#11: Lighting

1. #11: Lighting CSE167: Computer Graphics Instructor: Ronen Barzel UCSD, Winter 2006

2. Outline for today • Overview of lighting • Notes on color • Local Illumination • Light Sources • Shading • Advanced Lighting

3. Where we are so far… • We know how to rasterize: • Given a 3D triangle (or a bunch of triangles) • Given a 3D camera… • …we know which pixels represent the triangles • But what color should those pixels be?

4. Lighting (non-teapot images by Henrik Wann Jensen)

5. Lighting • To create a photorealistic image: • Simulate the interaction of light with the objects in the scene • Simulate the interaction of light with the eye or camera • I.e., simulation of physics and optics • Advanced rendering course (CSE168) • Many aspects “solved” in principle but still an area of active research • Very slow to compute fully • Use global illumination techniques: examine the whole scene at once • Known as physically-based rendering • For interactive computer graphics: • Use a simplified model for speed • Empirical/perceptual -- approximate interesting observed phenomena • Use local illumination techniques: • only direct effect of lights on surfaces

6. Basic Components of Lighting • Light sources • AKA emitters • Color and intensity • Geometric attributes: Position, Direction, Shape • Spatial attenuation • Advanced properties: Spectrum, Polarization, … • Surfaces • Geometric attributes: Position, Orientation • Material properties: reflectance • color • shininess, glossiness, … • texture • Advanced: translucency, microstructure, sub-surface scattering, …

7. Lighting vs. Shading • Lighting: compute the result of light illuminating surfaces • Shading: assign colors to pixels • For photorealistic rendering: • in principle, shading==lighting: perform lighting at every pixel • In practice: • may take shortcuts • may include non-lighting effects • fog • illustration • cartoon shading

8. Vertex Lighting • Each vertex goes through lighting process • Lighting computation determines final color at the vertex • Based on initial “unlit” vertex color • Based on lights in the scene • Based on material properties of the surface • Based on surface normal • Interpolate colors using Gouraud shading • (Same lighting computation for per-pixel lighting)

9. Outline for today • Overview of lighting • Notes on color • Local Illumination • Light Sources • Shading • Advanced Lighting

10. What is light? What is color? • Light is electromagnetic energy • a continuous range of wavelengths • varying intensity at each wavelength • Color is a property of the visual system • Not an inherent property of light • Human eyes have Red, Green, Blue receptors (cones) • Each receptor responds to a range of wavelengths • Gives rise to “primary colors”: • all colors expressed as combination of red, green, blue cone stimulation • Lots of perceptual, psychophysical effects: • adaptation, inhibition, illusion • Physically correct computation • requires computing interactions at all wavelengths • Perceptually correct computation • Requires taking into account psychophysics • Pretty good approximation: • Separate light into red, green, blue components • Process each component independently

11. Color illusion • The squares marked A and B are the same shade of gray

12. Color Illusion -- proof

13. Color Spaces

14. Material Colors • Inherent “material color” which is the color that the object reflects • Material reflects different wavelengths of light different amounts • In RGB, have a reflectivity amount for each of red, green, blue • An object can’t reflect more light than it receives • Maximum: reflect 100% of light in all wavelengths--bright white • Reasonable: reflect 95% of light, material color =(0.95, 0.95, 0.95) • Material colors range from 0.0 to 1.0 in RGB

15. Light Color • No limit to total light intensity reflecting from surface • Can make individual light source brighter • Can add more lights • Represent a light source using intensity in RGB • Range from 0.0 up • There is no upper limit to the intensity of light • In other words, a bright white light might have color (10,10,10) • Units? • physically-based rendering: photon power flux density • in practice: arbitrary units (“my light goes up to 11”)

16. Color & Intensity • Distinction between material color and light color: • Material colors represent the proportion of light reflected • Light colors represent the actual intensity of a beam of light • We never perceive the inherent material color • All we see is the light reflected off of a material • Shine a red light… • on a white or red surface: the object appears red • on a grey surface: the object appears dark red • on a blue surface: the object appears black

