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2IV60 Computer graphics set 8: Illumination Models and Surface-Rendering Methods

Learn about illumination models and surface rendering methods in computer graphics. This tutorial covers the basics of lighting sources, reflections, and the Phong illumination model.

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2IV60 Computer graphics set 8: Illumination Models and Surface-Rendering Methods

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  1. 2IV60 Computer graphicsset 8: Illumination Models and Surface-Rendering Methods Jack van Wijk TU/e

  2. OpenGL Illumination example GlfloatlightPos[] = {2.0, 0.0, 3.0, 0.0}; GlfloatwhiteColor[] = {1.0, 1.0, 1.0, 1.0}; GlfloatpinkColor[] = {1.0, 0.5, 0.5, 1.0}; glShadeModel(GL_SMOOTH); // Use smooth shading glEnable(GL_LIGHTING); // Enable lighting glEnable(GL_LIGHT0); // Enable light source #0 glLightfv(GL_LIGHT0, GL_POSITION, lightPos); // position LS 0 glLightfv(GL_LIGHT0, GL_DIFFUSE, whiteColor); // set color LS 0 glMaterialfv(GL_FRONT, GL_DIFFUSE, pinkColor); // set surface // color glBegin(GL_TRIANGLES); glNormal3fv(n1); glVertex3fv(v1); // draw triangle, give glNormal3fv(n2); glVertex3fv(v2); // first normal, followed glNormal3fv(n3); glVertex3fv(v3); // by vertex glEnd(); What is going on here?

  3. Introduction 1 Illumination model: Given a point on a surface, what is the perceived color and intensity? Known as Lighting Model, or Shading Model Surface rendering: Apply the Illumination model to color all pixels of the surface. H&B 17:531-532

  4. Introduction 2 Example: • Illumination model gives color vertices, • Surface is displayed via interpolation of these colors. H&B 17:531-532

  5. Introduction 3 Illumination: • Physics: • Material properties, light sources, relative positions, properties medium • Psychology: • Perception, what do we see • Color! • Often approximating models H&B 17:531-532

  6. Light sources 1 Light source: object that radiates energy. Sun, lamp, globe, sky… Intensity I = (Ired , Igreen , Iblue) If Ired = Igreen = Iblue : white light H&B 17-1:532-536

  7. Light sources 2 Simple model: point light source • position P and intensity I • Light rays along straight lines • Good approximation for small light sources H&B 17-1:532-536

  8. Light sources 3 Simpler yet: point light source at infinity • Direction V and intensity I • Sunlight V H&B 17-1:532-536

  9. Light sources 4 Damping: intensity of light decreases with distance Energy is distributed over area sphere, hence Il = I / d2, with d distance to light source. In practice often too ‘agressive’, hence Il = I / (a0 +a1d+a2d2) If light source at infinity: No damping with distance d H&B 17-1:532-536

  10. Q a l light cone Vlight Light sources 5 Directed light source, spotlight: Light is primarily send in direction of Vlight . P H&B 17-1:532-536

  11. Q a l light cone Vlight Light sources 6 More subtle: Let I decrease with increasing angle a. P H&B 17-1:532-536

  12. reflection transmission Surface illumination 1 • When light hits a surface, three things can happen: absorption H&B 17-2:536-537

  13. Surface illumination 2 • Suppose, a light source radiates white light, consisting of red, green and blue light. If only red light is reflected, then we see a red surface. reflection absorption transmission H&B 17-2:536-537

  14. Surface illumination 3 • Diffuse reflection: Light is uniformly reflected in all directions • Specular reflection: Light is stronger reflected in one direction. specular reflection diffuse reflection H&B 17-2:536-537

  15. Surface illumination 4 • Ambientlight: light from the environment. Undirected light, models reflected light of other objects. H&B 17-2:536-537

  16. Basic illumination model 1 Basic illumination model: • Ambient light; • Point light sources; • Ambient reflection; • Diffuse reflection; • Specular reflection. H&B 17-3:537-546

