CAP 4703 Computer Graphic Methods

1. CAP 4703Computer Graphic Methods Prof. Roy Levow Chapter 6

2. Light and Matter • Surface may • emit light • reflect light • Rendering equation • Recusrively applying the effect from a source to the viewer yields the • Equation cannot be solved in general • Approximate approaches can be computationally intensive

3. Approximations • Ray tracing • trace light from source to eye • most rays don’t reach the eye • Radiosity • Treat surfaces as small regions each emitting fixed light • Calculate emission independent of viewer • Simplified ray tracing • Single interaction between light source and each surface

4. Light and Surfaces

5. Interaction with Surfaces • Specular surface • Similar to mirror • Light leaves atsame angle as entry

6. Interaction with Surfaces (cont) • Diffuse Surface • Light leaves inall directions

7. Translucent Surface some light penetrates surface leaves at modified angle – refraction some reflected Interaction with Surfaces (cont)

8. Six variable illumination function I(x,y,x,theta, phi, lambda) Generally too complex to compute for all points Light Sources

9. Light Sources • Consider four simpler light sources • Ambient lighting • Omnidirectional; constant everywhere • Point source • Radiates in all directions • Spot lights • Small cone of constant light • Distant light • rays are parallel

10. Color Sources • Describe color with 3-dimensional RGB vector • (Ir, Ig, Ib)T

11. Characteristics of Light Sources • Ambient light • every point in scene receives same light in every direction • Point source • Intensity decreases with inverse square of distance • Often approximate with form (a + bd +cd2)-1where d is distance to soften effect

12. Characteristics of Light Sources • Spot light • Light limited to cone of angle • diminishes toward edges of cone as cose(theta) for some value of e • intensity as for point source • Distant source • constant intensity • fixed direction

13. Described by 4 vectors n – normal to p v – to viewer l – to source r – reflected ray Phong Reflection Model

14. Phong Reflection Model • Supports • ambient • diffuse • specular or combination of these • Uses 3 x 3 matrix • columns for r – g – b • rows for ambient, diffuse, specular • Color source and reflection matrices

15. Phong Reflection Model • resulting intensity is sum or products of corresponding terms Iir = RiraLira+RirdLird+RirsLirs = Iira+Iird+Iirs for red, etc • Sum over all points for complete effect

16. Ambient Reflection • Ra = ka, 0 <= ka <= 1 • a fraction of light is reflected • one coefficient for each color

17. Diffuse reflection • Lambertian surfaces • reflect only vertical component • Rd proportional to cos(theta) • Id = kd (l . n) Ld/ (a + bd +cd2)

18. Specular Reflection • Phong model equation Is = ks Ls (R . v)a ks is shininess coefficient

19. OpenGL Shading • Polygonal shading • Generate shading on individual polygons • Flat shading glShadeModel(GL_FLAT); • select one vertex of polygon for normal • same color throughout polygon • tends to produce artificial looking surfaces with distinctly colored regions • eye is very sensitive to change • Mach Bands

20. Flat Shading

21. Interpolative and Gourand Shading • GL_SMOOTH • average normals • example: plates 4 & 5