Yes, this is new to me also.

I was referring to ImgSampler by Wayne Hill, Jan 2020, a node. (As per Michael's links)
The nod imgSampler.js code can be examined in notepad++, and the relevant portions utilized in a regular MoI script.
That is, if we can understand what is going on. It also refers to Heightmap by Max, so maybe just look at Heightmap?

I'll study some more tonight.

- Brian

Is calculus really needed to do the luminance?