  17. Basic illumination model 2 • Ambientlight: environment light. Undirected light, models reflected light of other objects. H&B 17-3:537-546

  18. dA  dA/cos  Basic illumination model 3 Perfect diffuse reflector: light is reflected uniformly in all directions. H&B 17-3:537-546

  19. dA Basic illumination model 4 Perfect diffuse reflector: light is reflected uniformly in all directions. . • Lambert’s law: • Reflected energy is • proportional with cos , where • denotes the angle between the • normal N and a vector to the light • source L. N L   dA/cos  H&B 17-3:537-546

  20. Basic illumination model 5 Perfect diffuse reflector: light is reflected uniformly in all directions. Psource N Il L Psurf H&B 17-3:537-546

  21. Basic illumination model 6 Perfect specular reflector: light is only reflected in one direction. Angle of incidence is angle of reflection. N L R q q H&B 17-3:537-546

  22. Basic illumination model 7 Imperfect specular reflector: light is distributed in the direction of the angle of reflection, dependent on the roughness of the surface. N N L L R R q q q q glad ruw H&B 17-3:537-546

  23. Basic illumination model 8 Phong model: empirical model for specular reflection N L R V q q  H&B 17-3:537-546

  24. Basic illumination model 9 Phong model: empirical model for specular reflection N L R V q q  H&B 17-3:537-546

  25. L V N.L Basic illumination model 10 Phong model: calculating the vectors N L R H&B 17-3:537-546

  26. Basic illumination model 11 Phong model: variant with halfway vector H. Use a instead of f. N H L R a V  If light source and viewer far away: H  constant. H&B 17-3:537-546

  27. Basic illumination model 12 All together: H&B 17-3:537-546

  28. Basic illumination model 13 Color (reprise): Light intensity I and reflection coefficients k: (r,g,b) triplets So for instance: Plastic: kd is colored (r,g,b), ks is grey (w,w,w) Metal: kd and ks same color Basic model: simple but effective. It can be done much better though… H&B 17-3:537-546

  29. Transparancy 1 Transparant object: • reflected and transmitted light • refraction • scattering H&B 17-4:546-549

  30. Transparancy 2 Snell’s law of refraction: N L R qi qi T qr H&B 17-4:546-549

  31. Transparancy 3 Thin surface: • double refraction • shift of light ray H&B 17-4:546-549

  32. Poor result for silhouette edges… Transparancy 3 Very thin surface: • Discard shift H&B 17-4:546-549

  33. Atmospheric effects 1 Atmospheric effects: • dust, smoke, vapor • colors are dimmed • objects less well visible H&B 10-5:549-550

  34. = 0.25 + [ 1  0.25 ] Atmospheric effects 2 H&B 10-5:549-550

  35. Rendering polygons 1 Basic illumination model: Can be used per point, but that’s somewhat expensive More efficient: Illumination model gives color for some points; Surface is filled in using interpolation of these colors. H&B 17-10:559-564

  36. Rendering polygons 2 Constant-intensity rendering aka flat surface rendering: • Determine color for center of polygon; • Fill the polygon with a constant color. • Ok if: • Object consists of planar faces, and • Light sources are far away, and • Eye point is far away, • or • Polygons are about a pixel in size. H&B 17-10:559-564

  37. Rendering polygons 2 Constant-intensity rendering aka flat surface rendering: • Determine color for center of polygon; • Fill the polygon with a constant color. Highlights not visible, Facetted appearance, increased by Mach banding effect. H&B 17-10:559-564

  38. Mach banding • Human perception: edges are given emphasis, contrast is increased near edges. Angel (2000) H&B 17-10:559-564

  39. N2 N3 N1 V N4 Rendering polygons 2 Gouraud surface rendering: • Determine average normal on vertices; • Determine color for vertices; • Interpolate the colors per polygon (incrementally). H&B 17-10:559-564

