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Contour Lines
Another useful thresholding function does not try to select pixels with brightness or color values between limits, but instead draws contour lines that separate brighter from darker pixels. Contour lines are always continuous and are directly useful for measurement of structures, as will be seen in the next section. For sections through three-dimensional structures (typical sections examined in the light or electron microscope) the lines between structures represent the boundary surfaces. For surface images (e.g. from an atomic force microscope) the lines represent iso-elevation contours as on a topographic map. This interactive tutorial illustrates the drawing of iso-brightness contour lines based on the pixel brightness values in an image.
The tutorial initializes with a randomly selected specimen 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 brightness values in the image is shown at the right. When the Contour Lines button is clicked, a superimposed line representing a threshold setting is shown on the histogram, which can be adjusted using the Threshold slider. The image display shows continuous white lines that mark boundaries separating pixels brighter than this threshold from ones darker than it.
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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