17. Exposure and Display • What do we mean by “white”? • Human eyes (and digital cameras) adjust exposure settings automatically • In a moderatly lit room, intensity 0.5 might appear as white • In bright sunlight, intensity 100 might appear as (same) white • The monitor has an upper limit to the brightness it can display • RGB units: 0=no light at the pixel, 1=full intensity at the pixel • exact color light that emerges depends on monitor properties • brightness, contrast, white point, color balance, … • Final result of lighting calculation shouldn’t be more than 1.0 • Advanced techniques: exposure control, AKA “tone mapping” • In practice: • Assume intensity (1,1,1) is white • clamp all final color values to 0.0-1.0 range before storing in pixel

18. Outline for today • Overview of lighting • Notes on color • Local Illumination • Light Sources • Shading • Advanced Lighting

19. Local Illumination • AKA Local Lighting Models • Light on a point on the surface (vertex) • Assume we have an incident ray of light • Light coming from a known direction • With a given RGB color (intensity) • We will build up empirical material properties • Fancy name: Bidirectional Reflectance Distribution Function “BRDF”

20. Reflectivity • White sheet of paper might reflect 95% of incident light • A mirror might reflect 95% of incedent light • Yet, these two things look completely different: • They reflect light in different directions • The paper is a diffuse reflector • The mirror is a specular reflector

21. “Standard” Lighting Model • Consists of three terms linearly combined: • Diffuse component for the amount of incoming light reflected equally in all directions • Specular component for the amount of light reflected in a mirror-like fashion • Ambient term to approximate light arriving via other surfaces • This is very simple approximation • particularly good for plastic • particularly good for metal • That’s why CG images tend to look like plastic and metal

22. Diffuse Reflection • An ideal diffuse reflector receives light from some direction and bounces it uniformly in all directions • very rough at microscopic level • Diffuse materials have a dull or matte appearance • example: chalk

23. Diffuse Reflection • Assume a beam of parallel rays shining on the surface • Consider area of the surface covered by the beam • varies based on the angle between the beam and the normal • The larger this area, the less incident light per area • The incident light per unit area is proportional to the cosine of the angle between the normal and the light rays • Object darkens as normal turns away from light • This is known as Lambert’s cosine law • Diffuse surfaces AKA Lambertian surfaces n

24. Lambert’s Cosine Law

25. Diffuse Reflection • Notes: • Depends on light and normal directions • Doesn’t depend on eye position • diffuse reflection is same in every direction • Don’t want to illuminate from rear • use cl kd

26. Diffuse Lighting Examples • A Lambertian sphere at several different lighting angles: • Diffuse lighting provides visual cues • indicates 3D depth • indicates surface curvature

27. Multiple Lights • Can have many light sources in a scene • Light (generally) behaves additively • Add up the contribution of each light

28. Ambient Light • In the real world, light gets bounced all around the environment • Resulting light illuminates surface from every direction. • Global illumination techniques attempt to compute this. Complex. • Simple approximation (hack): Ambient light • Assume net effect is a constant color shining from every direction • Add to the net color, attenuated by reflectance coefficient • Effect of ambient light: • Keeps unlit areas from going completely black • Makes things look flatter • with ambient and no diffuse: object has solid color, is completely 2D • kaor ca usually small (.1 or less)

29. Specular Reflection • Shiny surfaces exhibit specular reflection • Polished metal • Glossy car finish • Plastics • A light shining on a specular surface causes a bright spot: • known as a specular highlight • essentially, a rough reflection of the light source • Highlight location depends viewer position relative to surface & lights

30. Specular Reflection • An ideal specular reflector = mirror • perfectly smooth surface • bounces an incoming light ray in a single direction • angle of incidence equals the angle of reflection

