Max's Heightmap script is not easy for me to understand.
g_imageData refers to the image pixel data.

He is getting the average of 3 colors of 1 pixel.

dX and dY are used.
?

Ed's links look promising.
I'll try to read them after work.
Nodeeditor uses a canvas, which might help.

I know little about MoI's image command and how pixels correspond to Geometry points.

- Brian