Module 5: Video Modeling Lecture 28: Illumination model. The Lecture Contains: Diffuse and Specular Reflection. Objectives_template

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1 The Lecture Contains: Diffuse and Specular Reflection file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_1.htm[12/30/2015 4:22:29 PM]

2 Diffuse and Specular Reflection As we see only objects that reflect light, their perceived color depend on the range of wavelengths reflected. In general, the reflection can be decomposed into two components: the diffuse reflection, which has equal energy distribution in all directions, and the specular reflection which is strongest in the mirror direction of the incident light. Surfaces that exhibit only diffuse reflection are known as Lambertian surfaces, more commonly described as dull or matte. Wood surfaces and cement walls belong to this category. Due to diffuse reflection, we can perceive the color of an object. Specular reflection can be observed with shiny surfaces and mirrors. Specular reflection does not show the color of the object but the color of the incident light is observed; therefore, we cannot actually perceive the color of an object with a surface that shows only specular reflections. Except for mirrors, surfaces usually have diffuse as well as specular reflections. Only the diffuse reflection determines the color of the object surface. file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_2.htm[12/30/2015 4:22:29 PM]

3 Radiance Distribution under changes in Illumination and Reflection Conditions: In video processing, the illumination model is primarily used to describe the temporal changes in the video sequence caused by the changing illumination of the real world. The illumination of a background may change because of an object that moves together with its cast shadow. Since the object surface reflects light, this reflecting source changes the overall illumination of the scene. When discussing the interaction of a light source with an object surface, there are three types of energy involved: (a) Incident flux, (b) incident irradiance, and (c) reflected radiance The incident flux refers to the rate at which energy is emitted from the light source. It is measured in watts (W). The incident irradiance is the incident flux per unit surface area on the object, with a unit of W/m 2. (Note that the irradiance at an object point depends on the angle between the incident light and the surface normal at that point.) Finally, reflected radiance measures the light energy reflected from an object surface. The distribution of the reflected radiance C depends on the distribution of incident irradiance E and the object surface reflection function r' at that point. The most general relation can be described by, file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_3.htm[12/30/2015 4:22:30 PM]

4 (5.1) where X is the location on the object surface, N is the surface normal vector at the location X, L is the illumination direction, V is the viewing direction connecting X to the focal point of the camera, and is the wavelength of light.obviously, L,V, and N are functions of X and t. The reflectance function r is defined as the ratio between the reflected light intensity (i.e. the flux) and the incident light intensity. This scalar function r is also known as the diffuse-reflection coefficient, or reflection coefficient. The reflectance function depends on the wavelength of the incident light, the surface geometry, and material properties. When the object is moving, the reflection coefficient changes in time at the same location. Note that is defined only for those X that belong to a surface at time t. We introduce several simplifying assumptions in order to learn more about the reflected radiance (Equation 5.1). We start by assuming opaque object surfaces and temporally invariant illumination (as well as viewing) direction. In this case, Equation (5.1) simplifies to (5.2) Note that although the illumination and viewing directions (V and L) are fixed, the incidence irradiance is still time varying because the object is moving. Consider the case of an ambient source. We know that am ambient source radiates the same amount of energy in every direction at any point; hence it illuminates objects without casting shadows. When the incident light is such an ambient source and the object surface is diffuse reflecting, the reflected radiance intensity distribution is (5.3) file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_4.htm[12/30/2015 4:22:30 PM]

5 Where represents the intensity of the ambient light at time t. Since the light source is ambient, does not depend on the surface location X or surface normal N. Since the surface is diffuse reflecting, the reflectance function r does not depend on the surface normal N. This ambient light source model is a local illumination model. This is so because we can no longer model global effects like shadows. In rooms, the illuminated white walls can often be modeled as ambient light sources. At outdoors, the sun, when covered by clouds, provides ambient illumination. We now consider the case of the reflected radiance due to a point source. We assume the light source is far away from the scene, such that the position of an object has no influence on the incident light. With a local illumination model and a diffuse-reflecting surface, the reflected radiance at any point X on object surface depends on the angle between the incident light direction L and the surface normal N at that point, denoted by. Obviously, holds. file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_5.htm[12/30/2015 4:22:30 PM]

6 Let represent the maximum irradiance from the light source. In other words, it is the irradiance intensity when the light is perpendicular to the surface. Then the irradiance with the light at an arbitrary angle is. Starting with Equation (5.2), the reflected radiance intensity at X simplifies to (5.4) The max operator is Equation (5.4) prevents negative reflected intensities from those parts of the object that do not receive light from the point light source. Typical point light sources are spotlights and the sun. When both ambient and point light sources are present, the total reflected radiance at any point is the superposition of the reflected radiance from each light source according to Equations (5.3) and (5.4). If a point light source is far away from the object surface, we can approximate the incident light as a parallel light. This approximation is valid for daylight and sometimes even at indoors. In this case, the maximum incident light irradiance E P is no longer dependent on the surface point X, but on the surface normal N. Assuming that the scene is illuminated by one stationary, distant point light source and an ambient light, both invariant in time and space, the description of the incident irradiance simplifies to: (5.5) This is referred to as the shading model used in early computer graphics. Assuming that the object has a homogeneous surface, that is, the corresponding reflected radiance becomes (5.6) file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2028/28_6.htm[12/30/2015 4:22:30 PM]