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] }

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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

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�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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