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**Visual Appearance (Shading & Texture mapping)**

Course Note Credit: Some of slides are extracted from the course notes of prof. J. Lee (SNU), prof. M. Desbrun (USC) and prof. H.-W. Shen (OSU) and others.

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**Shading (Local illumination)**

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**Photorealism in Computer Graphics**

Photorealism in computer graphics involves Accurate representations of surface properties, and Good physical descriptions of the lighting effects Modeling the lighting effects that we see on an object is a complex process, involving principles of both physics and psychology Physical illumination models involve Material properties, object position relative to light sources and other objects, the features of the light sources, and so on

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**Illumination and Rendering**

An illumination model in computer graphics also called a lighting model or a shading model used to calculate the color of an illuminated position on the surface of an object Approximations of the physical laws A surface-rendering method determine the pixel colors for all projected positions in a scene

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**Light Sources Point light sources Infinitely distant light sources**

Emitting radiant energy at a single point Specified with its position and the color of the emitted light Infinitely distant light sources A large light source, such as sun, that is very far from a scene Little variation in its directional effects Specified with its color value and a fixed direction for the light rays

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**Light Sources Directional light sources Area light sources**

Produces a directional beam of light Spotlight effects Area light sources

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**Light Sources Radial intensity attenuation**

As radiant energy travels, its amplitude is attenuated by the factor Sometimes, more realistic attenuation effects can be obtained with an inverse quadratic function of distance The intensity attenuation is not applied to light sources at infinity because all points in the scene are at a nearly equal distance from a far-off source

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**Light Sources Angular intensity attenuation**

For a directional light, we can attenuate the light intensity angularly as well as radially

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**Surface Lighting Effects**

An illumination model computes the lighting effects for a surface using the various optical properties Degree of transparency, color reflectance, surface texture The reflection (phong illumination) model describes the way incident light reflects from an opaque surface Diffuse, ambient, specular reflections Simple approximation of actual physical models

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Diffuse Reflection Incident light is scattered with equal intensity in all directions Such surfaces are called ideal diffuse reflectors (also referred to as Lambertian reflectors)

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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.

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**Diffuse Reflection : the intensity of the light source**

Ideally diffuse surfaces obey cosine law. Often called Lambertian surfaces. : the intensity of the light source : diffuse reflection coefficient, : the surface normal (unit vector) : the direction of light source, (unit vector)

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Ambient Light Multiple reflection of nearby (light-reflecting) objects yields a uniform illumination A form of diffuse reflection independent of the viewing direction and the spatial orientation of a surface Ambient illumination is constant for an object : the incident ambient intensity : ambient reflection coefficient, the proportion reflected away from the surface

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Ambient + Diffuse

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Specular Reflection Perfect reflector (mirror) reflects all lights to the direction where angle of reflection is identical to the angle of incidence It accounts for the highlight

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**Specular Reflection Phong specular-reflection model**

Note that N, L, and R are coplanar, but V may not be coplanar to the others : intensity of the incident light : color-independent specular coefficient : the gloss of the surface

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Specular Reflection Glossiness of surfaces

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Specular Reflection Specular-reflection coefficient ks is a material property For some material, ks varies depending on q ks =1 if q =90° Calcularing the reflection vector R

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**Specular Reflection Simplified Phong model using halfway vector**

H is constant if both viewer and the light source are sufficiently far from the surface

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**Ambient+Diffuse+Specular Reflections**

Single light source Multiple light source

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**Parameter Choosing Tips**

For a RGB color description, each intensity and reflectance specification is a three-element vector The sum of reflectance coefficients is usually smaller than one Try n in the range [0, 100] Use a small ka (~0.1) Example Metal: n=90, ka=0.1, kd=0.2, ks=0.5

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Atmospheric Effects A hazy atmosphere makes colors fade and objects appear dimmer Hazy-atmosphere effect is often simulated with an exponential attenuation function such as Higher values for r produce a denser atmosphere

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**Polygon Rendering Methods**

We could use an illumination model to determine the surface intensity at every projected pixel position Or, we could apply the illumination model to a few selected points and approximate the intensity at the other surface positions Curved surfaces are often approximated by polygonal surfaces So, polygonal (piecewise planar) surfaces often need to be rendered as if they are smooth

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**Constant-Intensity Surface Rendering**

Constant (flat) shading Each polygon is one face of a polyhedron and is not a section of a curved-surface approximation mesh

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**Intensity-Interpolation Surface Rendering**

Gouraud shading Rendering a curved surface that is approximated with a polygon mesh Interpolate intensities at polygon vertices Procedure Determine the average unit normal vector at each vertex Apply an illumination model at each polygon vertex to obtain the light intensity at that position Linearly interpolate the vertex intensities over the projected area of the polygon

