There are two approaches to elementary image processing to enhance an image: histogram operations and filtering.
Histogram operations aim at global contrast enhancement, in order to increase the visual distinction between features, while filter operations aim at enhancing local contrast (edge enhancement) and suppressing unwanted image detail. Histogram operations look at DN values without considering where they occur in the image and assign new values from a look-up table based on image statistics. Filtering is a “local operation” in which the new value of a pixel is computed based on the values of the pixels in the local neighbourhood.