This process aims to mask out the image background

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Unformatted text preview: s aims to mask out the image background, leaving gray-level information present in the objects. Threshold detection methods • If some property of an image after segmentation is known a priori, the task of threshold selection is simplified, since the threshold is chosen to ensure this property is satisfied. A printed text sheet may be an example if we know that characters of Faculty of Engineering Robotics Technology MECH 4041 6 B.Eng (Hons.) Mechatronics S. Venkannah Mechanical and Production Engineering Department the text cover 1/p of the sheet area. Using this prior information about the ratio between the sheet area and character area, it is very easy to choose a threshold (based on image histogram) such that 1/p of the image area has gray values less than T and the rest has gray values larger than T. This method is called p-tile thresholding. • More complex methods of threshold detection are based on histogram shape analysis. Bimodal histogram - if objects have approximately the same gray level that differs from the gray level of the background . Pixels of objects form one of its peaks, while pixels of the background form the second peak. The gray levels between the two peaks probably result from the border pixels between the objects and background. If the histogram is multi-modal, more thresholds may be determined at minima between any two maxima. Faculty of Engineering Robotics Technology MECH 4041 7 B.Eng (Hons.) Mechatronics S. Venkannah Mechanical and Production Engineering Department Bimodality of histograms To decide if a histogram is bimodal or multi-modal may not be so simple in reality, it is often impossible to interpret the significance of local histogram maxima. Bi modal histogram threshold detection algorithms usually find the highest local maxima first and detect the threshold as a minimum between them. This technique is called the mode method. Histogram bimodality itself does not guarantee correct threshold segmentation. A two part image with one half white and the second half black actually has the same histogram as an image with randomly spread white and black pixels i.e. a salt and pepper...
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