人机接口与图形学双语lighting.ppt

Lambert’s Cosine Law Ideal diffuse surfaces reflect according to Lambert’s cosine law: The energy reflected by a small portion of a surface from a light source in a given direction is proportional to the cosine of the angle between that direction and the surface normal These are often called Lambertian surfaces Note that the reflected intensity is independent of the viewing direction, but does depend on the surface orientation with regard to the light source Lambert’s Law Computing Diffuse Reflection The angle between the surface normal and the incoming light is the angle of incidence: Idiffuse = kd Ilight cos ? In practice we use vector arithmetic: Idiffuse = kd Ilight (n ? l) n l ? Diffuse Lighting Examples We need only consider angles from 0° to 90° (Why?) A Lambertian sphere seen at several different lighting angles: Specular Reflection Shiny surfaces exhibit specular reflection Polished metal Glossy car finish A light shining on a specular surface causes a bright spot known as a specular highlight Where these highlights appear is a function of the viewer’s position, so specular reflectance is view dependent The Physics of Reflection At the microscopic level a specular reflecting surface is very smooth Thus rays of light are likely to bounce off the microgeometry in a mirror-like fashion The smoother the surface, the closer it becomes to a perfect mirror The Optics of Reflection Reflection follows Snell’s Laws: The incoming ray and reflected ray lie in a plane with the surface normal The angle that the reflected ray forms with the surface normal equals the angle formed by the incoming ray and the surface normal: ?(l)ight = ?(r)eflection Non-Ideal Specular Reflectance Snell’s law applies to perfect mirror-like surfaces, but aside from mirrors (and chrome) few surfaces exhibit perfect specularity How can we capture the “softer” reflections of surface that are glossy rather than mirror-like? One option: model the microgeometry of the surface and e