Sorry for taking a while to get back to you. The reason I used the surface closest point to get the uvP was that it was the quickest/simplest way I could think to get the the uv parameter for all points inside of a single uv. domain; in this instance between 0 and 1.
You may have noticed that I changed the domain of your image so that the XY domains were also set to 0 To 1. So inputting any uv parameter within this domain will give the corresponding pixel value.
Yes, using the uv output from the surfaces provides parameters to evaluate the 2D image. It doesn’t necessary need to be a uv system, and it doesnt need to be with in a domain of 0 To 1.
For example, setting your image to an XY domain of 0 To 100, and inputting grid of 100x100 would give the following.
With the same domain, but a grid of 50x50 you would only get a quarter of the image.
You could set the domain of the image to the pixel dimensions if you wanted to evaluate every pixel.