple geometries for mesh lights will help noise clearing faster.

5. Lights near Bright & Reflective surfaces.

Lights that are very close to bright & reflective surfaces will result in added noise in the scene due to high-energy light particles originating from the light and spread throughout the scene. The only way to avoid these situations is by moving the light away from the surface to the point that there are no more bright pixels.

6. Tips for debugging scenes.

Below you will find general tips you can use in order to find the objects (lights, materials, geometries) that introduce noise while rendering.

  • Enable Clay Render mode. In this way, you will be able to know if noise is coming from materials.
  • Isolate lights to find the ones causing noise. Giving proper names to objects and materials will always make this step easier.
  • Check all materials created with Basic Layer for colors going above the safe values.
  • Hide geometry from the scene to the point where you still have noise issues. This will help you narrow down any bad geometry/light/material.