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Adjusting Image Contrast
This interactive tutorial illustrates the adjustment of the gain (contrast) and brightness (intensity) of an image by using the image histogram. Another useful tool, particularly when used in conjunction with the histogram, is the transfer function. This is a plot showing the original pixel values on the horizontal axis and the new value after adjustment on the vertical axis. In the tutorial, adjusting the gain (contrast) and brightness (level) changes the slope and position of the transfer function, and shifts the values in the image and 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 is shown on the right along with the transfer function that plots the output or display pixel brightness for each original or stored pixel value. The Gain and Brightness sliders control the slope and horizontal position of that function, respectively. As these are adjusted, either by sliding them directly or by using the arrow buttons shown next to each, 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 check boxes selects which channels are being adjusted together or individually.
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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