CycleAcoustics is a sound design addon for Blender artists, music producers, filmmakers, and more. It emulates sound reflections and transmission using Cycles raytracing to generate stereo reverb impulse responses from Blender models!

CycleAcoustics, a portmanteau of Cycles and psychoacoustics, actually has a psychoacoustic processing feature in its full edition.

Video Guide

Under the Hood: How to Accumulate Values Across Bounces in Blender Cycles Shaders

To render reverb reflections, CycleAcoustics materials use the Glossy BDSF shader node. Glossy materials alone are is far from sufficient to generate reverb impulse responses. Just reflecting light from an emitter to a camera gives you how bright, or how loud, a ray of light/sound is, but it doesn’t tell you how long it took for the sound to travel from the emitter to the listener. Both amplitude and timing are vital for making impulse responses.

How can we keep track of more information than luminosity in Cycles raytracing? We can "encode" other types of information into the red, green, and blue color channels. For example, the red and green color channels may be used to represent how loud a sound is, and the blue color channel may be used to store timing information. This would be done using the Light Path node:

The Light Path node’s "Ray Length" output gives the length of the last light ray in Blender units, which we can then divide by the speed of sound (343 meters per second) to calculate travel time. But there’s a problem: "Ray Length" is only the length of the ray since the last bounce, and the Light Path node provides no way to get the length of the whole traced path.

The Math Trick Behind CycleAcoustics

Every time light bounces off of a glossy material in Cycles, the ray’s color gets multiplied by the reflection color of the material it hit. For instance, if a white light that has a brightness of 2 (R=2, G=2, B=2) hits a bright blue surface (R=0, G=0, B=1), the light that gets reflected has the RGB values (R=0, G=0, B=2).

The way to accumulate values across more than one bounce in Cycles is to use exponents and logarithms to "add numbers by multiplying them".

To sum the Ray Length of every path segment to calculate the total path length, we can make one of the channels of the surface’s color a constant number (such as 0.95) raised to the power of the Ray Length. The light that comes from the emitter is pure white with a brightness of 1, so it has 1 in its blue channel. Each time the light bounces, the blue channel’s value gets multiplied by 0.99 raised to the power of the Ray Length.

Example: a ray travels 5 meters, bounces, travels 3 meters, bounces, and travels 1 meter.

0.95^5 * 0.95^3 * 0.95^1 = 0.6302494097

Then we "decode" the resulting value of 0.6302494097 by performing a logarithm on it with 0.95 as the log base.

log0.95(0.6302494097) = 9.0

The number that comes out of the logarithm operation is 9, which is the same as the sum of the distances of the path segments (5 + 3 + 1). And that’s how you sum values across bounces in Cycles.

What CycleAcoustics’s Materials Track

CycleAcoustics uses 2 render layers—each render layer has different material settings—to store the information it needs in 6 color channels.

Render Layer #1, red: amount of low-frequency sound energy
Render Layer #1, green: amount of high-frequency sound energy
Render Layer #1, blue: total travel distance
Render Layer #2, red: number of bounces
Render Layer #2, green: amount of diffusion (keeps track of how much a sound wave was "diffused" by bouncing off rough surfaces like carpet and grass)
Render Layer #2, blue: constant

The second render layer’s blue channel stores the color resulting from white light bouncing off perfectly white surfaces. What’s the purpose; wouldn’t it always be 1? No. Cycles multiplies all of the color channels of a light path by a factor based on the probability that the light takes the path. As a result, the second render layer’s blue channel’s value may come out being less than 1. This value is used in the compositing step to correctly calculate travel distance, bounce count, and diffusion.

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