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.
一幅图像的反向投影利用了其原始图像（或目标区域）的直方图，将该直方图作为一张查找表来找对应像素点的像素值，即将目标图像像素点的值设置为原始图像（或目标区域）直方图上对应的bin值。该bin值代表了（目标区域）上该像素值出现的概率。从而得到一幅图像的概率值。从而我们可以通过这幅概率图可以得知在这幅图像中，目标出现可能出现的位置。
原文地址：http://blog.csdn.net/lampqiu/article/details/38844231
