http://www.amazon.com/Learning-OpenCV-Computer-Vision-Library/dp/0596516134/ref=pd_bxgy_b_img_c#_
http://www.imagingbook.com/index.php?id=98
http://bubble.ro/How_to_create_the_histogram_of_an_image_using_PHP.html
http://www.softintegration.com/products/thirdparty/opencv/demos/
http://www.leavethatthingalone.com/examples/cfhistogram/
http://www.efg2.com/Lab/ImageProcessing/HistoStretchGrays.htm
http://clickdamage.com/sourcecode/index.php
http://rosettacode.org/wiki/Bitmap/Histogram#J
http://bubble.ro/How_to_convert_an_image_to_grayscale_using_PHP.html