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From local motion estimates to global ones - physiology:

From local motion estimates to global ones - physiology:. Motion fields for more complex patterns:. Hildreth (1985): Smoothness of velocity field along the contour. True motion field. Local motion estimates. Smoothest Velocity field. Motion fields for more complex patterns (contd.):.

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From local motion estimates to global ones - physiology:

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  1. From local motion estimates to global ones - physiology:

  2. Motion fields for more complex patterns: Hildreth (1985): Smoothness of velocity field along the contour True motion field Local motion estimates Smoothest Velocity field

  3. Motion fields for more complex patterns (contd.):

  4. Motion fields for more complex patterns (contd.): Ellipse demo

  5. Recovering 3D structure from motion: Kinetic Depth Effect [Wallach, 1953] Another possible percept Percept Inference: The human visual system has a preference for rigid interpretations

  6. Ullman’s model for recovering 3D structure from motion: • Establish correspondence between features in different frames • Recover transformation matrix and z values of points Key result: For a rigid structure, 4 non-coplanar points in 3 frames are sufficient to solve for all the unknowns [Ullman, 1979] Open questions: 1. Do these bounds apply to human observers too? 2. Does the rigidity assumption always hold? 3. How do we recover the 3D structure of non-rigid dynamic objects? Video 1: NR rotating object Video 2: Johansson

  7. Processing Framework Proposed by Marr Recognition 3D structure; motion characteristics; surface properties Shape From stereo Motion flow Shape From motion Color estimation Shape From contour Shape From shading Shape From texture Edge extraction Image

  8. Color

  9. Color Estimation: Goal: To recover the intrinsic surface reflectance of an object. And yet, we have good lightness constancy!

  10. Lightness Constancy: The constancy in perceived surface reflectance regardless of differences in illumination. Luminance (L) = Reflectance (R) * Illumination (I) Goal: Given L, recover R. Clearly underconstrained. Assumptions are needed for unique solutions. Helmholtz’s theory: Observer ‘knows’ I through past experience. Hering, Wallach, Land & McCann: Observer computes luminance ratios across edges. (some important hidden assumptions here) Explain fig above

  11. The perceptual importance of luminance ratios at edges: Cornsweet Illusion

  12. Explaining simultaneous contrast illusions via edge ratios:

  13. TANGENT ALERT! Are ratios taken with actual or perceived luminances?

  14. Land and McCann’s Retinex theory: I * R L Given L, recover R

  15. Land and McCann’s Retinex theory - Assumptions: • The world is flat and all sharp • luminance variations are due • to changes in reflectance. • Reflectance always changes • abruptly. • Illumination changes gradually • across a scene. Basic idea: Preserve luminance ratios at edges and discard slow variations.

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