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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