Illumination Lighting and Shading

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

2. Terminology • Illumination:1. luminous flux at any point on a surface exposed to incident light (direct or indirect)2. A light source3. Spiritual or intellectual enlightenment

3. Terminology • Illumination:1. luminous flux at any point on a surface exposed to incident light (direct or indirect)2. A light source3. Spiritual or intellectual enlightenment • Lighting:1. Method to provide artificial illumination2. Illumination3. The act of igniting

4. Terminology • Illumination:1. luminous flux at any point on a surface exposed to incident light (direct or indirect)2. A light source3. Spiritual or intellectual enlightenment • Lighting:1. Method to provide artificial illumination2. Illumination3. The act of igniting • Shading:1. produce gradations of light or color2. process of assigning colors to pixels

5. 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 illumination2. Illumination • Shading: 1. produce gradations of light or color2. process of assigning colors to pixels But you’ll hear them interchanged frequently!

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

7. 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/raygallery/stills.html

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

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

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

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

12. 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!

13. Elements of the Phong Model Light Source Properties All calculations based on idea thatRGB calculated independently

14. Elements of the Phong Model Light Source Properties • 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

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

16. Elements of the Phong Model Light Source Properties • ambient light • diffuse light • 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

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

18. Elements of the Phong Model Light Source Properties • ambient light • diffuse light • specular light • point source vs spotlight • 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 x y z w

19. Elements of the Phong Model Material properties

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

21. 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)

22. 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

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

24. 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!)

25. 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

26. Light & Material PropertiesExamples increasing diffuse increasing ambient increasing specular

27. Light & Material Properties absorption/reflectance influence on color Example: redbox will reflectred light absorbgreen and blue light

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

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

30. Notation: Material Properties Example material properties:

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

32. 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

33. 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

34. 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=0 theta=60

35. 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 product3 separate scalar products Note: independent of viewer

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

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

38. 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

39. 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)

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

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

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

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

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

45. 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)

46. 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

47. Setting up the Lighting Model OGL’s glLightModel has four settings … • local vs infinite viewer • one-sided vs two-sided • global ambient intensity • specular and texture interaction default: infinite affects highlights of specularinfinite: v vector for all vertices the same default: one-sided front-facingtriangle: on screen counterclockwise orientationone-sided = just shade front-facingtwo-sided = shade front and back-facing trianglesnice for inside and outside color effect default: none let’s revisit when wediscuss texture

48. OGL Calls Basic steps ... • create, position & enable lights • viewer local or infinite? • front and back polygon shading? • 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 !!

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

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