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What is Lighting? 2 Example 1. Find the cubic polynomial or that passes through the four points and satisfies 1.As a photon Metal Insulator

What is Lighting? 3 Light plays an important part in computer graphics for rendering realistic images. Using lighting models, we can simulate shading, reflection and refraction of light, comparable to what we see in the real world. 2. As a wave

What is a Illumination? 4 Example 1. Find the cubic polynomial or that passes through the four points and satisfies Interactions between light sources and the object surface An “illumination model” describes inputs, assumptions, and outputs used to calculate illumination (color / brightness) of surface elements Usually includes Light attributes (intensity, color, position, direction, shape) Object surface properties (color, reflectivity, transparency, etc.) Interaction between lights and objects

Basic Light Sources 5 Example 1. Find the cubic polynomial or that passes through the four points and satisfies  The amount that a light illuminates an object decreases with the square of the distance between them. This is known as Light Attenuation. As part of illumination models, one can specify whether the light intensity varies with the distance between the object and the light source

Illumination Models  Local Illumination  Takes only direct lighting information  refers to direct interaction between one light source and one object surface.  Global Illumination  refers to the interaction of light between all surfaces in a scene.  Most light striking a surface element comes directly from a light emitting source (direct illumination)  Sometimes light from a source is blocked by another object, resulting in shadows  However, objects in the shadow can still receive light from light bouncing off other objects (indirect illumination) 6

Example 1: Local Illumination Only considers the light, the observer position, and the object’s material properties OpenGL does this.

Example 2: Global Illumination Takes into account of the interaction of light from all the surfaces in the scene Recursive ray tracing is an example. It models light rays bouncing between objects

Local Illumination Model 9 In computer graphics, single object-light interaction is approximated through local illumination models. Basic model used is the Phong model which breaks local illumination into 3 components: Ambient reflection Diffuse reflection Specular reflection For every point, or small surface area, of an object, we want to calculate the light due to these three components.

Ambient Reflection 10 Example 1. Find the cubic polynomial or that passes through the four points and satisfies Ambient light = background light, it is used to simulate indirect lighting Ambient component Independent of Object’s position Viewer’s position Light source’s position Dependent of A constant factor (in each of the R, G, B channels)

Diffuse Reflection Diffuse light = illumination that a surface receives from a light source that reflects equally in all directions Independent of: Viewer’s position Dependent of: Light’s position Surface property (normal, reflectance property) 11

Lambert’s Law Need to know how much light a point on the object receives from the light source Solution based on Lambert’s Law 12 Point receives more light Point receives less light Diffuse = Kd*I*cos(theta)

Specular Reflection I  Specular light = light reflection from shiny surfaces  Color depends on material and how it scatters light  Shiny surfaces (metal, mirror, etc.) reflect more light  Specular light depends on both light source position and view position 13 Example 1. Find the cubic polynomial or that passes through the four points and satisfies

Specular Reflection II 14 Example 1. Find the cubic polynomial or that passes through the four points and satisfies

Variation in Diffuse and Specular in Local Model Putting Diffuse and Specular together in a local illumination model 15 Example 1. Find the cubic polynomial or that passes through the four points and satisfies

Phong Lighting Equation 16

Shading vs. Lighting  How is Shading different from Lighting?  Lighting evaluates the lighting equation at each point on the surface of an object  Shading is kind of a “hack” that computes only lighting equations on the vertices, and interpolates the pixels in between