shapefromshading-1

shapefromshading-1 - 1 Perception of Shape from Shading...

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2 Perception of Shape from Shading • Continuous image brightness variation due to shape variations is called shading • Our perception of shape depends on shading • Circular region on left is perceived as a flat disk • Circular region on right has a varying brightness and is perceived as a sphere
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3 From Image to Shape • Four main factors – Geometry of the scene – Reflectance of the visible surfaces – Illumination direction and distribution – Viewpoint • Can we compute scene geometry from distribution of pixel brightness in scene image? – Only in very simple situations – Too many unknowns in general Scene geometry Viewpoint Pixel brightness Illumination
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4 How Do We Do It? • Humans have to make assumptions about illumination: bump (left) is perceived as hole (right) when upside down Illumination direction is unknown. It is assumed to come from above
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5 Does Shading Play a Central Role? • Contour plays a more important role – Variations in intensity are same on both shapes – Upper region is perceived as composed of three cylindrical pieces illuminated from above – Lower region is perceived as sinusoidal, illuminated from one side • Note the ambiguities of the surface perceptions, depending on assumed illumination direction 2 possible illumination hypotheses
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6 Psychophysics • What assumptions do people make about surface reflectance? • Is an estimate of illumination direction necessary? • Stimuli: Shaded ellipsoids with varying – Elongations – Directions of light source – Reflectance – Cast shadows • Test: judge direction of light and surface orientations at discrete points
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7 Results • Task is hard: errors 15 to 20 degrees • No effect of glossiness, no Lambertian surface assumption • No correlation between judgement of light directions and shape – No prior estimate of light direction • Poor discrimination between elongated and rounded ellipsoids – Qualitative information
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8 Human Shading Interpretation • Is it metric or ordinal? – Metric: depth – Ordinal: depth order • Answer: – Ordinal, qualitative – Magnitude of shading gradient is not important
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9 Quantitative Shape Recovery • Orthographic projection • We have gray levels at pixels ( x, y ) • We want to recover the orientations of the normals at points ( x, y, z ) • By integration, we want to obtain z = f (x,y) q Image plane Orthographic projection (x, y, z) x z z = f (x,y) (x, y)
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10 • Fit a surface that is locally perpendicular to the normals From Normals to Surface Shape
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This note was uploaded on 01/12/2012 for the course CMSC 733 taught by Professor Staff during the Spring '08 term at Maryland.

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shapefromshading-1 - 1 Perception of Shape from Shading...

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