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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