1. Spatial Image Enhancement
March 10, 2000
Spatial Image Enhancement
FR 4262

2. 23. Image Enhancement
March 10, 2000
Spatial Enhancement
Area operation --- depends on the values of surrounding “neighborhood” pixels, as well as the pixel being processed.
Deals with “spatial frequency”
the difference between the highest and lowest values of a contiguous set of pixels; or the number of changes in brightness values per unit distance for any particular part of an image.
Spatial frequency
Zero
Low
High
FR 4262

3. Convolution Filtering
23. Image Enhancement
March 10, 2000
Convolution Filtering
Process of averaging small sets of pixels across an image to change the spatial frequency characteristics of the image.
FR 4262

4. 23. Image Enhancement
General procedure
March 10, 2000
Establish a “moving window” (the kernel) with an odd number of rows and columns
Move the kernel through the image and replace the central pixel value with a newly computed value
FR 4262

5. Kinds of Filters (Kernels)
23. Image Enhancement
March 10, 2000
Kinds of Filters (Kernels)
Low Pass Filters
reduce texture by suppressing high spatial frequencies; creates a smoothing effect
High Pass Filters
increase image texture by passing the high frequencies; enhances edges
FR 4262

6. Example of High Pass Convolution Filtering
23. Image Enhancement
March 10, 2000
Example of High Pass Convolution Filtering
Consider a 5 x 5 block of pixels in an image
and a 3 x 3 kernel applied to pixel 3, 3
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7. The output value for pixel 3, 3 is
23. Image Enhancement
March 10, 2000
The output value for pixel 3, 3 is
(-1 * 8) +
(-1 * 6) +
(-1 * 6) +
(-1 * 2) +
(16 * 8) +
(-1 * 6) +
(-1 * 2) +
(-1 * 2) +
(-1 * 8)
( )
= ( ) / (16 - 8)
= 88 / 8
= 11
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8. 23. Image Enhancement
March 10, 2000
When the 3 x 3 set of pixels in the center of the image is convolved, the output values are:
The lower values decrease and the higher values increase, increasing the spatial frequency of the image and accentuating edges.
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9. Image with no enhancement
23. Image Enhancement
Image with no enhancement
March 10, 2000
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10. Edge Sharpening with high pass filter
23. Image Enhancement
Edge Sharpening with high pass filter
March 10, 2000
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11. Smoothing with low pass filter
23. Image Enhancement
Smoothing with low pass filter
March 10, 2000
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12. Contrast Stretch and Edge Enhancement
23. Image Enhancement
March 10, 2000
Contrast Stretch and Edge Enhancement
FR 4262