1Lecture 15 6.837 Fall 2001Illumination and ShadingLight Sources Empirical Illumination Shading Transforming Normals Lecture 15 Slide 2 6.837 Fall 2001Illumination ModelsIlluminationThe transport luminous fluxfrom light sources between points via direct and indirect paths LightingThe process of computing the luminous intensity reflected from a specified 3-D point ShadingThe process of assigning a colors to a pixels Illumination ModelsSimple approximations of light transport Physical models of light transportLecture 15 Slide 3 6.837 Fall 2001Two Components of IlluminationLight Sources (Emitters)Emission Spectrum (color) Geometry (position and direction) Directional Attenuation Surface Properties (Reflectors)Reflectance Spectrum (color) Geometry (position, orientation, and micro-structure) Absorption ApproximationsOnly direct illumination from the emitters to the reflectorsIgnore the geometry of light emitters, and consider only the geometry of reflectorsLecture 15 Slide 4 6.837 Fall 2001Ambient Light SourceEven though an object in a scene is not directly lit it will still be visible. This is because light is reflected indirectly from nearby objects. A simple hackthat is commonly used to model this indirect illumination is to use of an ambient light source. Ambient light has no spatial or directional characteristics. Theamount of ambient light incident on each object is a constant for all surfaces in the scene. An ambient light can have a color. The amount of ambient light that is reflected by an object is independent of the object's position or orientation. Surface properties are used to determine how much ambient light is reflected.2Lecture 15 Slide 5 6.837 Fall 2001Directional Light SourcesAll of the rays from a directional light source have a common direction, and no point of origin. It is as if the light source was infinitely far away from the surface that it is illuminating. Sunlight is an example of an infinite light source. The direction from a surface to a light source is important for computing the light reflected from the surface. With a directional light source this direction is a constant for every surface. A directional light source can be colored. Lecture 15 Slide 6 6.837 Fall 2001Point Light SourcesThe point light source emits rays in radial directions from its source. A point light source is a fair approximation to a local light source such as a light bulb. The direction of the light to each point on a surface changes when a point light source is used. Thus, a normalized vector to the light emitter must be computed for each point that is illuminated.pldpl−=−Lecture 15 Slide 7 6.837 Fall 2001Other Light SourcesSpotlights Point source whose intensity falls off away from a given direction Requires a color, a point, a direction, parameters that control the rate of fall off Area Light Sources Light source occupies a 2-D area (usually a polygon or disk) Generates softshadows Extended Light Sources Spherical Light Source Generates softshadowsLecture 15 Slide 8 6.837 Fall 2001Ideal Diffuse ReflectionFirst, we will consider a particular type of surface called an ideal diffuse reflector. An ideal diffuse surface is, at the microscopic level a very rough surface. Chalk is a good approximation to an ideal diffuse surface. Because of the microscopic variations in the surface, an incoming ray of light is equally likely to be reflected in any direction over the hemisphere.3Lecture 15 Slide 9 6.837 Fall 2001Lambert's Cosine LawIdeal diffuse reflectors reflect light according to Lambert's cosine law, (there are sometimes called Lambertian reflectors). Lambert's law states that the reflected energy from a small surface area in a particular direction is proportional to cosine of the angle between that direction and the surface normal. Lambert's law determines how much of the incominglight energy is reflected. Remember that the amount energy that is reflected in any one direction is constant in this model. In other words the reflected intensity is independent of the viewing direction. The intensity does however depend on the light source's orientation relative to the surface, and it is this property that is governed by Lambert's law.Lecture 15 Slide 10 6.837 Fall 2001Computing Diffuse ReflectionThe angle between the surface normal and the incoming light ray is called the angle of incidence and we can express a intensity of the light in terms of this angle. The Ilightterm represents the intensity of the incoming light at the particular wavelength (the wavelength determines the light's color). The kdterm represents the diffuse reflectivity of the surface at that wavelength. In practice we use vector analysis to compute cosine term indirectly. If both the normal vectorand the incoming light vectorare normalized (unit length) then diffuse shading can be computed as follows: Lecture 15 Slide 11 6.837 Fall 2001Diffuse Lighting ExamplesWe need only consider angles from 0 to 90 degrees. Greater angles (where the dot product is negative) are blocked by the surface, and the reflected energy is 0. Below are several examples of a spherical diffuse reflector with a varying lighting angles. Why do you think spheres are used as examples when shading?Lecture 15 Slide 12 6.837 Fall 2001Specular ReflectionA second surface type is called a specular reflector. When we look at a shiny surface, such as polished metal or a glossy car finish, we see a highlight, or bright spot. Where this bright spot appears on the surface is a function of where the surface is seen from. This type of reflectance is view dependent. At the microscopic level a specular reflecting surface is very smooth, and usually these microscopic surface elements are oriented in the same direction as the surface itself. Specular reflection is merely the mirror reflectionof the light source in a surface. Thus it should come as no surprise that it is viewer dependent, since if you stood in front of a mirror and placed your finger over the reflection of a light, you would expect that you could reposition your head to look around your finger and see the light again. An ideal mirror is a purely specular reflector. In order to model specular reflection we need to understand the physics of reflection.4Lecture 15 Slide 13 6.837 Fall 2001Snell's LawReflection behaves according to Snell's law: The incoming ray, the surface normal, and the reflected ray all lie in a common plane. The angle that the
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