CS 148, Summer 2012 Introduction to Computer Graphics and Imaging

Transcription

1 CS 148, Summer 2012 Introduction to Computer Graphics and Imaging

2 f(~v 2 ) A 3 A 1 f(~v 1 ) A 2 f(~v 3 ) How to write shaders and relevant science, but not how to shade!

3 Interplay between: Efficiency of rendering Desired artistic outcome Physicality of material

4 How do physical materials interact with light?

5 Surface to render (assume opaque!)

6 Normal vector (determines orientation) ~N

7 ~N ~ko Camera direction (unit vector)

8 Light direction (unit vector) ~ ki ~N ~ko

9 ½( ~ k i ; ~ k o ; ~ N) Amount of light leaving the surface in each direction given input direction Bidirectional Reflectance Distribution Function

10 Conserves energy Z Positive S 2 ½( ~ k i ; ~ k o )( ~ N ~ k o ) d ~ k o 1 8 ~ k i ½ 0 Obeys Helmholtz reciprocity ½( ~ k i ; ~ k o ) = ½( ~ k o ; ~ k i )

11 Conserves energy Z Positive S 2 ½( ~ k i ; ~ k o )( ~ N ~ k o ) d ~ k o 1 8 ~ k i ½ 0 Obeys Helmholtz reciprocity ½( ~ k i ; ~ k o ) = ½( ~ k o ; ~ k i )

12 Goniophotometer

13 L( ~ k o ) = Z S 2 ½( ~ k i ; ~ k o ; ~ N)L i ( ~ k i )( ~ N ~ k i ) d ~ k i

14 L( ~ k o ) = Z S 2 ½( ~ k i ; ~ k o ; ~ N)L i ( ~ k i )( ~ N ~ k i ) d ~ k i

15 Z L( ~ k o ) = S 2 ½( ~ k i ; ~ k o ; ~ N)L i ( ~ k i )( ~ N ~ k i ) d ~ k i Outgoing intensity

16 Z L( ~ k o ) = S 2 ½( ~ k i ; ~ k o ; ~ N)L i ( ~ k i )( ~ N ~ k i ) d ~ k i Outgoing intensity Sum over light source directions

17 Z L( ~ k o ) = S 2 ½( ~ k i ; ~ k o ; ~ N)L i ( ~ k i )( ~ N ~ k i ) d ~ k i Outgoing intensity BRDF Sum over light source directions

18 Z L( ~ k o ) = ½( ~ k i ; ~ k o ; N)L ~ i ( ~ k i )( N ~ ~ k i ) d ~ k i S 2 Incoming Outgoing light BRDF intensity Sum over light source directions

19 Z L( ~ k o ) = ½( ~ k i ; ~ k o ; N)L ~ i ( ~ k i )( N ~ ~ k i ) d ~ k i S 2 Incoming Outgoing light BRDF intensity Normal component Sum over light source directions

20 We rarely evaluate the BRDF or rendering integral but rather know the form of L directly.

21 Reflection Refraction

22 Reflection Refraction

23 L = c r c l max( ~ N ~ k i ; 0) Light and surface color Cosine term Lambertian ( Cosine ) shading

24 L = c r c a Surface color Ambient light color Constant regardless of light position

25 L = c l max(0; ~ k o ~r) p ~r = ~ k i + 2( ~ k i ~ N) ~ N ~N ~ko ~ ki Reflected light direction ~r Adds glossy shine

26 L = c l max(0; ~ k o ~r) p ~r = ~ k i + 2( ~ k i ~ N) ~ N Increasing p Adds glossy shine

27 Ambient-diffuse-specular (ADS)

28 Can specify different ambient, diffuse, and specular reflected colors, plus Phong exponent p.

29 Lambertian Specular Combined

30 glenable(gl_lighting); //important! GLfloat light0_position[] = { 1, 1, 1, 0 }; GLfloat light0_diffuse[] = { 1,1, 1, 1 }; GLfloat light0_specular[] = { 1, 1, 1, 1 }; gllightfv( GL_LIGHT0, GL_POSITION, light0_position); gllightfv( GL_LIGHT0, GL_DIFFUSE, light0_diffuse); //if you set these to 0's you won't see diffuse color //regardless of your material gllightfv( GL_LIGHT0, GL_SPECULAR, light0_specular); //if you set these to 0's you won't see specular //highlights regardless of your material glenable(gl_light0);

