From 2011:
https://moi3d.com/forum/index.php?webtag=MOI&msg=4238.2
4238.2 In reply to 4238.1
Hi Janne - the background images displayed in a viewport have to be generated as a texture that is sent to your video card, and it used to be common for many video cards to have a limit on the size of textures that they supported and to also only have a limited amount of space available on them to hold the textures.

So, what is "texture", versus luminance.
A guess would be that "texture" does not have any luminance value?
Can the image "texture" below a point in MoI screen be directly acquired? Would it be of any use as "luminance" gradient?

If so, converting point coordinates from MoI, to pixel coordinates in canvas image, would be required to work from image in MoI?

Or else do all work in nodeeditor, on nodeeditor canvas.
?