It's always interesting to me to see how image processing operators get applied to terrain systems to generate interesting results (in this case, an HDR tone mapping operator of the sort described in Local Laplacian Filters: Edge-aware Image Processing with a Laplacian Pyramid - it's a goo dpaper because of the review of lots of other tone mapping algorithms). I wonder if pushing alpha high, thresholding the image and using that output as input to a distance algorithm would yield interesting results...