http://en.wikipedia.org/wiki/Histogram_equalization#Back_projection

Backprojection

The back projection (or" project") of a histogrammed image is the re-application of themodified histogram to the original image, functioning as a look-up table forpixel brightness values.

For each group of pixels taken from the same position fromall input single-channel images, the function puts the histogram bin value tothe destination image, where the coordinates of the bin are determined by thevalues of pixels in this input group. In terms of statistics, the value of eachoutput image pixel characterizes the probability that the corresponding inputpixel group belongs to the object whose histogram is used.

反向投影的意义