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RGB Slicing
This interactive tutorial illustrates the use of thresholding or slicing RGB channels to create a binary image. For color images, thresholds may be applied to each color channel, and this may be done for RGB, LAB, HSI or any other set of color coordinates. In most cases, HSI coordinates correspond best to human interpretations of color. Selecting a target color from a point on an image and then adjusting tolerances to hue, saturation and intensity around that point is often a good strategy, and much simpler to understand than trying to set limits on red, green and blue.
The tutorial initializes with a randomly selected specimen 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 brightness values in the image is shown in the Grayscale Histogram window. When the Display Image setting is set to Original by clicking on the button, this histogram shows the average pixel brightness. When the Binary image button is clicked, the histogram shows the values for the Red, Green or Blue channel as selected with the labeled buttons. The superimposed lines represent the upper and lower level threshold settings for that channel. These are adjusted using the Threshold Levels sliders for the Upper and Lower brightness threshold values. The binary image that is displayed shows the pixels whose values lie between all three sets of individual channel threshold settings as white, and the background pixels as black. It is recommended that you begin with all channels set with the limits at the ends (all pixel values selected) and move the limits one channel at a time to attempt to select just the pixels of interest.
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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