How can I increase brightness a image region ?

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Multiply the image by a constant factor. For this image, I would do something pretty high, like 16, or 32. This is called gain, and it makes everything brighter, including noise.

To modify just a region, use the operator(cv::Rect) that allows you to extract sub-images. Or any one of several other functions on that page that does the same thing.

If you're trying to binarize the license plate, you may want to try the adaptiveThreshold or the niBlackThreshold functions.