A couple of years ago, I worked on an implementation of Burley's Normalized Diffusion (a.k.a. Disney SSS). The original paper claims that the CDF is not analytically invertible. I have great respect for both authors, Brent Burley and Per Christensen, so I haven't questioned their claim for a second. Turns out, "Question Everything" is probably a better mindset.

I've been recently alerted by @stirners_ghost on Twitter (thank you!) that the CDF is analytically invertible. In fact, the inversion process is almost trivial, as I will demonstrate below.

The diffusion model is radially symmetric, and is defined as

$$ \tag{1} R(r) = \frac{A s}{8 \pi r} \Big( e^{-s r} + e^{-s r / 3} \Big). $$

The PDF must be normalized:

$$ \tag{2} \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} \frac{R(\sqrt{x^2+y^2})}{A} dx dy = \int_{0}^{2 \pi} \int_{0}^{\infty} \frac{R(r)}{A} r dr d\theta = \int_{0}^{2 \pi} \int_{0}^{\infty} p_r(r, \theta) dr d\theta = 1. $$

Therefore, in polar coordinates, the PDF is given as

$$ \tag{3} p_r(r, \theta) = \frac{R(r)}{A} r = \frac{s}{8 \pi} \Big( e^{-s r} + e^{-s r / 3} \Big). $$

The marginal CDF is then

$$ \tag{4} P(r) = \int_{0}^{2 \pi} \int_{0}^{r} p_r(z, \theta) dz d\theta = 1 - \frac{1}{4} e^{-s r} - \frac{3}{4} e^{-s r / 3}. $$

Let us define \(x = s r\). If we ask Mathematica to solve \(y = P(x)\) for \(x\) in order to obtain the inverse \(x = P^{-1}(y)\), we get a somewhat terrifying (but, of course, correct) output:

The trick is to solve \(P(x) = 1 - u\) instead (where \(u = 1 - P(x)\) is the complementary CDF):

To get rid of the imaginary part, we can set the free constant \(c_{1} = 0\), which results in the following inverse:

$$ \tag{5} x = s r = 3 \log{\Bigg(\frac{1 + G(u)^{-1/3} + G(u)^{1/3}}{4 u} \Bigg)}, $$

where

$$ \tag{6} G(u) = 1 + 4 u \Big( 2 u + \sqrt{1 + 4 u^2} \Big). $$

We can verify that it works by feeding the complementary CDF with its inverse.

For importance sampling, we can uniformly sample either the complementary or the regular CDF - it makes no difference.

## Acknowledgments

I would like to thank @stirners_ghost for informing me that the CDF is invertible, and Brent Burley for spotting a missing minus sign in the derivation.