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Illumination Models and Shading Illumination Models and Shading Foley & Van Dam, Chapter 16 • Light Source Models • Ambient Illumination • Diffuse Reflection • Specular Reflection • Polygon Rendering Methods • Flat Shading • Gouraud Shading • Phong Shading Illumination Models Illumination Model Parameters • Lighting effects are described with models that consider the • Motivation: In order to produce realistic images, interaction of light sources with object surfaces we must simulate the appearance of surfaces under various lighting conditions • The factors determining the lighting effects are: – The light source parameters: • Positions • Illumination Model: Given the illumination • Electromagnetic Spectrum incident at a point on a surface, quantifies the • Shape reflected light – The surface parameters • Position • Reflectance properties • Position of nearby surfaces – The eye (camera) parameters • Position • Sensor spectrum sensitivities Illumination Models and Rendering Light Source Models • An illumination model is used to calculate the • Point Source (a): All light rays originate at a point and intensity of the light that is reflected at a given point radially diverge. A reasonable approximation for sources whose dimensions are small compared to the object size on a surface • Parallel source (b): Light rays are all parallel. May be • A rendering method uses intensity calculations modeled as a point source at infinite distance (the sun) from the illumination model to determine the light • Distributed source (c): All light rays originate at a finite intensity at all pixels in the image area in space. It models a nearby source, such as a fluorescent light c b a Illumination Models Ambient Illumination • Simplified and fast methods for calculating • Assume there is some non-directional light surfaces intensities, mostly empirical in the environment (background light) • Calculations are based on optical properties of surfaces and the lighting conditions (no • The amount of ambient light incident on reflected sources nor shadows) each object is constant for all surfaces and over all directions • Light sources are considered to be point sources • Very simple model, not very realistic • Reasonably good approximation for most scenes • OpenGL default Ambient Illumination Ambient Illumination • The reflected intensity Iamb of any point on the • Example: surface is: Iamb = Ka Ia Ia - ambient light intensity Ka [0,1] - surface ambient reflectivity • In principle Ia and Ka are functions of color, so we have IRamb, IGamb and IBamb Diffuse Reflection Diffuse Reflection • Diffuse (Lambertian) surfaces are rough or grainy, • Brightness is proportional to cos( ) because a like clay, soil, fabric surface (a) perpendicular to the light direction is more illuminated than a surface (b) at an oblique • The surface appears angle equally bright from all a b viewing directions • The brightness at each L L N N point is proportional to cos( ) Diffuse Reflection Diffuse Reflection • The reflected intensity Idiff of a point on the • Example: surface is: Idiff = Kd Ipcos( ) = Kd Ip(N L) Ip - the point light intensity. May appear as attenuated source fatt(r)IP Kd [0,1] - the surface diffuse reflectivity N - the surface normal L - the light direction NOTE: If N and L have unitary length: cos( ) = N L Diffuse Reflection Diffuse Reflection • Example: diffuse reflection from different • Commonly, there are two types of light sources: light directions – A background ambient light – A point light source • The equation that combines the two models is: I = Idiff + Iamb = Kd Ip N L + Ka Ia • Note this is the model for one color and it should be replicated for each channel: IR, IG and IB Diffuse Reflection Specular Reflection • Example: • Models shiny and glossy surfaces (like metal, 0 0.3 0.6 Kd plastic, etc..) with highlights • Reflectance intensity changes with reflected angle 0.3 • An ideal specular surface (mirror) reflects light exclusively in one direction: R • Glossy objects are not ideal mirrors and reflect 0.5 in the immediate vicinity of R N N L R L R V 0.7 Ideal specular surface Non-ideal specular surface Ka Specular Reflection Specular Reflection • The Phong Model: reflected specular intensity • The Phong Model: plots of cosn( ) for three falls off as some power of cos ( ): values of the specular parameter n 1 Ispec = Ks Ipcosn( ) = Ks Ip(R V)n n=1 0.8 n=8 Ks - the surface specular reflectivity n=64 N 0.6 L R n – specular reflection parameter, determining V the deviation from ideal specular surface 0.4 (for a perfect mirror n= ) 0.2 Specular surface N L R 0 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 V Specular surface Specular Reflection Specular Reflection • The illumination equation combined with diffuse • Example: Ks 0.2 0.5 0.8 reflection is: I = Iamb+Idiff+Ispec= 0 Ka Ia + Ip (Kd N L + Ks (R V)n) 0.3 • If k light sources are present in the scene: I= Iamb+ k k k (I diff+I spec) 0.7 Kd Specular Reflection Specular Reflection • Example: effects of the specular parameter n • Example: Ambient Illumination n=50 Ambient + Diffuse n=10 Ambient + Diffuse + Specular n=3 Composing Light Sources Polygon Rendering Methods • Example: • A freeform surface can be approximated by polyhedra • Rendering: calculate the illumination at each surface point • Applying the illumination model at each surface point is computationally expensive Flat Shading Gouraud Shading • A single intensity is calculated for each surface • Renders the polygon surface by linearly polygon interpolating intensity values across the surface • Fast and simple method • Gives reasonable result only if all of the following Gouraud Shading Algorithm: assumptions are valid: – The object is a polyhedron 1. Determine the normal at each polygon vertex – Light source is far away 2. Apply an illumination model to each vertex to from the surface so that calculate the vertex intensity N•L is constant over each 3. Linearly interpolate the vertex intensities over polygon the surface polygon – Viewing position is far away from the surface so that V•R is constant over each polygon Gouraud Shading Gouraud Shading • The normal Nv of a vertex is an average of all • Interpolation of the vertex intensities neighboring normals: y N k I3 k N V I1 IP N k scan line k I4 I5 I2 x y 4 y 2 y 1 y 4 I4 I1 I2 y 1 y 2 y 1 y 2 y 5 y 2 y 3 y 5 I5 I3 I2 y 3 y 2 y 3 y 2 x 5 x p x p x 4 Ip I4 I5 x 5 x 4 x 5 x 4 Gouraud Shading Phong Shading • Example: Gouraud shading of a sphere • A more accurate method for rendering a polygon surface is to interpolate normal vectors, and then apply the illumination model to each surface point Phong Shading Algorithm: 1. Determine the normal at each polygon vertex 2. Linearly interpolate the vertex normals over the surface polygon 3. Apply the illumination model along each scan line to calculate intensity of each surface point Phong Shading Polygon Rendering Methods • Example: Phong shading of a sphere • Example: Flat Gouraud Phong Polygon Rendering Methods Polygon Rendering Methods • Example: • Example: Flat Gouraud Flat Gouraud Phong Phong

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Computer Graphics, Illumination Models, light sources, Computer Science, Applied Geometry for Computer Graphics and CAD, Gouraud Shading, Specular Reflection, Introduction to Computer Graphics, Raster Graphics, Ray Tracing

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posted: | 3/30/2011 |

language: | English |

pages: | 6 |

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