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Perceptual Grouping (contd.) 
• Approaches 
• General: 
• No specific domain knowledge used 
• examples: dot grouping, symmetry grouping 
• Model-based: 
• Generic, invariant properties of classes of objects 
• Search for features consistent with such properties 
• Hierarchical grouping reduces computational complexity 
Example of Rectilinear Building Detection 
• Projective Properties 
• Roofs project to parallelograms 
(orthography, para-perspective) 
• Given one side in image, direction of other is constrained 
• Projection of vertical lines is in a certain direction 
(vertical, with no roll, and orthographic projection) 
• Relations between: 
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cast shadow lines and shadow casting lines 
roof and wall lines