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An Efficient and Fast Active Contour Model for Salient Object Detection Authors: Farnaz Shariat, Riadh Ksantini, Boubakeur Boufama shariatf@uwindsor.ca ksantini@uwindsor.ca boufama@uwindsor.ca University of Windsor May 2009

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2 Presentation Outline Introduction Active contours Level sets A variational level set method Polarity information The active contour model using polarity information Experiments An Efficient and Fast Active Contour Model for Salient Object Detection

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3 Active Contours Image Segmentation solution Based on Deformable models Find equation Parametric Represent curves and surfaces explicitly in their parametric forms during deformation; (Kass 1987 ) Geometric Based on curve evolution and the level set method, Represent curves and surfaces implicitly as a level set of a function; (Caselles 1993) An Efficient and Fast Active Contour Model for Salient Object Detection

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Active Contours(cont’d) limitations of parametric AC Initial contour dependant Same topology Geometric ACs provide elegant solution Based on level set, curve evolution An Efficient and Fast Active Contour Model for Salient Object Detection 4

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5 Level sets Main Idea: Closed interface Γ, velocity v Goal: motion of the interface Osher and Sethian1988 idea: Represent the interface by implicit smooth function φ φ = (x, t) =0, Γ φ = (x, t) <0, Γ in φ = (x, t) >0, Γ out An Efficient and Fast Active Contour Model for Salient Object Detection

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6 Level sets(cont’d) Remains a valid function change topology Computationally simple Start far from boundaries An Efficient and Fast Active Contour Model for Salient Object Detection

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7 Level sets (cont’d) Classical vs. Variational Variational methods are suitable for incorporating additional information originated from a certain evolution PDE of a parameterized curve originated from minimizing the energy function

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An Efficient and Fast Active Contour Model for Salient Object Detection 8 Level sets (cont’d) Reshaping (re-initialization) φ can develop shocks inaccurate computation To avoid Initialize φ as a signed distance function Reshape φ as a signed distance function regularly Drawbacks Displacement of the zero level set Increasing of the number of iteration Expensive, Complex Ad hoc manner

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9 Variational level set C. Li, C. Xu, C. Gu, M.D. Fox, “Level set evolution without re-initialization: a new variational formulation”, CVPR, 2005 Energy function : Keeping the function close to sign distance function Moving toward the boundaries An Efficient and Fast Active Contour Model for Salient Object Detection

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10 Variational level set (cont‘d) Advantages Initialization is automatic No need for reinitialize Computationally effective An Efficient and Fast Active Contour Model for Salient Object Detection Active contour result using Li’s algorithm

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11 Variational level set (cont‘d) Problem Noisy background Textured background Proposed Solution Proposed Solution Using “ Polarity information ” instead of gradient with “ Level sets ” An Efficient and Fast Active Contour Model for Salient Object Detection Active contour result using Li’s algorithm

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12 Polarity information Common edge detectors Polarity [Carson, 1997], discriminates boundaries A measure of the extent to which the gradient vectors in a certain neighbourhood all point in the dominant orientation. An Efficient and Fast Active Contour Model for Salient Object Detection #gradient vectors in are in + side of dominant orientation #gradient vectors in are in - side of dominant orientation

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13 Polarity Values An Efficient and Fast Active Contour Model for Salient Object Detection Edge Noise Texture i.e. E - =0, E + !=0 E + ~ 0 E - ~ 0 E + = E -

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14 The Active Contour Model Using Polarity Information Instead of Gradient in E ext use Polarity Combine “Polarity based stopping function” with “Variational Level Set” An Efficient and Fast Active Contour Model for Salient Object Detection

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15 The Active Contour Model Using Polarity Information The final energy function is An Efficient and Fast Active Contour Model for Salient Object Detection

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16 The Active Contour Model Using Polarity Information Then by using energy minimization method to minimize the total energy it can reach to: And by using gradient descent, the approximation of the above formula is: An Efficient and Fast Active Contour Model for Salient Object Detection

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17 Results An Efficient and Fast Active Contour Model for Salient Object Detection

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18 Results An Efficient and Fast Active Contour Model for Salient Object Detection

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19 Results An Efficient and Fast Active Contour Model for Salient Object Detection

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20 Results An Efficient and Fast Active Contour Model for Salient Object Detection

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21 Results An Efficient and Fast Active Contour Model for Salient Object Detection

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