Contrast Limited Adaptive Histogram Equalization (CLAHE)

CLAHE is a useful tool for preprocessing images (or video) for computer vision/pattern recognition tasks. It more or less helps you "see" areas of the image that are in shadows.

There are many available implementations of this but I like the one in open CV:

http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_imgproc/py_histograms/py_histogram_equalization/py_histogram_equalization.html

Note: it is usually better to convert images to HSV colorspace first.

Before

After

Additional info:

http://fiji.sc/wiki/index.php/Enhance_Local_Contrast_(CLAHE)