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Image enhancement is the process by which we try to improve an image so that it looks subjectively better. We do not really know how the image should look, but we can tell whether it has been improved or not, by considering, for example, whether more detail can be seen, or whether unwanted flickering has been removed, or the contrast is better etc. | Image Processing The Fundamentals. Maria Petrou and Panagiota Bosdogianni Copyright 1999 John Wiley Sons Ltd Print ISBN 0-471-99883-4 Electronic ISBN 0-470-84190-7 Chapter 4 Image Enhancement What is image enhancement Image enhancement is the process by which we try to improve an image so that it looks subjectively better. We do not really know how the image should look but we can tell whether it has been improved or not by considering for example whether more detail can be seen or whether unwanted flickering has been removed or the contrast is better etc. How can we enhance an image The approach largely depends on what we wish to achieve. In general there are tw o major approaches those which reason about the statistics of the grey values of the image and those which reason about the spatial frequency content of the image. Which methods of the image enhancement reason about the grey level statistics of an image Methods that manipulate the histogram of the image for the purpose of increasing its contrast. The method of principal component analysis of a multispectral image for obtaining a grey level version of it with the maxinum possible contrast. Methods based on rank order filtering of the image for the purpose of removing noise. What is the histogram of an image The histogram of an image is a discrete function that is formed by counting the number of pixels in the image that have a certain grey value. When this function is normalized to sum up to 1 for all the grey level values it can be treated as a probability density 126 Image Processing The Fundamentals function that expresses how probable is for a certain grey value to be found in the image. Seen this way the grey value of a pixel becomes a random variable which takes values according to the outcome of an underlying random experiment. When is it necessary to modify the histogram of an image Suppose that we cannot see much detail in the image. The reason is most likely that pixels which represent different .
