Illumination Lighting and Shading CSE 470/598 Introduction to Computer Graphics Arizona State University Dianne Hansford

Terminology Illumination: 1. luminous flux at any point on a surface exposed to incident light (direct or indirect) 2. A light source 3. Spiritual or intellectual enlightenment

Terminology Illumination: 1. luminous flux at any point on a surface exposed to incident light (direct or indirect) 2. A light source 3. Spiritual or intellectual enlightenment Lighting: 1. Method to provide artificial illumination 2. Illumination 3. The act of igniting

Terminology Illumination: 1. luminous flux at any point on a surface exposed to incident light (direct or indirect) 2. A light source 3. Spiritual or intellectual enlightenment Lighting: 1. Method to provide artificial illumination 2. Illumination 3. The act of igniting Shading: 1. produce gradations of light or color 2. process of assigning colors to pixels

Terminology Illumination: 1. luminous flux at any point on a surface exposed to incident light (direct or indirect) 2. A light source Lighting: 1. The method used to provide artificial illumination 2. Illumination Shading: 1. produce gradations of light or color 2. process of assigning colors to pixels But you’ll hear them interchanged frequently!

Lighting Overview CG lighting models Elements of a lighting model The Phong illumination model Application of the Phong model Shading methods: flat, Gouraud, Phong OpenGL aspects

CG Lighting Models: Global Multiple interaction of light & objects Not real-time (yet) Examples: Raytracing, radiosity, photon mapping … From: llery/stills.html llery/stills.html

CG Lighting Models: Local Single interaction of light & objects Real-time Supported by OGL Example: Phong illumination model

Elements of a Lighting Model: light sources: number, type (desk lamp vs sun), color

Elements of a Lighting Model: light sources: number, type, color reflections

Elements of a Lighting Model: light sources: number, type, color reflections material properties: reflection & absorption of light

Elements of a Lighting Model: light sources: number, type, color reflections material properties: reflection & absorption of light 3D feel, depth perception lighting model == approximation of real-world lighting!

Elements of the Phong Model Light Source Properties All calculations based on idea that RGB calculated independently

Elements of the Phong Model Light Source Properties 1.ambient light > scattered > no detectable direction > backlighting in a room > can use to give a feel for the main color in a room > not dependent on viewpoint

Elements of the Phong Model Light Source Properties 1.ambient light 2.diffuse light > directional > scatters equally in all directions once hits object > closest to the color of light > not dependent on eye position

Elements of the Phong Model Light Source Properties 1.ambient light 2.diffuse light 3.specular light > comes from a detectable direction > bounces off object in preferred direction > plays a role in shininess > dependent on viewpoint diffuse and specular normally set the same

Elements of the Phong Model Light Source Properties 1.ambient light 2.diffuse light 3.specular light 4.point source vs spotlight > point source: light emitted in all directions > spotlight: cone-shaped

Elements of the Phong Model Light Source Properties 1.ambient light 2.diffuse light 3.specular light 4.point source vs spotlight 5.positional vs directional > positional: like a desk lamp > directional: like the sun all rays parallel when reach object > homogeneous coordinate to distinguish > location transformed by modelview matrix xyzwxyzw

Elements of the Phong Model Material properties

Elements of the Phong Model Material properties 1. reflectance of light a. ambient > amount of ambient light > most visible where no direct light hits

Elements of the Phong Model Material properties 1. reflectance of light a. ambient b. diffuse > degree of scattering of light on surface > matte vs flat paint finish Color of object == ambient and diffuse (typically set the same)

Elements of the Phong Model Material properties 1. reflectance of light a. ambient b. diffuse c. specular > degree of mirror-like quality > typically set to white so highlights produced are color of light

Elements of the Phong Model Material properties 1. reflectance of light a. ambient b. diffuse c. specular d. translucent (opaque)

Elements of the Phong Model Material properties 1. reflectance of light a. ambient b. diffuse c. specular d. translucent (opaque) 2. surface normals (unit length!)

Elements of the Phong Model Material properties 1. reflectance of light a. ambient b. diffuse c. specular d. translucent (opaque) 2. surface normals 3. emissive color

Light & Material Properties Examples increasing diffuseincreasing ambient increasing specular

Light & Material Properties absorption/reflectance influence on color Example: red box will reflect red light absorb green and blue light

Notation: Light Properties Model is computed independently for red, green, blue components Light’s luminance represented by boldface vectors: Ld := diffuse Ls := specular rgbrgb 0 <= r,g,b <= 1  % of full intensity La = := ambient

Notation: Material Properties Material’s properties represented by boldface vectors: ka := ambient kd := diffuse ks := specular Each vector takes form rgbrgb Represents % of reflection of light source’s corresponding property 0 <= r,g,b <= 1

Notation: Material Properties Example material properties:

Geometry of the Phong Model ppoint on surface l (light – p) vector n normal to surface r reflection vector v (viewpoint – p) vector theta angle of incidence phi angle between v and r v p All vectors normalized Recall: angle of incidence equals angle of reflection

Phong Model in OGL vertex color = material emission‡ + (global ambient light scaled by material ambient property) + (ambient, diffuse, specular of lights, attenuated by material properties, viewer location and light position) ‡ at vertex