31. Law of Reflection • Angle of reflectance = angle of incidence p

32. Specular (Glossy) Reflection • Many materials not quite perfect mirrors • Glossy materials look shiny and will show specular highlights • In CG, this is sometimes referred to as glossy reflection • Many formulations for this • First: most basic and famous: Phong lighting model (1973) • Then: most popular: Blinn lighting model (1977)

33. Shiny materials • The surface roughness will vary from material to material • Smooth surfaces have sharp highlights • Rougher surfaces have larger, more blurry highlights • Assume surface composed of microfacets with random orientation • Smooth surfaces: microfacet normals very close to surface normal • Rough surfaces: microfacet normals are spread around more • on average, microfacet normals close to surface normal • Polished: • Smooth: • Rough: • Very rough:

34. Empirical observation • In general, we expect most reflected light to travel in direction of exact reflection • But because of microscopic surface variations, some light may be reflected in a direction slightly off the ideal reflected ray • So: • Most reflected light in direction of ideal reflection • Brightest when eye vector (view vector) is aligned with reflection • Intensity decreases as eye vector angle from reflection increases • Use dot product of eye vector with reflection vector

35. Phong Lighting Model • parameters: • specular reflectance coefficient, ks • Phong Exponent p controls the apparent size of the specularity • Higher p, smaller highlight cl ks

36. Phong Lighting Model Examples p=2 p=1 p=4 p=8 p=32 p=16 p=64 p=128 p=256

37. Blinn Lighting Model cl ks

38. cl ks Blinn Lighting Model

39. Complete Blinn Lighting Model • Add to ambient and diffuse • Add specular contribution for each light • It appears in a few slightly different forms and in a wide variety of notations…

40. Note on color • Do this in parallel for R,G,B • Coefficients ka, kd, ks can be different for each of R,G,B • This defines the material ambient color, diffuse color, and specular color. • Other (expensive) terms in expression are shared for each of R,G,B • Generally, use ambient color = diffuse color • For metals, specular color = diffuse color • highlight is color of the material • For plastics, specular color = white • highlight is the color of the light

41. Note on normals and spaces • Lighting depends on angles between normals, vectors • Must be in space that preserves angles • World Space or Camera Space • Not normalized view space: perspective doesn’t preserve angles • Conveniently, we can put world-space normals as per-vertex data • Doesn’t get transformed by projection. • But remember, when taking normals from object to world space: • if world transform has nonuniform scale, normals must use inverse-transpose • if world transform has uniform scale, normals must be renormalized • if world transform has no scales, normals transform like vectors

42. General Lighting Models • General form: • Bidirectional Reflectance Distribution Function BRDF, B() • There are many lighting models • Phong • Blinn • Cook-Torrance • includes “roughness”, other parameters • Gooch • non-photorealistic, for illustrations • Ward • includes anisotropy • etc…

43. Cook-Torrance

44. Gooch

45. Anisotropy • Material reflects differently in different directions • E.g., brushed metal; Ward’s model isotropic anisotropic

46. Outline for today • Overview of lighting • Notes on color • Local Illumination • Light Sources • Shading • Advanced Lighting

47. Light Sources • In general, light sources can have complex properties • Geometric area over which light is produced • Anisotropy in direction • Variation in color • Some very simple light sources models are standard

48. Light Sources • Two aspects of light sources are important for a local shading model: • Where is the light coming from (the L vector)? • How much light is coming (the I values)? • Various light source types give different answers to the above questions: • Directional: Light from a specific direction • Point light source: Light from a specific point • Spotlight: Light from a specific point with intensity that depends on the direction

49. Directional Light • When light is coming from a distant source • light rays are parallel • light ray direction is the same everywhere in the scene • as if the source were infinitely far away • good approximation to sunlight • Specified by a unit length direction vector, and a color csrc cl

50. Point Lights • For closer light sources, such as light bulbs • Model as a point that radiates light in all directions equally • Light vector varies across the surface • Intensity from a point light source drops off proportionally to the inverse square of the distance from the light p csrc cl cl v v