31 GLfloat mat_ambient[] = {1,.1,.8, 1 }; GLfloat mat_diffuse[] = {.2f,.2f,.6f, 1 }; GLfloat mat_specular[] = {.8f,.8f,.8f, 1 }; GLfloat mat_shininess[] = { 128 }; //range[0,128] glmaterialfv(gl_front, GL_AMBIENT, mat_ambient); glmaterialfv(gl_front, GL_DIFFUSE, mat_diffuse); glmaterialfv(gl_front, GL_SPECULAR, mat_specular); glmaterialfv(gl_front, GL_SHININESS, mat_shininess);

32 glenable(gl_color_material); //when you want to draw something with a new //diffuse AND ambient color glcolormaterial(gl_front, GL_AMBIENT_AND_DIFFUSE); glcolor4f(0,0,1,1); drawsphere(); //now we just change the diffuse to red glcolormaterial(gl_front, GL_DIFFUSE); glcolor4f(1,0,0,1); drawsphere();

33 glenable(gl_color_material); //when you want to draw something with a new //diffuse AND ambient color glcolormaterial(gl_front, GL_AMBIENT_AND_DIFFUSE); glcolor4f(0,0,1,1); drawsphere(); //now we just change the diffuse to red glcolormaterial(gl_front, GL_DIFFUSE); glcolor4f(1,0,0,1); drawsphere();

34 Spatially-varying BRDF (SVBRDF)

35 B Subsurface Scattering RDF (BSSRDF)

36 B Subsurface Scattering RDF (BSSRDF)

37 B Subsurface Scattering RDF (BSSRDF)

38 B Subsurface Scattering RDF (BSSRDF)

39 B Subsurface Scattering RDF (BSSRDF)

40 B Subsurface Scattering RDF (BSSRDF)

41 Algorithm: 1. Calculate color c of triangle. 2. Fill whole triangle with c Flat shading: One color per triangle

42 Algorithm: 1. Calculate color c i of each vertex. 2. Interpolate pixel colors using barycentric weights. Gouraud shading: One color per vertex

43 Algorithm: 1. Calculate normal N i of each vertex. 2. Interpolate normal using barycentric weights, normalize, and use it for shading. Phong shading: One color per fragment

44

45 ~v 3 ~N = (~v 2 ~v 1 ) (~v 3 ~v 1 ) k(~v 2 ~v 1 ) (~v 3 ~v 1 )k ~v 3 ~v 1 ~v 2 ~v 2 ~v 1 ~v 1

46 ~F 2 ~F3 1 N NX i=1 ~F i ~ 1 ~F 4 ~F 5

47 ~F 2 ~F3 1 N NX i=1 ~F i ~ 1 ~F 4 ~F 5

48 Takes care of conversion between object and light/camera coordinates for normal! glbegin(gl_triangles); glnormal(1.f,1.f,1.f); glvertex3f(0.f,0.f,1.f); glnormal(1.f,1.f,1.f); glvertex3f(1.f,0.f,1.f); glnormal(1.f,1.f,1.f); glvertex3f(1.f,1.f,1.f); glend();

49 Do vertex normals on a cube make sense? When should normals be smoothed?

50 Can choose artistic effects to accentuate surface properties rather than simulate light transport.

51 Nonphotorealistic [non-foh-toh-ree-uh-lis-tik]: Rendering techniques that do not attempt to mimic light transport.

52 ( ~ k o ~ N 0 )( ~ k o ~ N 1 ) 0 Face normals on either side of edge change direction relative to eye.

53 ( ~ k o ~ N 0 )( ~ k o ~ N 1 ) 0

54 ~N 0 ~ N 1 threshold Faces adjacent to the edge have normals form a large angle.

55 ~N 0 ~ N 1 threshold

56 Use curvature to choose better contour lines.

57

58

59 CS 148, Summer 2012 Introduction to Computer Graphics and Imaging