Specialised lighting is required for silhouettes it

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Unformatted text preview: ised lighting is required for silhouettes: it is difficult to obtain reliable binary images without restricting the environment. • Thresholding: Gray level thresholding is the simplest segmentation process. Many objects or image regions are characterized by constant reflectivity or light absorption of their surfaces; a brightness constant or threshold can be determined to segment the objects and background. Thresholding is a binary conversion technique in which each pixel is converted into a binary value, either black or white. This is accomplished by utilising a frequency histogram of the image and establishing what intensity (gray level) is to be the border between black and white. The histogram plots the frequency (number of pixels) versus the gray level for the image. For histograms that are bimodal in shape, each peak of the histogram represents either the object itself or the background upon which the object rests. Since we are trying to differentiate between the object and background, the procedure is to establish a threshold (typically two peaks) and assign, for example, a binary 1 for the object and 0 for the background. To improve the ability to differentiate , special lighting techniques must often be applied to generate a high contrast. Faculty of Engineering Robotics Technology MECH 4041 5 B.Eng (Hons.) Mechatronics S. Venkannah Mechanical and Production Engineering Department It should be pointed out that the above method of using a histogram to determine a threshold is only one of a large number of ways to threshold an image. Such a method is said to use a global threshold for the entire image. And in some cases this is not possible and a local thresholding may be employed. When it is not possible to find a single threshold for an entire image (for example, if many different objects occupy the same scene, each having different levels of intensity), one approach is to partition the total image into smaller rectangular areas and determine the threshold for each window being analysed. Thresholdin...
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