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binary_threshold — Segment an image using binary thresholding.
binary_threshold(Image : Region : Method, LightDark : UsedThreshold)
binary_threshold segments a single-channel Image using an automatically determined global threshold and returns the segmented region in Region. This is, e.g., useful for the segmentation of characters on a homogeneously illuminated background. binary_threshold also returns the used threshold in UsedThreshold.
The used threshold is determined by the method that is given in Method. Currently the operator provides the following two methods: 'max_separability' and 'smooth_histo'. Both methods should only be used for images that have a bimodal histogram.
The method 'smooth_histo' provides the same functionality that has been provided by the operator bin_threshold. The method 'max_separability' tends to determine smaller values for UsedThreshold. Furthermore, it is less sensitive to thin isolated peaks in the histogram that are far from the rest of the spectrum and often, it is faster than 'smooth_histo'.
By selecting Method = 'max_separability', automatic thresholding based on the gray-level histogram according to Otsu (see the paper in References) is invoked. The algorithm first calculates the histogram of the image and then uses statistical moments to find the optimal threshold that divides the pixels into foreground and background and maximizes the separability between these two classes. This method is only available for byte and uint2 images.
If LightDark = 'light' all pixels with gray values greater or equal to are selected.
If LightDark = 'dark' all pixels with gray values smaller than are selected.
By selecting Method = 'smooth_histo' binary_threshold determines the threshold in the following way: First, the relative histogram of the gray values is determined. Then, relevant minima are extracted from the histogram, which are used as parameters for a thresholding operation. In order to reduce the number of minima, the histogram is smoothed with a Gaussian, as in auto_threshold. The mask size is enlarged until there is only one minimum in the smoothed histogram. Then, the threshold is set to the position of this minimum.
If LightDark = 'light' all pixels with gray values greater or equal to are selected.
If LightDark = 'dark' all pixels with gray values smaller than are selected.
Input Image.
Segmented output region.
Segmentation method.
Default value: 'max_separability'
List of values: 'max_separability', 'smooth_histo'
Extract foreground or background?
Default value: 'dark'
List of values: 'dark', 'light'
Used threshold.
connection, select_shape, select_gray
auto_threshold, char_threshold, local_threshold
N. Otsu, “A threshold selection method from gray level histograms",
IEEE Trans. Syst. Man. Cybern., Vol. SMC-9, 62-66 (1979)
Foundation
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