October 17, 2022 Hana Trieu

Reduce noise to optimize renders in Cycles for Blender (Part 2)

When performing a final render, it is important to reduce noise as much as possible. In the previous article, we learned some tricks to reduce noise to optimize renders in Cycles. Today, we will go through some more ones. Let’s explore!

Some tricks to reduce noise for optimizing renders in Cycles

5. Light falloff

In a vacuum, light always falls off at a rate of 1/(distance^2). In reality, light in a vacuum always decays at the rate of 1/(distance2). However, as the distance decreases to zero, this value increases to infinity, resulting in very bright spots in the image. These are mostly a problem with indirect lighting, as the probability of hitting such a small but incredibly bright point is low and seldom occurs. This is a common recipe for Fireflies.

Source: Blender Documentation (docs.blender.org)

To reduce this effect, you can use a Smooth factor in the Light Falloff node to reduce the maximum intensity a light can contribute to near surfaces. The images above demonstrate the default falloff and smooth value of 1.0.

6. Multiple importance sampling

Materials with emission shaders can be configured to utilize Multiple Importance Sampling (Material Settings). This means that rays will be sent directly at them rather than rays randomly bouncing around them. For very bright mesh light sources, this can significantly reduce noise. However, this will take samples away from other brighter light sources that must be found in this manner when the emission is not particularly bright. 

The best setting here is difficult to predict; it may be a matter of trial and error. However, it is often clear that a slightly glowing object may only contribute light locally, whereas a mesh light utilized as a light would require this option to be enabled. Let’s look at the below example images where the emissive spheres contribute slightly to the lighting.  By disabling Multiple Importance on the emissive spheres, the image renders with slightly less noise. 

Source: Blender Documentation (docs.blender.org)

The world background also has a Multiple Importance (Settings) option. This is mostly useful for environment maps that have small bright spots in them, rather than being smooth. This option will then, in a preprocess, determine the bright spots, and send light rays directly toward them. Enabling this option, once again, may take samples away from more essential light sources if it is not required.

7. Glass and transparent shadows

Glass shadows may look too dark when caustics are disabled. On the other hand, caustics may appear too soft when the filter is glossy. We can create a glass shader that uses a Glass BSDF when viewed directly and a Transparent BSDF when viewed indirectly. The Transparent BSDF can be used to identify light sources directly through surfaces and produce properly colored shadows without caustics. The Light Path node determines which of the two shaders to use.

Below we will see the node setup for the glass transparency trick.

We can watch the two following images. The render on the left has dark shadows because of missing caustics. The right image has applied the trick.

Source: Blender Documentation (docs.blender.org)

8. Light portals

When rendering a daylight indoor scene where the majority of the light comes in from a window or door opening, it is hard for the integrator to find its way to them. Light Portals can help with this. You will then need to adjust its shape to match that of the opening you are attempting to fill.

9. Denoising

Even with all of the above settings, there will always be some render noise, no matter how many samples you utilize. To fix this, however, you can use a post-processing method to eliminate the remaining bit of noise. To use this method, enable Denoising in the Properties’ Render tab. Let’s  see the denoising effect in the following images (rendered by The Pixelary)

Source: Blender Documentation (docs.blender.org)

10. Clamp fireflies

Ideally, we can apply all of the above tricks to eliminate fireflies. However, they can still happen. When that happens, we can clamp the intensity that any individual light ray sample will deliver to a pixel to a maximum value using the integrator Clamp setting. Setting this value too low can result in losing highlights in the image, which may be necessary to keep for camera effects like bloom or glare. To fix this problem, it’s typically helpful to clamp just indirect bounces, leaving highlights that are directly visible to the camera untouched.

Above are all important tricks to reduce noise in Cycles that iRender would like to share with you. Hope you find them useful to optimize your final animation renders.

Source: Blender Documentation (docs.blender.org)

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Reference source: Blender Documentation

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