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Unformatted text preview: 1 Image Processing : 4. 4. Radiometry and Radiometry and Shading Models Shading Models Aleix M. Martinez [email protected] Radiometry • By understanding how light travels from a source to a surfaces and how this creates a brightness pattern, we will be able to estimate additional data from an image. • Our goal is to understand the principals and how these can be used. •We will used them to build what’s call shading models . Illumination ^ n ^ s a I(u,v) A 3D object illuminated with a light source. Intensity at image point (u,v) T . Reconstruction • Since an image depends on its illumination, it should be possible to recover its 3D information. Images under different but known illumination. Convex Objects y 45o x’ y’ S5 S3 S2 S4 S1 45o 45o 60o Object x z Camera I 1 I 2 I 3 I 4 I 5 Hemisphere of Direction 2 • Questions: –how “bright” will a surfaces be? –what is “brightness”? • measuring light, • interactions between light and surfaces. • Core idea : think the light arriving at a surface around any point defines a hemisphere of directions. • Simplest problems can be dealt with by reasoning about this hemisphere. Simulating the hemisphere Construct a 3D sphere to simulate a large number of illumination directions. Eliminate the shadows caused by the known illumination 3D reconstruction of the face. Generate a new view with a new illumination • By analogy with angle (in radians), the solid angle subtended by a region at a point is the area projected on a unit sphere centered at that point. • The solid angle subtended by a patch area dA is given by: . sin , cos 2 d d d r dA d Solid Angle Radiance • The power (amount of energy per unit time) traveling at some point in a specified direction, per unit area perpendicular to the...
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- Fall '10
- Image processing, Trigraph, Phong shading, 3D computer graphics, Bidirectional reflectance distribution function