Light Reflection Models

Transcription

1 Light Reflection Models Visual Imaging in the Electronic Age Donald P. Greenberg October 21, 2014 Lecture #15

2 Goal of Realistic Imaging From Strobel, Photographic Materials and Processes Focal Press, 186.

3 Rendering Framework 17

4 Direct Lighting and Indirect Lighting Direct Indirect Direct

5 Direct Lighting and Indirect Lighting

6 Lighting Jeremy Birn, [digital] Lighting & Rendering, 2000 New Riders Publishers The three dimensional shape is only inferred with this lighting.

7 Lighting Jeremy Birn, [digital] Lighting & Rendering, 2000 New Riders Publishers The geometry is better understood with correct lighting and shading.

8 Assumptions In Direct Lighting Light travels directly from light source to all object surfaces (no occlusion) no shadows All light sources are point light sources (no geometric area) No interreflections from any surfaces Lights maybe directional, spot or omni lights

9 Diffuse Reflections Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 12.

10 Specular Reflections Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 12.

11 Glossy Reflections Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 12.

12 Smooth Surface, Rough Surface, Combination Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 12.

13 Reflectance - Three Forms Ideal diffuse (Lambertian) Ideal specular Directional diffuse

14 Diffuse Reflections Observer I N q Light Source L I = k d cosq

15 How do you find the angle β? If you know the surface definition (it s planar equation), you can find it s normal direction N. A unit normal in this direction is N N If you know the location of the light source L, you can find the illumination direction L. A unit normal in this direction is L L

16 Cosine Calculations Dot Product Definition N L N L cosq cosq N L N L N N L L Usually, the normal and light source vector directions are given as unit normals.

17 Gouraud Flat Polygon Shading Each polygon is shaded based on a single normal. Gouraud Thesis

18 Gouraud Smooth Shading Four polygons approximating a surface in the vicinity of point A. The shading at point R is computed as two types of successive linear interpolations: across polygon edges: P between A and B, Q between A and D; across the scan line: R between P and Q. B 2 A P R Q 1 C 3 D 4 scan line Gouraud Thesis

19 Gouraud Smooth Shading Each pixel is shaded by interpolating intensities computed at each of the polygon s vertices. Gouraud Thesis

20 Steps in Gouraud Shading For each polygon Compute vertex intensities (using any illumination model) Compute slopes in spatial (image) domain and intensity domain (linear interpolation) Increment by scan line For each scan line Compute slope in intensity domain Render each pixel Note the intensity computations are based on object space data, but all interpolation is done in image space.

21 Diffuse Shading Jeremy Birn. Digital Lighting & Rendering, p. 74.

22 Specular Shading Viennese Siler, Modern Design ) Teapot, Jakob Krautauer, Vienna 1802 Silver, fruitwood, H 14.8 cm/5. in.

23 Phong Model Assumptions The reflection function can be represented by three components: a constant ambient term, and diffuse and specular components Isotropic (rotationally symmetric) Point or parallel light source (one vector direction) Computationally simple

24 Phong Model Specular Reflection Observer V I R b q r N q i Light Source L I = k s cos n b

25 How do you find the angle β? If you know the illumination direction L, you can find the reflection direction R (angle of reflection = angle of incidence) If you know the location of the observer, you can find the view direction V The specular reflection component is a function of the angle β, the angle between the view direction and the reflection vector

26 Variation of cos n b

27 Phong Reflection Model I j Diffuse Specular Mirror Reflection Vector R V Diffuse = Specular = k d s ( N L) k ( R V ) n

28 Phong Goblet Bui Toung Phong Thesis

29 Phong Equation I = I a + I d + I s = [k a +k d (N L)](object color) + k s (R V) 2 (light color) Where k a = constant ambient term and k a +k d + k s = 1

30 Phong Model with Constant Ambient Term and Variations of Specular Exponent Roy Hall

31 Phong Model with Constant Specular Exponent and Variation of Ambient Term Roy Hall

32 Reflection Geometry (BRDF) dw i q i q r dw r j i j r Bidirectional Reflection Distribution Function

33 Light Measurement Laboratory

34 Gonioreflectometer Spectroradiometer Mirror Light Source Mirror Sample

35 Bidirectional Reflectometer

36 Reflection Processes First surface reflections Multiple surface reflections Subsurface reflections

37 Gaussian Distribution m = 0.2 m = 0.6 Where m=root mean square slope of the microfacets

38 Comparison of experiment and theory Aluminum s 0 = 0.28m, t =1.77m

39 Bidirectional Reflectance (BRDF) l directional diffuse specular q uniform diffuse t s

40 Retro-Reflection

41 Retroreflection

42 Retroreflection

43 Reflectance of Copper Mirror

44 Light Reflected from Copper

45 Cook s Fresnel Approximation

46 Cook s Copper Spheres

47 Cook-Torrance Renderings

48 Copper Vase Copper-colored plastic Copper

49 Reflection from Plastic Incident Light Specular Reflection (white) Diffuse Reflection (colored) Vinyl Substrate (white) Pigment Particles (colored)

50 The geometry of scattering from a layered surface ACM Computer Graphics, SIGGRAPH 13 p. 166

51 Henrik Wann Jensen, Stephen R. Marschner, Marc Levoy, Pat Hanrahan. A Practical Model for Subsurface Light Transport, ACM Siggraph 2001, August 2001, Los Angeles, CA, pp

52 Schematic model of the image process ACM Transactions on Graphics, SIGGRAPH 2003 p. 773

53 Schematic flow of the imaging process in proposed imagebased skin color and texture analysis/synthesis ACM Transactions on Graphics, SIGGRAPH 2003 p. 771

54 3D Studio Max: Material Editor

55 3D Studio Max: Material Editor

56 End...