[android-developers] vignette formula

I noticed the vignette effect on Android looks really good.

In the code, it boils down to this surprising formula:

float lumen = shade / (1.0 + exp((dist * inv_max_dist - range) * slope)) + (1.0 - shade);

Anyone know where this formula comes from?  I was definitely surprised to see a cameo from the irrational number e.

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