  40. Rendering polygons 3 Gouraud surface rendering: • Much better result for curved surfaces • Errors near highlights • Linear interpolation still gives Mach banding • Silhouettes are still not smooth Gouraud Flat

  41. Fast Phong surface rendering: Like Phong surface rendering, but use 2nd order approximation of color over polygon: Rendering polygons 4 Phong surface rendering: • Determine average normal per vertex; • Interpolate normals per polygon (incrementally); • Calculate color per pixel. H&B 17-10:559-564

  42. Rendering polygons 5 Phong surface rendering: • Even better result for curved surfaces • No errors at high lights • No Mach banding • Silhouettes remain coarse • More expensive than flat or Gouraud shading H&B 17-10:559-564

  43. Rendering polygons 5 Flat Gouraud Phong H&B 17-10:559-564

  44. OpenGL Illumination GlfloatlightPos[] = {2.0, 0.0, 3.0, 0.0}; GlfloatwhiteColor[] = {1.0, 1.0, 1.0, 1.0}; GlfloatpinkColor[] = {1.0, 0.5, 0.5, 1.0}; glShadeModel(GL_SMOOTH); // Use smooth shading glEnable(GL_LIGHTING); // Enable lighting glEnable(GL_LIGHT0); // Enable light source #0 glLightfv(GL_LIGHT0, GL_POSITION, lightPos); // position LS 0 glLightfv(GL_LIGHT0, GL_DIFFUSE, whiteColor); // set color LS 0 glMaterialfv(GL_FRONT, GL_DIFFUSE, pinkColor); // set surface // color glBegin(GL_TRIANGLES); glNormal3fv(n1); glVertex3fv(v1); // draw triangle, give glNormal3fv(n2); glVertex3fv(v2); // first normal, followed glNormal3fv(n3); glVertex3fv(v3); // by vertex glEnd(); H&B 17-11:564-574

  45. OpenGL Light-sources 1 First, enable lighting in general: glEnable(GL_LIGHTING); OpenGL provides (at least) eight light-sources: lightName = GL_LIGHT0, GL_LIGHT1, … , GL_LIGHT7 Enable the one(s) you need with: glEnable(lightName); Set properties with glLight*(lightName, lightProperty, propertyValue); * = i, f, iv, or fv (i: integer, f: float, v vector) H&B 17-11:564-574

  46. OpenGL Light-sources 2 • Position light-source: • GlfloatsunlightPos[] = {2.0, 0.0, 3.0, 0.0}; • GlfloatlamplightPos[] = {2.0, 0.0, 3.0, 1.0}; • glLightfv(GL_LIGHT1, GL_POSITION, sunlightPos); • glLightfv(GL_LIGHT2, GL_POSITION, lamplightPos); • Fourth coordinate = 0: source at infinity • Fourth coordinate = 1: local source • Specified in world-coordinates, according to the currentModelView specification – just like geometry. Hence, take care when you specify the position. • Light from above looks more natural H&B 17-11:564-574

  47. OpenGL Light-sources 3 • Color light-source: • GlfloatgreyColor[] = {0.3, 0.3, 0.3, 1.0}; • GlfloatpinkColor[] = {1.0, 0.7, 0.7, 1.0}; • GlfloatwhiteColor[] = {1.0, 1.0, 1.0, 1.0}; • glLightfv(GL_LIGHT1, GL_AMBIENT, greyColor); • glLightfv(GL_LIGHT1, GL_DIFFUSE, pinkColor); • glLightfv(GL_LIGHT1, GL_SPECULAR, whiteColor); • OpenGL light-source has three color properties, dependent on reflection surface. Not realistic, can be used for special effects. • If you don’t have ambient light, things often appear black. • Colors are always 4-vectors here: Fourth coordinate is alpha. Most cases: set it to 1.0. • More settings: See book H&B 17-11:564-574

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