The goal of image enhancement is to improve the overall quality of an image for use in an autonomous pattern recognition system or for viewing by the human visual system. The first part of this chapter discusses the enhancement of an image by the direct manipulation of its graylevels using a constant or a set of graylevels from a second image. The latter part provides additional image processing methods based upon the histogram of an image to automatically improve the image's brightness and contrast.
Image enhancement can be accomplished in either the spatial domain or in the frequency domain using the discrete Fourier transform, as discussed in Chapter 2. Image enhancement in the spatial domain provides the benefit of being less extensive computationally since the DFT does not have to be computed. Because of the computational simplicity of processing an image in the spatial domain, many image processing algorithms are easily implemented in specialized digital hardware, providing real-time electronic image processing. Pointwise operations can be separated into the modification of a pixel's graylevel by a constant, by algebraic or Boolean methods, and by nonlinear or histogram techniques. Each of these operations will be discussed in detail in this chapter. Pointwise operations provide a means of making an image lighter or darker, enhancing features that would not be otherwise observable.
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