How is it non-differentiable? They explicitly point out that after prefiltering (convolution with a pixel filter), it is differentiable. Moreover, these deriva

How is it non-differentiable? They explicitly point out that after prefiltering (convolution with a pixel filter), it is differentiable. Moreover, these derivatives can be obtained via methods like Automatic Differentiation, so they are true derivatives, not finite difference approximations: https://en.wikipedia.org/wiki/Automatic_differentiation https://en.wikipedia.org/wiki/Automatic_differentiation