Is there a simple way to see the intensity of each point of an image?

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I am using threshold on a batch of images(more than one hundred). If I could to see the intensity of each point by clicking mouse on the gray image, it would be very fantastic and easy way to find good threshold numbers for all the images. But I am new on OpenCV and don't know how can I do it? May someone guides me or put an existing code if there is?