04-cameraproj

04-cameraproj - Lecture 5 Cameras, Projections, and...

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Lecture 5 Cameras, Projections, and Clipping
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Reading • Hearn Baker (3/E). 7.2-6, 7.9 • Foley et al. Chp. 6 Recommended: • OGL Red book Chp. 3
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Lecture outline: 1. Pin-hole camera 2. Family of Projections 3. Parallel Projection 4. Perspective Projection 5. OpenGL Projection Model 6. Clipping and Perspective Division * Some material of this lecture is from Stanford (courtesy of Levoy)
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• Last lecture: we built a 3D geometric world • Now, we need some sort of sensor to receive and record a picture. • Is this all we need? • Do we get a useful image? Camera: Forming an image
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Pin-hole Camera • Advantages: – easy to simulate – everything is in focus • Disadvantages: – needs a bright scene (or long exposure) – in real life – everything is in focus (no depth of field) – not realistic (photography) • Supported by most graphics API a hole
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Camera with Lens • Advantages: – no need to have bright scene - in real life – not everything is in focus (more realistic in terms of photography) • Disadvantages: – need more programming to simulate – not everything is in focus (can only capture objects within a certain depth range) • Not supported by most graphics APIs focal length focal point
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What is a Projection? • From a higher dimension to a lower dimension – E.g. from n-space to m-space such that m < n • So, in 3D computer graphics, usually n=3 and m=2 2D film 3D object eye/camera
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Family of Projections Note: all these are planar projections (other projections: planetarium, etc.) [Levoy] The taxonomy: Parallel Perspective: 1, 2, 3-point, etc. Orthographic Oblique: cavalier & others Orthogonal: side, top, front, etc. Axonometric: isometric & others
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Parallel VS Perspective Projections • In 3D, we map points from 3-space to the projection plane (PP) along projection lines emanating from the center of projection (COP): • The center of projection (COP) is exactly the same as the pinhole in a pinhole camera [Levoy] * Finite distance between PP and COP * Infinite distance between PP and COP 1. Perspective 2. Parallel COP scene PP (no need to center of view) Film plane scene Pinhole Observer (COP) at scene PP (no need to center of view)
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Parallel Projection 1. Oblique Projection (DOP NOT perpendicular to PP) 2. Orthographic Projection (DOP perpendicular to PP) In Parallel Projection, we can specify Direction of Projection (DOP) because all projection lines are parallel . Camera or eye at infinity DOP PP Camera or eye at infinity DOP PP
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Oblique Projection (parallel) 1. Cavalier Projection - DOP makes 45 o angle with PP - Does not foreshorten lines perpendicular to PP - So, it preserves lengths 2. Cabinet Projection - DOP makes 63.4 o angle with PP - Foreshorten lines perpendicular to PP by one-half * Also known as “Chinese Perspective” - It is like shearing the geometry along PP - Two standards of Oblique Projection: Examples:
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This note was uploaded on 10/19/2009 for the course COMP 341 taught by Professor Qu,huamin during the Spring '09 term at HKUST.

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04-cameraproj - Lecture 5 Cameras, Projections, and...

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