Image Enhancement in Spatial Domain Methods (Image Plane)

• • Techniques are based on direct manipulation of pixels in an image

• Frequency Domain Methods
  • Techniques are based on modifying the Fourier transform of the image.

• Combination Methods
  • There are some image enhancement in spatial domain techniques based on various combinations of methods from the first two categories

Statistical Order/Non-Linear Filters

Some simple neighbourhood operations include:

Min: Set the pixel value to the minimum in the neighbourhood

Max: Set the pixel value to the maximum in the neighbourhood

Median: The median value of a set of numbers is the midpoint value in that set (e.g. from the set [1, 7, 15, 18, 24] 15 is the median). Sometimes the median works better than the average

Smoothing Spatial Filters

• One of the simplest spatial filtering operations we can perform is a smoothing operation
• Simply average all of the pixels in a neighbourhood around a central value
• Especially useful in removing noise from images
• Also useful for highlighting gross detail

Image Smoothing Example

• The image at the top left is an original image of size 500*500 pixels
• The subsequent images show the image after filtering with an averaging filter of increasing sizes
• 3, 5, 9, 15 and 35
• Notice how detail begins to disappear

Weighted Smoothing Filters

• More effective smoothing filters can be generated by allowing different pixels in the neighbourhood different weights in the averaging function
• Pixels closer to the central pixel are more important
• Often referred to as a weighted averaging

• By smoothing the original image we get rid of lots of the finer detail which leaves only the gross features for thresholding

Averaging Filter Vs Median Filter Example

• Filtering is often used to remove noise from images
• Sometimes a median filter works better than an averaging filter

Strange Things Happen At The Edges!

• There are a few approaches to dealing with missing edge pixels:
• Omit missing pixels
• Only works with some filters
• Can add extra code and slow down processing
• Pad the image
• Typically with either all white or all black pixels
• Replicate border pixels
• Truncate the image
• Allow pixels wrap around the image
• Can cause some strange image artifacts.