Out, d*mn spot!
The NOAA weather radar images from the National Weather Service have a reflectivity key on the side. To get rid of the ground clutter & other low intensity noise, just remove the colors from the image corresponding to returns below a certain threshold dBZ. It sounds like it ought to be simple, but they threw a small monkey wrench into things.
Here are a copy of the key clipped from the large composite map along with a part of the transparent radar-only composite on a checkered background. If everything at or below 5 or 10 dBZ is removed, most of the noise goes away, but the interesting weather remains. However, the composite national image does not consist of the pure colors shown in the key. Areas of the “same” color consist of a smattering of pixels that have almost the same value. It’s easy for your eye to see them as identical, but not quite as simple for a computer when comparing bits.
I entertained the idea of looking at the colors in terms of hue, saturation, and lightness instead of red, green blue. All pixels above a certain brightness and/or saturation could be removed. Those would correspond to white and grey areas, and to the light blue/cyan areas (below 15 dBZ) if the parameters are chosen carefully. Some initial tests looked promising, but then I discovered that the graphics tools I use (ImageMagick and GraphicsMagick) have a method that seems custom-made for this purpose — a method to turn transparent all pixels in an image of a given color, with a fuzz factor. The fuzz factor allows for matching all pixels close to the given color, and that’s just what is needed.
That’s enough of the bit-twiddling details — see the library documentation and my utility’s source code for the nitty-gritty. Lets look at the results.
