cropCompare lets you define two zoomed-in windows on your image to enlarge, visually compare alongside one another, and even graph the values of. It's designed to assist in matching the black levels of two disparate elements in a comp, but it can be used for all kinds of image analyses.
Add the cropCompare node downstream of your comp and view it. You'll see four position controls and two little windows in the lower right of the image.
You have separate Show controls for Crops 1 and 2. For each Crop, you define its location and Sample Size and choose where on the image to overlay it and how big. Put them next to each other and maybe bump up the Gamma the Crops control to make sure your values are matching.
Or, to really make sure, turn on Do Graphs. You now have an RGB graph of the centerline of each Crop. Want to see those graphs unobstructed? Check Do Graphs and Only together.
For the graphs to look their best, please enable Fusion's HiQ mode.
Please give it a try and let me know what you think!
Download O-cropCompare_v2.2 (8kb .rar file)