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An edge contour extraction techniqueextraction technique
Ivan Cabezas and Maria TrujilloUniversidad del Valle, Cali - Colombia
Panos LiatsisCity University, London - UK
Outline
�Introduction
�The Proposed Technique
� Edge region detection� Edge contour extraction
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� Edge contour extraction� Effects of Outliers
�Experimental Validation
� Inter-technique evaluation� Intra-technique evaluation
Introduction
�An edge can be describedas a local luminancechange
�Edge detectors producedisjoint edge segments
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disjoint edge segmentsstored in binary maps
�Edge contours arerelatively invariant to thechanges of illumination,and sensor characteristics,among others
Introduction
� A contour can be represented using an ordered chain of symbols which links an initial edge point to a final edge point
�For instance: { [70,45],2,3,2,3,3,2,3,3,3,2, . . . 3,4,3,4,3,4,3,4,3,4,[17,32] }
Slide 4 / 16CLEI 2008 - An edge contour extraction technique
3 2 1
4 C 0
5 6 7
[70,45]
[17,32]
Edge Region
�An edge region is a set of adjacent points witha gradient magnitude higher than a thresholdand a common quantised gradient direction
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GradientMagnitude 0 0 0 10 20 10 0 0 0
DirectionGradient
Magnitude 0 0 0 40 40 0 0 0
Direction
Edge Region Detection
�A quantisation is used in order to establishcommon gradient directions
90º135º 45º
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180º
225º 270º 315º
0º
Quantisation
Edge Contour Extraction
�There is a region when at leasttwo contiguous points with highgradient magnitude have thesame quantised gradient direction
�An edgel is located by performing
Slide 7 / 16CLEI 2008 - An edge contour extraction technique
�An edgel is located by performingnon-maximal suppression
�The continuity of the edge regionand the adjacency of locatededgels allow tracking edgels
Innovative aspect
�The proposed technique does not process theentire image for extracting edge contours
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Points processed by traditional edge
extraction techniques
Points processed by the proposed technique
Edge contours extracted by the proposed technique
Effects of Outliers
Original Signal Gradient Directions
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Signal plus Noise
Edge Contour Extraction with Noise
�An outlier point can have a high gradient
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�An outlier point can have a high gradientmagnitude but it causes a gradient directionpeculiarity in its neighbourhood
�The presence of outliers in an edge regioncauses deterioration in edge maps computedby the proposed technique
Inter-Technique Evaluation
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FPP = FP / (TP+FN) TPR = TP / (TP+FN) FPR = FP / (FP+TN)
Inter-Technique Evaluation
“Egg” Ground Truth
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The Sobel The Canny The proposed technique
Intra-Technique Evaluation
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FPP = FP / (TP+FN)
Intra-Technique Evaluation
Edge Contour Maps
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Search densities and processed points
247 3162 16
Conclusions
�Edge contours are thin chains of edgels withmaximum gradient magnitude and a commongradient direction
�The computation effort is reduced using a set
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�The computation effort is reduced using a setof columns and/or rows
�It is possible to obtain a edge contours mapby processing 50% or less of an image,depending on the image content
Questions
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