Adaptive Light Sampling

Adaptive light sampling is a mode of light sampling where the most important lights are preferentially sampled for each intersection. You can toggle this mode on using the following scene variables:

SceneVariables {
    ["light_sampling_mode"] = "adaptive",
    ["light_sampling_quality"] = 0.6 #see the section below on how to choose this value
}

Uniform vs. Adaptive Light Sampling

Uniform

The default light sampling mode in MoonRay is “uniform”. This means that:

For each pixel sample in pixel_samples^2 we take light_samples^2 samples from every light in the scene.

So, we sample all lights equally, regardless of their position and orientation in the scene. This leads to inefficient light sampling, because we spend time drawing light samples that contribute comparatively little to no radiance.

Adaptive

The more intelligent method is “adaptive” light sampling, where

For each pixel sample in pixel_samples^2 we take light_samples^2 samples preferentially, from the most important lights.

This generally leads to more efficient light sampling, because we focus our time on sampling the lights that matter most. That said, there is some overhead associated with choosing the ideal lights, so adaptive light sampling typically performs best on scenes that have a lot of lights.

What makes a light “important”?

How do we determine whether the light is likely to contribute more to the radiance than other lights? There are several factors that play a role:

• Energy (brighter lights are more important)
• Distance (closer lights are more important)
• Orientation (lights that face the point (and whose normal is facing the light, in turn) are more important)

Light Sampling Quality

How do we determine the threshold at which a light is considered “important”? We have a user-supplied value that acts as this threshold, which we call "light_sampling_quality". This is a value between 0 and 1, where 0 represents the strictest adherence to importance (very few lights are sampled), and 1 represents the loosest adherence to importance (all lights are sampled). We call this threshold "light_sampling_quality" because the higher the threshold, the more lights that are sampled and the higher the render quality, at the expense of performance.