Computer Graphics Lighting

Outline Lighting Lighting models Ambient Diffuse Specular Surface Rendering Methods

What we know We already know how to render the world from a viewpoint.

“Lighting” Two components: Lighting Model or Shading Model - how we calculate the intensity at a point on the surface Surface Rendering Method - How we calculate the intensity at each pixel

Jargon Illumination - the transport of light from a source to a point via direct and indirect paths Lighting - computing the luminous intensity for a specified 3D point, given a viewpoint Shading - assigning colors to pixels Illumination Models: Empirical - approximations to observed light properties Physically based - applying physics properties of light and its interactions with matter

The lighting problem… What are we trying to solve? Global illumination – the transport of light within a scene. What factors play a part in how an object is “lit”? Let’s examine different items here…

Two components Light Source Properties Object Properties Color (Wavelength(s) of light) Shape Direction Object Properties Material Geometry Absorption

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Global Effects shadow multiple reflection translucent surface Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Local vs Global Rendering Correct shading requires a global calculation involving all objects and light sources Incompatible with pipeline model which shades each polygon independently (local rendering) However, in computer graphics, especially real time graphics, we are happy if things “look right” Exist many techniques for approximating global effects Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Light Source Properties Color We usually assume the light has one wavelength Shape point light source - approximate the light source as a 3D point in space. Light rays emanate in all directions. good for small light sources (compared to the scene) far away light sources?

Distributed Lights Light Source Shape continued distributed light source (not supported natively in OpenGL) - approximating the light source as a 3D object. Light rays usually emanate in specific directions good for larger light sources area light sources

Light Source Direction In computer graphics, we usually treat lights as rays emanating from a source. The direction of these rays can either be: Omni-directional (point light source) Directional angle (spotlights) Directional (parallel rays)

Light Position We can specify the position of a light with an x, y, and z coordinate. What are some examples? These lights are called positional lights Q: Are there types of lights that we can simplify? A: Yep! Think about the sun. If a light is significantly far away, we can represent the light with only a direction vector. These are called directional lights. How does this help?

Contributions from lights We will breakdown what a light does to an object into three different components. This APPROXIMATES what a light does. To actually compute the rays is too expensive to do in real-time. Light at a pixel from a light = Ambient + Diffuse + Specular contributions. Ilight = Iambient + Idiffuse + Ispecular

Ambient Term - Background Light The ambient term is a HACK! It represents the approximate contribution of the light to the general scene, regardless of location of light and object Indirect reflections that are too complex to completely and accurately compute Iambient = color

Diffuse Term Contribution that a light has on the surface, regardless of viewing direction. Diffuse surfaces, on a microscopic level, are very rough. This means that a ray of light coming in has an equal chance of being reflected in any direction. What are some ideal diffuse surfaces?

Lambert’s Cosine Law Diffuse surfaces follow Lambert’s Cosine Law Lambert’s Cosine Law - reflected energy from a small surface area in a particular direction is proportional to the cosine of the angle between that direction and the surface normal. Think about surface area and # of rays

Diffuse Term To determine how much of a diffuse contribution a light supplies to the surface, we need the surface normal and the direction on the incoming ray What is the angle between these two vectors? Idiffuse = kdIlightcos = kdIlight(N . L) Ilight = diffuse (intensity) of light kd [0..1] = surface diffuse reflectivity What CS are L and N in? How expensive is it?

Example What are the possible values for theta (and thus the dot product?) http://graphics.lcs.mit.edu/classes/6.837/F98/Lecture18/Slide11.html

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Normal for Triangle n plane n ·(p - p0 ) = 0 p1 n = (p1 - p0 ) × (p2 - p0 ) p p0 p2 normalize n  n/ |n| X Note that right-hand rule determines outward face (programatically: ‘winding’ or order of vertices) Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Specular Reflection Specular contribution can be thought of as the “shiny highlight” of a plastic object. On a microscopic level, the surface is very smooth. Almost all light is reflected. What is an ideal purely specular reflector? What does this term depend on? Viewing Direction Normal of the Surface

Snell’s Law Specular reflection applies Snell’s Law. We assume l = r

Snell’s Law is for IDEAL surfaces Most surfaces are not ideal. Think about the amount of light reflected at different angles. N R L V 

Different for shiny vs. dull objects

Snell’s Law is for IDEAL surfaces Think about the amount of light reflected at different angles. N R L V   

Phong Model Phong Reflection Model An approximation: set the intensity of specular reflection proportional to (cos )shininess What are the possible values of cos ? What does the value of shininess mean? How do we represent shinny or dull surfaces using the Phong model? Ispecular = ksIlight (cos )shininess = ksIlight (V.R)shininess

The Shininess Coefficient Values of a between 100 and 200 correspond to metals Values between 5 and 10 give surface that look like plastic cosa f -90 f 90 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

How do we compute R? N*(N.L) R+L=2N(N.L) R = 2N(N.L)-L L N R N(N.L) V  L 

Simplify this Instead of R, we compute halfway between L and V. We call this vector the halfway vector, H. H N R V  L 

Combining the terms Ambient - the combination of light reflections from various surfaces to produce a uniform illumination. Background light. Diffuse - uniform light scattering of light rays on a surface. Proportional to the “amount of light” that hits the surface. Depends on the surface normal and light vector. Sepecular - light that gets reflected. Depends on the light ray, the viewing angle, and the surface normal.

