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Image Contrast Adjustment
Nonlinear functions selectively expand contrast in one part of the brightness range by contracting contrast elsewhere. This can also be used to compensate for the characteristics of the acquisition device, which may be linear, logarithmic, or have some other response. These adjustments are best understood in terms of the transfer function. This interactive tutorial illustrates the nonlinear adjustment of the display transfer function for an image. In the tutorial, the slider changes the shape of the transfer function without shifting the midpoint or end points. Applying such changes to the individual RGB channels can produce color shifts, and so operating on the intensity only while leaving color alone is the preferred method.
The tutorial initializes with a randomly selected specimen imaged in the microscope appearing in the left-hand window entitled Specimen Image. 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 is shown in the right-hand window along with the transfer function that plots the output or display pixel brightness for each original or stored pixel value. The Contrast Level slider controls the shape of that function. As this is adjusted, either by sliding it directly or by using the arrow buttons shown at either end, the histogram shifts, expands or contracts and the image display adjusts accordingly.
Clicking on the Grayscale Contrast button shows just the monochrome version of the image, with its corresponding histogram. Clicking on the HSI Contrast button shows the color version of the image, but the histogram still shows just the monochrome intensity and the slider adjustments of gain and brightness affect only the intensity and not the color values. Clicking on the RGB Contrast button allows the sliders to manipulate the individual color channels. Clicking on the Red, Green and/or Blue button selects which channels is being adjusted individually. Adjustments may be made sequentially, one channel at a time.
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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