Rendering. Illumination Model. Wireframe rendering simple, ambiguous Color filling flat without any 3D information

Transcription

1 llumination Model Wireframe rendering simple, ambiguous Color filling flat without any 3D information Requires modeling interaction of light with the object/surface to have a different color (shade in 3D

2 llumination Model Light on a surface is Absorbed Reflected Transmitted The amount reflected determines the color and brightness of the object light material (surface interaction

3 llumination Model The reflected light is scattered depending upon the surface properties and incident light Ambient light comes from all directions, is scattered in all directions Diffuse light comes from one direction and is scattered in all directions Specular light comes from one direction and is scattered in preferred direction

4 llumination Model Diffuse Reflection Lambert s Law L θ N d d. cos = kdl cos = k ( L N diffuse d l L: Light vector N: Normal θ: Angle between L and n k d diffuse reflection coefficient

5 llumination Model Diffuse Reflection

6 llumination Model Diffuse Reflection

7 llumination Model Diffuse Reflection Amount of light reflected depends on the direction to the light source and not on the direction to the viewer Viewer independent Distance from light source q can also be incorporated d = kdl ( L N kd = a + bq + cq ( L N 2 l

8 llumination Model Specular Reflection Highlights / Shininess Viewing Direction

9 llumination Model Specular Reflection N L θ θ α R V L: Light vector N: Normal θ: Angle between L and N α: Angle between R and V s s n = ksl cos. k n s specular reflection coefficient = ksl ( R V n specular reflection exponent

10 llumination Model Specular Reflection n= large n: metals small n: paper

11 llumination Model Specular Reflection Example

12 llumination Model Ambient Reflection Light from distributed light sources (and surroundings Also approximates effects of diffusely reflected light from outer bodies / objects. = k ambient a a k a ambient reflection coefficient a ambient incident light

13 llumination Model Phong llumination Model = + + = + + = + + = + + = m i n i i s i i d a a n l s l d a a n l s l d a a total V R k N L k k V R k N L k k. k k k reflection specular diffeuse reflection reflection ambient 1 ( ( ( ( cos cos

14 llumination Model Phong llumination Model Local computation for obtaining color (intensity at a point of the surface Basic inputs are light(s, material properties

15 llumination Model Reflection Vector N L θ i N θ r R L R 2 (L N N -L R = 2( L N N L

16 llumination Model Half Way Vector L N H V R nstead of L + V H = L + V R V can use N H

17 llumination Model Normal Vector Plane ax + by + cz + d = 0 p n n ( p p 0 = n = [ a b c] Normalize 0 p 0

18 llumination Model Normal Vector Plane v 3 n = ( v3 v1 ( v2 v1 Normalize v 1 v 2

19 llumination Model Normal Vector Sphere mplicit Equation = = + + = z f y f x f n z y x z y x f,, 0 1,, ( 2 2 2

20 llumination Model Normal Vector Parametric Surface, (, (, (, (, (,,,, v u b v v u b u v u b v v u b u v u n n m n m n m n m = u v n

21 llumination Model Light Sources Point light source Given by a point Light emitted in all directions Direction light source Given by a vector Spotlight light Given by a cone

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23 llumination Model Phong llumination Model = + + = + + = + + = + + = m i n i i s i i d a a n l s l d a a n l s l d a a total V R k N L k k V R k N L k k. k k k reflection specular diffeuse reflection reflection ambient 1 ( ( ( ( cos cos

24 Polygon Shading Shading Process of applying illumination model to surface points Approaches Polygon (approximates the 3D shape/surface Flat Shading Gouraud Shading Phong Shading

25 Polygon Shading Flat Shading One intensity for the whole polygon constant shading For each face/polygon n p P y Select a point P on the face y Find normal to the face n p y Find intensity at P y Fill the polygon with Not smooth

26 Polygon Shading Flat Shading Example

27 Polygon Shading Flat Shading Computationally fast Not smooth Mach Band effect

28 Polygon Shading Gouraud Shading Smooth shading Compute intensity at vertices of a polygon ÖNeeds vertex normal Fill the interior with shade (intensity using interpolation

29 Polygon Shading Gouraud Shading Vertex Normal N 2 N v N 4 Normal at the vertex is average of normals of the faces incident at the vertex N 1 N 3 N v = P i = 1 n n i i

30 Polygon Shading Gouraud Shading Vertex Normal

31 Polygon Shading Gouraud Shading nterpolation (x 1, y 1 1 Scan line a s b 4 (x 2, y 2 (x 4, y 4 Scan conversion! 2 (x 3, y 3 3

32 Gouraud Shading nterpolation a b s Scan line Polygon Shading 1 (x 1, y 1 (x 2, y 2 (x 3, y 3 (x 4, y 4 ] ( ( [ 1 ] ( ( [ 1 ] ( ( [ a s b s b a a b s s s b s s a x x x x x x y y y y y y y y y y y y + = + = + = (x a, y s (x b, y s (x s, y s

33 Polygon Shading Gouraud Shading Example

34 Polygon Shading Gouraud Shading Handling Specular Reflections- Highlights Not Right

35 Polygon Shading Phong Shading nterpolate normals and then compute intensity Not to confuse with Phong llumination Model

36 Polygon Shading Phong Shading (x 1, y 1 N 1 llumination using N s Scan line N a N s N b N 4 N s s (x 2, y 2 (x 4, y 4 N 2 (x 3, y 3 N 3

37 Polygon Shading Phong Shading More accurate specular component Reduced Mach band effect Better shape approximation N 1 N 2 Original surface Computationally more intensive

38 Polygon Shading Phong Shading Example

39 Polygon Shading Phong Shading Example

40 Polygon Shading Problems Polygon Silhouette nterpolated shading

41 Polygon Shading Problems nterpolated shading nterpolation naccuracy (screen space vs world space Linear nterpolation Perspective nterpolation

42 Polygon Shading Problems Animation nterpolated shading V 1 Rotate V P 4 V 1 V 2 P V 2 V 3 V 3 V 4

43 Polygon Shading Problems nterpolated shading Vertex Normal Vertex Normals Face Normals

44 Polygon Shading Graphics Pipeline Order llumination computation is done early after modeling transformation Shading is done towards the end with rasterization (scan conversion

45 Polygon Shading Transparency Simple Model Non refractive = k + ( 1 k reflected transmitted k is the opacity factor (coefficient

46 Polygon Shading mplementation OpenGL Enable light sources JO(QDEOH*/B/,*+7 Light properties {r g b a} Glfloat light_diffuse[] = {1.0, 1.0, 1.0, 1.0} JO/LJKWY*/B/,*+7*/B',86(OLJKWBGLXVH Light position {x y z w} Glfloat light_position[] = {-1.0, 1.0, -1.0, 1.0} JO/LJKWY*/B/,*+7*/B326,7,21OLJKWBSRVLWLRQ

47 Polygon Shading mplementation OpenGL Material properties {r g b a} Glfloat mat_d[] = {0.1, 0.2, 0.8, 1.0} JO0DWHULDOY*/B5217*/B',86(PDWBG Normal vector JO1RUPDOQ[Q\Q] JO9HUWH[Q[Q\Q]