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Region Detection
Defining regions of an image
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Introduction All pixels belong to a region
Object Part of object Background
Find region Constituent pixels Boundary
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Region Detection A set of pixels P An homogeneity predicate H(P) Partition P into regions {R}, such
that
iRi1
n
P
H iR True
H iR jR False
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Point based methods – thresholding If
Regions are different brightness or colour
Then Can be differentiated using this
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Global thresholds Compute threshold from whole image
Incorrect in some regions
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Local thresholds Divide image into regions Compute threshold per region Merge thresholds across region
boundaries
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Region Growing All pixels belong to a region Select a pixel Grow the surrounding region
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Slow Algorithm If a pixel is
Not assigned to a region Adjacent to region Has colour properties not different to
region’s Then
Add to region Update region properties
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Split and Merge Initialise image as a region While region is not homogeneous
Split into quadrants and examine homogeneity
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Recursive SplittingSplit(P){ If (!H(P)) {
P subregions 1 … 4;Split (subregion 1);Split (subregion 2);Split (subregion 3);Split (subregion 4);
}}
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Recursive Merging If adjacent regions are
Weakly split Weak edge
Similar Similar greyscale/colour properties
Merge them
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Edge Following Detection
Finds candidate edge pixels Following
Links candidates to form boundaries
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4/8 ConnectivityProblem
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Contour Tracking Scan image to find first edge point Track along edge points
Spurs? Endpoints?
Join edge segments
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Edge Linking Aggregate collinear chain codes
Colinear?• Sequential least squares• tolerance band
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Sequential Least Squares Accumulate best fitting line to segments and
error When error exceeds a threshold, finish
segment
Tolerance Band Accumulate best fitting line to segments If new point lies more than from line, finish
segment
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1. Take the first k edges from the list 2. Fit a line segment between the first and last 3. If the normalised maximum error is too large, shorten the sublist to the point of maximum error and return to step 2. 4. If the fit succeeds, compare this and the previous segments, if they are colinear, join them. 5. Advance the window of edges to get another k edges in the sublist and return to step 2.
Hop Along Algorithm
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Examples An example would show an edge
detected image There would be a record of the
edge points constituting each edge segment
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Scale Based Methods Structures observed depend on
scale of observation
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Analysis Processing of an image should be
at a level of detail appropriate to structures being sought Image pyramid Wavelet transform
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Image PyramidReducing resolution
Pixels in each layer computed by averaging groups of pixels in layer below. OrUse scale dependent operators – e.g. Marr Hildreth.
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Wavelet Transform Transform image data Select coefficients Reverse transform
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Watersheds of Gradient Magnitude Compare geographical watersheds
Divide landscape into catchment basins
Edges correspond to watersheds
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Algorithm Locate local minima Flood image from these points When two floods meet
Identify a watershed pixel Build a dam Continue flooding
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Examplewatersheds
local minima
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watershed point
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watershed point dam
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Representing Regions Constituent pixels Boundary pixels
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Region map As an array of region labels
Pixel value = region label
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Summary Region detection
Growing Edge following Watersheds
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I think there is a world market for maybe five computersThomas J Watson, chairman IBM, 1943