Rendering (彩現 渲染)

Content Light-Material Interaction Phong Reflection model Gouraud vs. Phone Shading Fall 2013

Rendering The computation required to convert 3D scene to 2D display photo-realistically Fall 2013

Shading the gradation (of color) that give the 2D images the appearance of being 3D Fall 2013

Light-Material Interaction specular diffuse translucent Fall 2013

Light point, spot, directional lights ambient light: to account for uniform level room lighting describe a light source through a three-component (RGB) intensity Fall 2013

Phong Reflection Model Diffuse (漫射) Specular Ambient Fall 2013

Those Were the Days… “In trying to improve the quality of the synthetic images, we do not expect to be able to display the object exactly as it would appear in reality, with texture, overcast shadows, etc. We hope only to display an image that approximates the real object closely enough to provide a certain degree of realism.” – Bui Tuong Phong, 1975 Fall 2013

Lambert’s Cosine Law The reflected luminous intensity in any direction from a perfectly diffusing surface varies as the cosine of the angle between the direction of incident light and the normal vector of the surface. Intuitively: cross-sectional area of the “beam” intersecting an element of surface area is smaller for greater angles with the normal. Fall 2013

Lambert’s Cosine Law Ideally diffuse surfaces obey cosine law. Often called Lambertian surfaces. Id = kd Iincident cos  = kd Iincident (N·L). kd is the diffuse reflectance of the material. L N  Fall 2013

Phong Lighting Model Phong adds specular highlights. His original formula for the specular term: W(i)[cos s ]n s is the angle between the view and specular reflection directions. “W(i) is a function which gives the ratio of the specular reflected light and the incident light as a function of the the incident angle i.” Ranges from 10 to 80 percent. “n is a power which models the specular reflected light for each material.” Ranges from 1 to 10. Fall 2013

Phong Lighting Model More recent formulations are slightly different. Replace W(i) with a constant ks independent of the incident direction. What do we lose when we do this? Is= ks Iincident cosn = ks Iincident (V·R)n R = 2(N·L)N – L Fall 2013

Ambient Reflection Local illumination models account for light scattered from the light source only Light may be scattered from all surfaces in the scene. We are missing a lot of light, typically over 50% Ambient term = a coarse approximation to this missing flux This is a constant everywhere in the scene Fall 2013

Johann Heinrich Lambert (1728 – 1777) was a Swiss mathematician, physicist and astronomer. Diffuse Reflection Lambertian scatters (wikipedia): the irradiance landing on the area element is proportional to the cosine of the angle between the illuminating surface and the normal. When a Lambertian surface is viewed from any angle, it has the same radiance. Fall 2013

Specular Reflection a Effect of Shininess Coefficient a. v Fall 2013

Phong Reflection Model L: light source property (RGB) R: material property (RGB) ambient reflection diffuse reflection specular reflection final result To consider distance attenuation a: shininess coefficient Fall 2013

Phong Model (cont) For multiple light sources: Fall 2013

Blinn-Phong Model Popular variation of Phong model. Uses the halfway vector, H. Is = ks Iincident (N·H)n. H = (L+V) / | L+V | What are the advantages? L N  H V Fall 2013

Blinn-Phong Model Popular variation of Phong model. Uses the half vector, H. Is = ks Iincident (N·H)n. H = (L+V) / | L+V | Faster to compute than reflection vector. Still view-dependent since H depends on V. Jim Blinn (1949 - now) L N  H V Fall 2013

Blinn-Phong Model Time-Consuming! n r l An alternate formulation employs the half vector H Time-Consuming! Fall 2013 20

Blinn-Phong Highlights Does using N.H vs. R.V affect highlights? Yes, the highlights “spread” (Wikipedia) Why? Is this bad? Fall 2013

Blinn-Phong Highlights Does using N.H vs. R.V affect highlights? Yes, the highlights “spread”. Why? Is this bad? Not really, for two reasons. Can always adjust the exponent. Phong and Blinn-Phong are not physically based, so it doesn’t really matter! Fall 2013

Target of Shading: Polygon, Vertex or Fragments Recall the rendering (OpenGL) pipeline Fall 2013

Shading Modes Flat vs. Smooth Local vs. Global Flat: single color per face Gouraud (intensity interpolation) Phong (normal interpolation) Local vs. Global Fall 2013

Gouraud vs. Phong Most h/w implement Gouraud shading Phong shading can better imitate specular effects (∵normals are interpolated) Fall 2013

Fall 2013

Compare: Flat, Gouraud, Phong Fall 2013

Rendering Pipeline Tutorial No longer on line [local copy]

Rendering Pipeline (Foley and van Dam) Fall 2013