Image Processing: Spatial filters

Spatial Filters
Mr. A. B. Shinde

A. B. Shinde
Contents…
 Smoothing spatial filters:
 Smoothing linear,
 Order statistic filters,
 Sharpening spatial filters:
 Use of second derivatives for
enhancement,
 Use of first derivatives for
enhancement
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A. B. Shinde
Introduction
• 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 prior to (large) object extraction, and bridging of small
gaps in lines or curves.
• Noise reduction can be accomplished by blurring with a linear filter and
also by nonlinear filtering.
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A. B. Shinde
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Smoothing Linear Filters

A. B. Shinde
• Smoothing, linear spatial filter computes the average of the pixels
contained in the neighborhood of the filter mask.
• These filters sometimes are called averaging filters and sometimes they
are also referred as lowpass filters.
• During such filtration, the value of every pixel in an image is replaced by
the average of the intensity levels in the neighborhood defined by the
filter mask.
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A. B. Shinde
• This results in an output image with reduced “sharp” transitions in
intensities.
• Random noise typically consists of sharp transitions in intensity levels,
hence smoothing linear filter is used for noise reduction.
• Averaging filters have the undesirable side effect that they blur edges.
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A. B. Shinde
• Figure shows 3 x 3 smoothing filter.
• Use of this filter yields the standard average of the
pixels under the mask.
• Equation:
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which is the average of the intensity levels of the
pixels in the 3 x 3 neighborhood defined by the
mask.

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• The coefficients of the filter are all 1s to make it
computationally more efficient.
• At the end of the filtering process the entire image
is divided by 9.
• An m x n mask would have a normalizing constant
equal to 1/mn.
• A spatial averaging filter in which all coefficients
are equal is called a box filter.

A. B. Shinde
• This mask uses weighted average, (pixels are
multiplied by different coefficients i.e. giving more
importance (weight) to some pixels at the expense
of others).
• In the mask shown, the pixel at the center is
multiplied by a higher value than any other, thus
giving this pixel more importance in the calculation.
• The other pixels are inversely weighted as a function
of their distance from the center of the mask.
• The diagonal terms are further away from the center
than the orthogonal neighbors and, thus, they are
weighed less than the immediate neighbors of the
center pixel.
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A. B. Shinde
• Highest weighing given to the center and the
value of the coefficients are decreased as a
function of increasing distance is an attempt to
reduce blurring in the smoothing process.
• The sum of all the coefficients in the mask of
figure is equal to 16, an attractive feature for
computer implementation because it is an
integer power of 2.
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A. B. Shinde
• With reference to earlier equation, the general implementation for
filtering an M x N image with a weighted averaging filter of size m x n
(m and n odd) is given by the expression
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Where, x = 0, 1, 2, … , M - 1 and y = 0, 1, 2, …., N – 1
The denominator in equation is simply the sum of the mask coefficients
and, therefore, it is a constant that needs to be computed only once.

A. B. Shinde
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Original image, of size
500 x 500 pixels
Result of averaging
filter masks of size
m = 3
Result of averaging
m = 5
Result of averaging
m = 15
Result of averaging
m = 35
Result of averaging
m = 9

A. B. Shinde
14
Image of size
528 x 485 pixels from
the Hubble Space
Telescope
Image filtered with a
15 x 15 averaging mask
Original image courtesy of NASA

A. B. Shinde
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Smoothing Nonlinear Filters

A. B. Shinde
• Order-statistic filter is nonlinear spatial filter.
• The response is based on ordering (ranking) the pixels contained in the
image encompassed by the filter and then 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, which replaces
the value of a pixel by the median of the intensity values in the
neighborhood of that pixel.
• Median filters are popular because, for certain types of random noise,
they provide excellent noise-reduction capabilities, with considerably
less blurring.
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A. B. Shinde
• Median filters are particularly effective in the presence of impulse noise,
(salt-and-pepper noise) because of its appearance as white and black
dots superimposed on an image.
• The median ξ, of a set of values is such that half the values in the set
are less than or equal to ξ and half are greater than or equal to ξ .
• In order to perform median filtering at a point in an image, we first sort
the values of the pixel in the neighborhood, determine their median, and
assign that value to the corresponding pixel in the filtered image.
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A. B. Shinde
• For example:
• In a 3 x 3 neighborhood the median is the 5th largest value, in a 5 x 5
neighborhood, 13th largest value, and so on.
• When several values in a neighborhood are the same, all equal values
are grouped.
• For example, suppose that a 3 x 3 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 results in a median of 20.
• The principal function of median filters is to force points with distinct
intensity levels to be more like their neighbors.
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A. B. Shinde
• The median filter is the most useful order-statistic filter in image
processing.
• The median represents the 50th percentile of a ranked set of numbers.
• For example, using the 100th percentile results in the so-called max filter,
which is useful for finding the brightest points in an image.
• The response of a 3 x 3 max filter is given by
R = max{zk| k = 1, 2, ....., 9}.
• The 0th percentile filter is the min filter.
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A. B. Shinde
20
X-ray image of circuit
board corrupted by
salt-and-pepper noise
Noise reduction with
3 x 3 averaging mask
Noise reduction with a
3 x 3 median filter
image courtesy of Mr. Joseph E. Pascente, Lixi, Inc.
The averaging filter blurred the image and its noise reduction performance
was poor.
The superiority of median over average filtering is quite good.
In general, median filtering is much better than averaging for the removal
of salt-and-pepper noise.

