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Levels Adjustment
Setting the contrast and brightness of a digital image to get maximum contrast without significant clipping of values to white or black is difficult. It is generally better to use the image histogram to set limits that maximize the image contrast. If this is done on individual color channels, it can produce unwanted color shifts in the image. Instead, the image should be converted to a different color space such as HSI or LAB where the brightness can be adjusted without altering colors, and then the result converted back for correct display.
Clipping of more than a tiny fraction of the pixels to black and or white should be avoided since those pixels lose any information they may have had in the original image. It is better to acquire an image with slightly less than full contrast rather than one in which significant areas exceed the dynamic range of the camera and are clipped to black or white. This interactive tutorial shows the effect of stretching the contrast to limits based on allowing a small fraction of the pixels to be clipped. In the tutorial, the gain (contrast) and brightness (intensity) of an image are adjusted for maximum contrast based on a permitted amount of clipping at bright and dark ends of the histogram.
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. The histogram of the image brightness values is shown in the Image Histogram window, along with the transfer function that plots the output or display pixel brightness for each original or stored pixel value. The Clipping Amount slider controls the number of pixels that can be set to black and white to produce maximum contrast, up to a maximum of 1 percent. As this is adjusted, the histogram expands and the image display adjusts accordingly.
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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