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Histogram Displays
The image histogram is a vitally important tool for understanding and adjusting image contrast. As shown in the following interactive tutorial, this is a plot of the number of pixels (or the fraction of the image area) as a function of the brightness level. For color images, the average of red, green and blue, or the weighted luminance that corresponds to human visual response (approximately: (25 percent x red) + (60 percent x green) + (10 percent x blue)), or any of the various color space axes shown above, may be used. The cumulative histogram, which can also be selected in the example, shows the integral of the histogram and is used for some of the adjustments shown below. This interactive tutorial can be used to examine histogram displays for a variety of images.
The tutorial initializes with a randomly selected specimen imaged in the microscope appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of specimen images, in addition to the initial randomly chosen one. For a color image, the individual R, G or B channels, or the Average of the three, or the perceptually-weighted Luminance value can be selected with the radio buttons. The Choose A Histogram pull-down menu selects either the conventional histogram showing the Number of Pixels with each value, or the Cumulative histogram showing the percentage of pixels with values less than or equal to the value.
Contributing Authors
John C. Russ - Materials Science and Engineering Dept., North Carolina State University, Raleigh, North Carolina, 27695.
Matthew Parry-Hill, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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