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

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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

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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

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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

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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!

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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

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CG Lighting Models: Global Multiple interaction of light & objects Not real-time (yet) Examples: Raytracing, radiosity, photon mapping … From: http://jedi.ks.uiuc.edu/~johns/raytracer/rayga llery/stills.html http://jedi.ks.uiuc.edu/~johns/raytracer/rayga llery/stills.html

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

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

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

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

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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!

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

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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

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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

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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

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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

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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

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Elements of the Phong Model Material properties

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

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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)

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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

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

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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!)

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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

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Light & Material Properties Examples increasing diffuseincreasing ambient increasing specular

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

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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

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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

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Notation: Material Properties Example material properties:

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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

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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

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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

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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

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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

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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)

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

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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

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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)

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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

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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

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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

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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

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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

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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)

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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

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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

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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 !!

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

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

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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

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Resources Many figures for these slides were taken from Pascal Vuytsteker’s website: http://escience.anu.edu.au/lecture/cg/Illumination/ind ex.en.html Of course, OGL Red book Chapter 5

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Material Properties r,g,b,alpha

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Material Properties

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