07-lightshade

07-lightshade - Lecture 8 Lighting, Material, and Shading...

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Lecture 8 Lighting, Material, and Shading
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Reading • Hearn Baker (3/E). sections 10.1-3, 10.10 • Foley Section 16.1-2
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Lecture outline: 1. Introduction 2. Light source 3. Phong Illumination Model 4. Interpolative Shading 5. Lighting in OGL
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Introduction • So far, we’ve talked exclusively about geometry. – What is the shape of an object? – How do I place it in a virtual 3D space? – How do I view it through my virtual camera? – How do I know which pixels it covers? • Once we’ve answered all those, we have to ask one more important question: – What value do I set each pixel to? • Answering this question is the job of the shading . • (Of course, people also call it a lighting model, a light reflection model, a local illumination model, a reflectance model, etc., etc.) light
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Tedious Reality • Properly determining the right color is really hard. • Look around the room. Each light source has different characteristics. Trillions of photons are pouring out every second. • These photons can: – interact with the atmosphere, or with things in the atmosphere – strike a surface and • be absorbed •b e r e f l e c t ed • cause fluorescence or phosphorescence • Of course, none of the surfaces here are perfectly flat. At some microscopic level (very important for photons) they’re all really bumpy. • Also, everything depends on wavelength.
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Light Source What is a Light source? - Any object that emits radiant energy Type of Light source: - Directional -P o i n t - Spot light - More complicated: extended and environment
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Directional Light Source • The light source is infinity away • Homogeneous Coordinate: ( X, Y, Z, 0 ) • Incoming light rays are parallel and of same intensity • Easy to compute with because the incoming light direction is the same for all surface points surface Sun (infinity away)
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Point Light Source • The light source (zero-volume) locates at a finite distance from the surface • Homogeneous Coordinate: ( X, Y, Z, 1 ) • Light rays goes out from the source in all directions • Harder to compute with because incoming light direction varies at different surface point • Attenuation – we may specify how the light goes dimmer with distance surface Point Light
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Spot Light Source • Special Point Light pointing at a particular direction (inherit all properties of point light source) • cutOffAngle defines the angle of the 3D cone , which restricts the shape of light emitting from the source • In addition to distance attenuation, we may attenuate the intensity of light by measuring its deviation from the spotlight pointing direction surface Spot Light
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Extended Light Source • Extended Light Source, e.g. Line or Area (fluorescent lamp) • Every point along the line or within the area is a point light source • More Realistic, but computation is very hard • Produce soft shadow: umbra and penumbra • No direct support in OGL, but can be approximated using ray tracing [Sillion] * May try this in your ray tracing project
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Environment as a Light Source • Environment assumed to be infinity away •T
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07-lightshade - Lecture 8 Lighting, Material, and Shading...

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