In my efforts to make the shadows soft, based on the formulas for penumbras, I finally realized that my method does not support physically correct penumbra softness, but I will explain the reason for this when I do the tutorial.
Until then, here’s what I came up with, after applying some Gaussian blur to the final shadow image. (variable degree of softness)
Even if it’s not physically correct, I still think the softness looks good enough.
There’s also an interesting looking result when the radius of the Gaussian Blur depends on the distance from the light…