I'm writing an app that will offer OCR of an image captured by the iPhone camera. I want to test the image before I perform the OCR analysis to see if there is sufficient lighting. Anyone have any ideas?
The most flexible way is to generate a histogram for the red green and blue color channels, and then use it to determine the average lightness, median lightness, black/white points, contrast or other custom functions.
Use CGBitmapContextCreate to create a bitmap context backed by a buffer of your creation, draw your image into it, then loop through each pixel in the buffer to populate the histograms.
来源：https://stackoverflow.com/questions/3992790/good-way-to-calculate-brightness-of-uiimage