I would like to be able to open standard (most likely grayscale) image formats (jpg especially, but also tiff, png or gif) for viewing and have an interactive cursor that could be moved over the image. The cursor would display the grayscale intensity modified by some simple algebra of the pixel(s) under the cursor. The parameters for the modification would be user supplied, either in response to questions about parameters on start of the program, or as fill-in boxes in the interface. I would need to be able to vary the zoom level.
I have no idea how of how difficult this is and have not been successful with simple searches to find programs that do it. I did find a tool in Photoshop that reports the %K value, which would be a start, but hardly an economic one. I don't think there is, or a could not find, the same capability in GIMP, but I am even less experienced in GIMP than in Photoshop.
This would be for a Windows platform, say Win XP and later. Image sizes would vary widely, from small at about 60KB to very large, potentially multiple GBs.
Your thoughts and suggestions would be appreciated.