Ambient + Diffuse + Specular

Lighting Equation N R L V  Ilambient = light source l’s ambient component Ildiffuse = light source l’s diffuse component Ilspecular = light source l’s specular component kambient = surface material ambient reflectivity kdiffuse = surface material diffuse reflectivity kspecular = surface material specular reflectivity shininess = specular reflection parameter (1 -> dull, 100+ -> very shiny) N R L V 

Attenuation One factor we have yet to take into account is that a light source contributes a higher incident intensity to closer surfaces. What happens if we don’t do this?

Subtleties What’s wrong with: What’s a good fix?

Full Illumination Model Run demo

Steps in OpenGL lighting Enable lighting and select model Specify normals Specify material properties Specify lights Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Normal for Triangle n plane n ·(p - p0 ) = 0 p1 n = (p1 - p0 ) × (p2 - p0 ) p p0 p2 normalize n  n/ |n| X Note that right-hand rule determines outward face (programatically: ‘winding’ or order of vertices) Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Normals In OpenGL the normal vector is part of the state Set by glNormal*() glNormal3f(x, y, z); glNormal3fv(p); Usually we want to set the normal to have unit length so cosine calculations are correct Length can be affected by transformations Note that scaling does not preserved length glEnable(GL_NORMALIZE) allows for autonormalization at a performance penalty Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Shading Shading is how we “color” a triangle. Constant Shading Gouraud Shading

Constant Shading Constant Intensity or Flat Shading One color for the entire triangle Fast Good for some objects What happens if triangles are small? Sudden intensity changes at borders

Gouraud Shading Intensity Interpolation Shading Calculate lighting at the vertices. Then interpolate the colors

Gouraud Shading Relatively fast, only do three calculations No sudden intensity changes What can it not do? What are some approaches to fix this? Question, what is the normal at a vertex?

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Enabling Shading Shading calculations are enabled by glEnable(GL_LIGHTING) Once lighting is enabled, glColor() ignored Must enable each light source individually glEnable(GL_LIGHTi) i=0,1….. Can choose light model parameters glLightModeli(parameter, GL_TRUE) GL_LIGHT_MODEL_LOCAL_VIEWER do not use simplifying distant viewer assumption in calculation GL_LIGHT_MODEL_TWO_SIDED shades both sides of polygons independently Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Defining a Point Light Source For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the position GL float diffuse0[]={1.0, 0.0, 0.0, 1.0}; GL float ambient0[]={1.0, 0.0, 0.0, 1.0}; GL float specular0[]={1.0, 0.0, 0.0, 1.0}; Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0}; glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); glLightv(GL_LIGHT0, GL_POSITION, light0_pos); glLightv(GL_LIGHT0, GL_AMBIENT, ambient0); glLightv(GL_LIGHT0, GL_DIFFUSE, diffuse0); glLightv(GL_LIGHT0, GL_SPECULAR, specular0); Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Distance and Direction The source colors are specified in RGBA The position is given in homogeneous coordinates If w =1.0, we are specifying a finite location If w =0.0, we are specifying a parallel source with the given direction vector The coefficients in the distance terms are by default a=1.0 (constant terms), b=c=0.0 (linear and quadratic terms). Change by a= 0.80; glLightf(GL_LIGHT0, GLCONSTANT_ATTENUATION, a); Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Spotlights Use glLightv to set Direction GL_SPOT_DIRECTION Cutoff GL_SPOT_CUTOFF Attenuation GL_SPOT_EXPONENT Proportional to cosaf f -q q Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Global Ambient Light Ambient light depends on color of light sources A red light in a white room will cause a red ambient term that disappears when the light is turned off OpenGL also allows a global ambient term that is often helpful for testing glLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient) Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Moving Light Sources Light sources are geometric objects whose positions or directions are affected by the model-view matrix Depending on where we place the position (direction) setting function, we can Move the light source(s) with the object(s) Fix the object(s) and move the light source(s) Fix the light source(s) and move the object(s) Move the light source(s) and object(s) independently Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Material Properties Material properties are also part of the OpenGL state and match the terms in the modified Phong model Set by glMaterialv() GLfloat ambient[] = {0.2, 0.2, 0.2, 1.0}; GLfloat diffuse[] = {1.0, 0.8, 0.0, 1.0}; GLfloat specular[] = {1.0, 1.0, 1.0, 1.0}; GLfloat shine = 100.0 glMaterialf(GL_FRONT, GL_AMBIENT, ambient); glMaterialf(GL_FRONT, GL_DIFFUSE, diffuse); glMaterialf(GL_FRONT, GL_SPECULAR, specular); glMaterialf(GL_FRONT, GL_SHININESS, shine); Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Front and Back Faces The default is shade only front faces which works correctly for convex objects If we set two sided lighting, OpenGL will shade both sides of a surface Each side can have its own properties which are set by using GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK in glMaterialf back faces not visible back faces visible Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009

Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Emissive Term We can simulate a light source in OpenGL by giving a material an emissive component This component is unaffected by any sources or transformations GLfloat emission[] = 0.0, 0.3, 0.3, 1.0); glMaterialf(GL_FRONT, GL_EMISSION, emission); Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009