Diffuse Intensity Calculation Lambert’s Law: light reflected is proportional to the cosine of the angle (theta) between surface normal n and light vector l theta is called the angle of incidence

Diffuse Intensity Calculation Lambert’s Law: light reflected is proportional to the cosine of the angle (theta) between surface normal n and light vector l theta is called the angle of incidence theta=0theta=60

Diffuse Intensity Calculation Id := intensity of reflected diffuse light Id = kd x Ld x cos(theta) cos(theta) = l n theta in [-90,90°] are of interest Id = kd x Ld x (max{l n, 0}) “x” is not cross product 3 separate scalar products Note: independent of viewer

Specular Intensity Calculation cos(phi) = v rr = [2(ln)]n - l Is := intensity of reflected specular light Basic idea: Is = ks x Ls x cos^s(phi) Focus of specular influenced by s Note: depends on viewpoint phi = 0°  full specular |phi| > 90°  no specular (never compute angle directly)

Specular Intensity Calculation s: Phong constant or “shininess” coefficient 90°- 90° s=10 focus s=1 s=0.1 spread cos^s(phi)

Specular Intensity Calculation Blinn-Torrence modification – simplification for faster computation h = (l + v) / || l + v || “halfway” vector cos(alpha) = h n alpha ~ ½ phi so good approximation

Specular Intensity Calculation Blinn-Torrence specular is implemented in OGL Is = ks x Ls x [max{h n, 0}]^s if l n < 0 then Is = 0 (no diffuse, no specular)

Attenuation Function For a positional light... d := distance of light source to vertex for directional light, f(d) = 1 inverse distance functions  diminish intensity d increases ogl choices for functions f(d) = 1/a f(d) = 1/(a + b*d) f(d) = 1/(a + b*d + c*d^2) constant linear quadratic

Spotlight Effect Cone-shaped spotlight defined by: c position gamma “cut-off” angle d direction cos(delta) = -l d if delta > gamma sp = 0 else sp = (max{-l d, 0})^p sp := spotlight effect for a light source defined by angle between -l and d : p influence similar to Phong constant; focus of intensity d gamma c

Putting It All Together Phong Model in OGL I = e + (ka x Ma) + for each light { [f(d) * sp * (ka*La + Id + Is)] } I := intensity at a vertex e := emission intensity at a vertex Ma := ambient intensity for the entire model Remember: boldface indicates r,g,b values

Shading Methods Phong model  color of vertex Shading methods  color of triangle Methods: 1.Flat shading 2.Gouraud (smooth) shading 3.Phong‡ Shading ‡ confusing, but different from Phong illumination model in ogl Recall: triangle normal.vs. averaged vertex normal

Flat Shading One normal per triangle glShadeModel(GL_FLAT) Simulates viewer and light source distant then v, n, l same over triangle  one shading calculation

Gouraud (smooth) Shading One normal per vertex Lighting calculation made at each vertex  I1, I2, I3 Lighting at any point p within triangle v1, v2, v3 I = b1*I1 + b2*I2 + b3* I3 where b1, b2, b3 are the barycentric coordinates of p wrt v1, v2, v3 p = b1*v1 + b2*v2 + b3*v3 (b1 + b2 + b3 = 1)

Phong Shading One normal per vertex... however a normal is calculated for each rendered point p in triangle vertex normals n1, n2, n3 p = b1*v1 + b2*v2 + b3*v3 n = b1*n1 + b2*n2 + b3*n3 Calculate intensity at p wrt n Not considered a real-time algorithm therefore, not in ogl

Setting up the Lighting Model OGL’s glLightModel has four settings … 1.local vs infinite viewer 2.one-sided vs two-sided 3.global ambient intensity 4.specular and texture interaction default: infinite default: one-sided default: none let’s revisit when we discuss texture front-facing triangle: on screen counterclockwise orientation one-sided = just shade front-facing two-sided = shade front and back-facing triangles nice for inside and outside color effect affects highlights of specular infinite: v vector for all vertices the same

OGL Calls Basic steps... 1.create, position & enable lights 2.viewer local or infinite? 3.front and back polygon shading? 4.set material properties Keep in mind that local viewer and lights require more computation Default camera: eye at origin and looking down –z axis (This is eye coordinates.) Reading: Chapter 5 !!

Controlling the Light’s Position Light stationary: glModelMatrixMode(GL_MODELVIEW) glLoadIdentity(); modeling and viewing here glLightfv(GL_LIGHT0, GL_POSITION, position)

Controlling the Light’s Position Rotate light about stationary object: glPushMatrix(); gluLookAt(…); glPushMatrix(); glRotate*(…); glLightfv (GL_LIGHT0, GL_POSITION, position ) glPopMatrix(); draw_object(); glPopMatrix();

Controlling the Light’s Position Move light with viewpoint: key: specify light position in eye coordinates before viewing transf. GLfloat position[] = {0, 0, 0, 1} … glModelMatrixMode(GL_MODELVIEW) glLoadIdentity(); glLightfv(GL_LIGHT0, GL_POSITION, position) glPushMatrix() gluLookAt( …) draw object() glPopMatrix() aka: minor’s hat

Resources Many figures for these slides were taken from Pascal Vuytsteker’s website: ex.en.html Of course, OGL Red book Chapter 5

Material Properties r,g,b,alpha 

Material Properties