A. B. Shinde
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Sharpening Filters

A. B. Shinde
Sharpening Filters
• Sharpening filters are used is to highlight transitions in intensity.
• Sharpening applications ranging from electronic printing and medical
imaging to industrial inspection and autonomous guidance in military
systems.
• Image blurring could be accomplished in the spatial domain by pixel
averaging in a neighborhood.
• As averaging is analogous to integration, it is logical to conclude that
sharpening can be accomplished by spatial differentiation.
• Thus, image differentiation enhances edges and other discontinuities
(such as noise) and deemphasizes areas with slowly varying intensities.
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A. B. Shinde
Sharpening Filters
• Derivatives:
• The derivatives of a digital function are defined in terms of differences.
• There are various ways to define these differences.
• First derivative
– (1) must be zero in areas of constant intensity;
– (2) must be nonzero at the onset of an intensity step or ramp; and
– (3) must be nonzero along ramps.
• Second derivative
• (1) must be zero in constant areas;
• (2) must be nonzero at the onset and end of an intensity step or ramp; and
• (3) must be zero along ramps of constant slope.
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A. B. Shinde
Sharpening Filters
• A basic definition of the first-order derivative of a one-dimensional
function f(x) is the difference
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• Second-order derivative of f(x) as the difference

A. B. Shinde
Sharpening Filters
• Second Derivative for Image Sharpening:
• This approach consists of defining a discrete formulation of the second-
order derivative and then constructing a filter mask based on that
formulation.
• We are interested in isotropic filters, whose response is independent of
the direction of the discontinuities in the image to which the filter is
applied.
• Isotropic filters are rotation invariant, in the sense that rotating the image
and then applying the filter gives the same result as applying the filter to
the image first and then rotating the result.
• The simplest isotropic derivative operator is the Laplacian, which, for a
function (image) f(x,y) of two variables, is defined as
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A. B. Shinde
Sharpening Filters
• Because derivatives of any order are linear operations, the Laplacian is
a linear operator.
• To express this equation in discrete form, in the x-direction, we have
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and, similarly, in the y-direction we have
Therefore, discrete Laplacian of two variables is

A. B. Shinde
Sharpening Filters
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Filter mask used
to implement
discrete Laplacian
of two variables
Isotropic result for
rotations in
increments of 900
Mask used to
implement an
extension of this
equation that
includes the
diagonal terms
Isotropic results in
increments of 45°
Implementations
of the Laplacian
Implementations
of the Laplacian

A. B. Shinde
Sharpening Filters
• As Laplacian is a derivative operator, its use highlights intensity
discontinuities in an image and deemphasizes regions with slowly
varying intensity levels.
• This will produce images that have grayish edge lines and other
discontinuities.
• If the definition used has a negative center coefficient, then we subtract,
rather than add, the Laplacian image to obtain a sharpened result.
• Thus, the basic way in which we use the Laplacian for image sharpening
is
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The constant is c = -1, if the Laplacian filters are mask 1 or 2 used, and
c = 1 if either of the other two filters is used.

A. B. Shinde
Sharpening Filters
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Blurred image
of the moon
Laplacian
without scaling
Laplacian with
scaling
Image
sharpened using
the mask 1
Result
of using the
mask 2
Mask 1 Mask 2

A. B. Shinde
Sharpening Filters
• Unsharp Masking and Highboost Filtering:
• Printing and publishing sharpen images consists of subtracting an
unsharp (smoothed) version of an image from the 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 the mask.)
3. Add the mask to the original.
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A. B. Shinde
Sharpening Filters
• Letting denote the blurred image, unsharp masking is expressed
in equation form as follows. First we obtain the mask:
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Then we add a weighted portion of the mask back to the original image
where we included a weight, k (k ≥ 0), for generality. When k = 1, we
have unsharp masking, as defined above. When k ≥ 1, the process is
referred to as highboost filtering. Choosing k < 1,de-emphasizes the
contribution of the unsharp mask.

A. B. Shinde
Sharpening Filters
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Example

A. B. Shinde
Sharpening Filters
• First-Order Derivatives (Nonlinear) Image Sharpening:
• First derivatives in image processing are implemented using the
magnitude of the gradient. For a function f(x, y), the gradient of f at
coordinates (x, y) is defined as the two-dimensional column vector
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This vector has the important geometrical property that it points in the
direction of the greatest rate of change of f at location (x, y).

A. B. Shinde
Sharpening Filters
• The magnitude (length) of vector denoted as M(x, y), where
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is the value at (x, y) of the rate of change in the direction of the gradient
vector. Note that M(x, y) is an image of the same size as the original,
created when x and y are allowed to vary over all pixel locations in f. It is
common practice to refer to this image as the gradient image (or simply
as the gradient when the meaning is clear).

A. B. Shinde
Sharpening Filters
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Optical image of contact lens
(with defects)
Sobel gradient.

This presentation is published only for educational purpose
abshinde.eln@gmail.com

Image Processing: Spatial filters

Image Processing: Spatial filters

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Image Processing: Spatial filters