This is a demo from late 2009. It’s an extension of the single-pixel eyedropper I wrote previously.
It’s powered by an early version of an old JS toolkit I wrote called JSImage. The latest version is available at my JSImage github repo. Don’t be fooled by the 2011 commits, those are just artifacts from svn->git migration. No guarantees that the histo’s are actually correct. :)
Click and drag to set the image border to the average of the selected pixels.