1. Chapter 3 (cont)

2.  In this section several basic concepts are introduced underlying the use of spatial filters for image processing.  Mainly spatial filtering is used for image enhancement.  The name filter is borrowed from frequency processing, which is the topic of the next chapter, where “filtering” refers to accepting (passing) or rejecting certain frequency components. For example filters that pass low frequencies are called a lowpass filters.  Spatial Filtering consist of : (1) A neighborhood, (typically a small rectangle). (2) A predefined operation that is performed on the image pixels encompassed by the neighborhood. If the operation performed on the image is linear, then the filter is called linear spatial filter. Otherwise, the filter is nonlinear.

4.  G(x,y) = w(-1,-1)f(x-1,y-1) + w(-1,0)f(x-1,y) + … +w(0,0)f(x,y) + … + w(1,1)f(x+1,y+1)  Observe that the center coefficient of the filter, w(0,0), aligns with the pixel at location (x,y).

5.  There are two closely related concepts that must be understood clearly when performing linear spatial filtering. One is Correlation and the other is Convolution.  Correlation :is the process of moving a filter mask over the image and computing the sum of products at each location.  Convolution: is the same except that the filter is first rotated by 180.

7.  Smoothing filters are used for blurring and for noise reduction. Blurring is used in preprocessing tasks, such as removal of small details from an image.  Smoothing Linear Filters  Order-Statistic (Nonlinear) Filters.

8.  The output of a smoothing, linear spatial filter is simply the average of the pixels contained in the neighborhood of the filter mask. These filters are called averaging filters. they also are referred to as lowpass filters. This process results in an image with reduced “sharp” transitions in intensities.

9.  A spatial averaging filter in which all coefficients are equal sometimes is called a box filter as seen in the first mask in the figure. 3.32.  The second mask is called weighted average

10. An important application of averaging is to blur an image for the purpose of getting a gross representation of objects of interest, such that the intensity of smaller objects blends with the background and larger objects become easy to detect.

11.  Order-statistic filters are nonlinear spatial filters whose response is based on ordering(ranking) the pixels contained in the image area encompassed by the filter, and replacing the value of the center pixel with the value determined by the ranking result. The best-known filter in this category is the median filter.  Median Filters: which replaces the value op a pixel by the median of the intensity values in the neighborhood of that pixel.  Median Filters : are quite popular because they provide excellent noise reduction capabilities with considerably less blurring than linear smoothing filters.  Median Filters: effective for impulse noise, also called salt- and-pepper noise because of its appearance as white and black dots in an image.

12. Suppose that a 3x3 neighborhood has values (10,20,20,20,15,20,20,25,100) these values are sorted as (10,15,20,20,20,20,20,25,100), which result in a median of 20.

13.  The principal objective of sharpening is to highlight transitions in intensity.  Uses of image sharpening include applications ranging from electronic printing and medical imaging.  In last section we saw that blurring can be done by averaging filters which are analogous to integration, it is logical to conclude that sharpening can be done by spatial differentiation.  We will consider here sharpening filters that are based on first and second order derivitaves.

14.  The approach basically consists of defining a discrete formulation of the second-order derivative and then constructing a filter mask based on that formulation.  The filter used here is the Laplacian

15.  A process that has been used for many years by printing and publishing industry to sharpen images consists of subtracting an unsharp (smoothed) version of an image from original image. This process, called unsharp masking, consists of the following steps: 1. Blur the original image. 2. Subtract the blurred image from the original (the resulting difference is called mask.) 3. Add the mask to the original.

17.  First derivatives in image processing are implemented using the magnitude of the gradient.  The filter used here is the Gradient as shown below:

18.  The Gradient is frequently used in industrial inspection, either to aid humans in the detection of defects or as preprocessing step in automated inspection.  The gradient is used for edge detection with sobel mask.  Also used to enhance the small discontinuities in an image.