This 'making of' tutorial covers lighting and rendering volumes as well as some other issues when encountering certain kinds of noise in the render. This scene has the potential for 'fireflies' to appear as it includes small visible light sources, highly specular surfaces, and volume scattering.
The scene is lit with an 'Arnold Area Light' with its Light Shape set to 'Disk' (moonlight). The UFOs lights are geometry that has been converted to mesh lights. There is also a spotlight pointing underneath the UFO and an area light on the right side. Volume Scattering has been enabled in the MtoA Render Settings window.
More information about rendering volumes as clouds can be found here.
Cloud shader assigned to a sphere that represents the cloud layer (rollover image).
Once rendered, the clouds should look like the image below:
Disk light representing Moonlight with Atmosphere Scattering enabled (rollover image).
This scene suffers from a form of sampling noise commonly referred to as 'spike noise', or 'fireflies'. These are isolated, super bright pixels that jump around from frame to frame in an animation. This noise is very difficult to remove by simply increasing the number of samples in the renderer.
Fireflies appearing in the render (concentrated around the light and glossy specular metallic UFO material).
This scene has some contributing factors to noise such as highly specular surfaces, atmosphere scattering, and small light sources. We will look at ways of fixing these 'noise' issues below:
The UFOs light's consist of geometry that has also been converted to a Mesh light. When a mesh light is created, it is positioned in the same place as the geometry from which it was generated. This can be a problem when sampling light rays and shadow noise may appear. One way round this is to turn off 'Casts Shadows' and 'Receive Shadows' for the geometry. Another solution is to move the light away from the object (see images below).
Another way to reduce fireflies is to make the mesh invisible to specular glossy rays. This can be achieved with the Ray Switch shader. This method gives you control over any shaders that could be contributing towards the appearance of 'fireflies'. For example:
An alternative method to reduce noise is to disable Diffuse Reflection and/or Specular Reflection attributes for the mesh that is causing the noise. These attributes can be found in the Arnold attributes of the mesh.
One option to reduce noise is to clamp the sample values of the rendered pixels. However, this will clamp the pixels for the entire image. One way around this would be to clamp the pixels for a particular shader. To do this:
Clamp node connected to Surface material attribute of the Shading Group of an Ai Standard shader
The shading network should look like this:
Try lowering the 'Max' value in the 'Clamp Color Attributes'
In the example below, the specular attribute of an Ai Standard has been connected to a Maya clamp node. The clamp node is then connected to the glossy attribute of the Ai Ray Switch shader. It will work as a secondary ray clamp instead of the general clamp in Render Settings window. This way you get the benefit of keeping your range in the direct highlights, but you can also control any secondary ray noise.
In the image below you can see the effect that enabling Atmosphere Volume has on the scene.
Enabling Atmosphere Volume has introduced some 'fireflies' on the ground material. This material has some Specularity which is contributing to the fireflies. This is more noticeable in the foreground of the image. A simple way to reduce the 'fireflies' would be to reduce the amount of Specularity on the ground material. If this is not possible, increasing the Specular Samples to 4 helps to remove the fireflies.