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**Intensity-Interpolation Surface Rendering**

Normal vectors at vertices Averaging the normal vectors for each polygon sharing that vertex 4 3 2 1 ) ( N v + =

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**Intensity-Interpolation Surface Rendering**

Intensity interpolation along scan lines I4 and I5 are interpolated along y-axis, and then Ip is interpolated along x-axis Incremental calculation is also possible y 3 1 p scan line 4 5 2 x

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**Intensity-Interpolation Surface Rendering**

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**Gouraud Shading Problems**

Lighting in the polygon interior is inaccurate Mach band effect Optical illusion

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Mach Band Effect These “Mach Bands” are not physically there. Instead, they are illusions due to excitation and inhibition in our neural processing The bright bands at 45 degrees (and 135 degrees) are illusory. The intensity of each square is the same.

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**Mach Band artifact (Gouraud Shading)**

Mach bands: Caused by interaction of neighboring retinal neurons. Acts as a sort of high-pass filter, accentuating discontinuities in first derivative. Linear interpolation causes first deriv. Discontinuities at polygon edges.

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**Normal-Vector Interpolation for Surface Rendering**

Phong shading Interpolate normal vectors at polygon vertices Procedure Determine the average unit normal vector at each vertex Linearly interpolate the vertex normals over the projected area of the polygon Apply an illumination model at positions along scan lines to calculate pixel intensities n1 n2 n3

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**Gouraud versus Phong Shading**

Gouraud shading is faster than Phong shading OpenGL supports Gouraud shading Phong shading is more accurate

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Texture Mapping

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Can you do this …

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Visual Realism

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**Texture Mapping Surfaces in real world are very complex**

Objects have properties that vary across surface Cannot model all the fine variations We need to find ways to add surface detail How?

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Texture Mapping Of course, one can model the exact micro-geometry + material property to control the look and feel of a surface But, it may get extremely costly So, graphics use a more practical approach – texture mapping

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**Texture Mapping Texture Mapping**

Want a function that assigns a color to each point The the surface is a 2D domain, so that is essentially an image can represent using any image representation raster texture images are very popular

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Photo-textures

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**OpenGL functions - demo**

Usually texture is a (2D/3D) image. During initialization read in or create the texture image and place it into the OpenGL state. glTexImage2D (GL_TEXTURE_2D, 0, GL_RGB, imageWidth, imageHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, imageData); Before rendering your textured object, enable texture mapping and tell the system to use this particular texture. glBindTexture (GL_TEXTURE_2D, 13);

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OpenGL functions During rendering, give the cartesian coordinates and the texture coordinates for each vertex. glBegin (GL_QUADS); glTexCoord2f (0.0, 0.0); glVertex3f (0.0, 0.0, 0.0); glTexCoord2f (1.0, 0.0); glVertex3f (10.0, 0.0, 0.0); glTexCoord2f (1.0, 1.0); glVertex3f (10.0, 10.0, 0.0); glTexCoord2f (0.0, 1.0); glVertex3f (0.0, 10.0, 0.0); glEnd ();

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**Texture and Texel Each pixel in a texture map is called a Texel**

Each Texel is associated with a (u,v) 2D texture coordinate The range of u, v is [0.0,1.0] due to normalization

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(u,v) tuple For any (u,v) in the range of (0-1, 0-1) multiplied by texture image width and height, we can find the corresponding value in the texture map

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**How do we get F(u,v)? We are given a discrete set of values:**

F[i,j] for i=0,…,N, j=0,…,M Nearest neighbor: F(u,v) = F[ round(N*u), round(M*v) ] Linear Interpolation: i = floor(N*u), j = floor(M*v) interpolate from F[i,j], F[i+1,j], F[i,j+1], F[i+1,j] Filtering in general !

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Interpolation Nearest neighbor Linear Interpolation

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**Specifying texture coordinates**

Texture coordinates needed at every vertex Hard to specify by hand Difficult to wrap a 2D texture around a 3D object

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**We would like a constant cost per pixel**

Texture Filtering Resampling using mip mapping Magnification: Interpolation Minification: Averaging Texture Image Texture => Image We would like a constant cost per pixel

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**Mip Mapping MIP = Multim In Parvo = Many things in a small place**

Constructs an image pyramid. Each level is a prefilteredversion of the level below resampled at half the frequency. Whilerasterizing use the level with the sampling rate closest to the desired sampling rate.

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**Trilinear interpolation**

Mip Mapping Used with bilinear/trilinear interpolation R R G B G B Trilinear interpolation

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Mip Mapping - Example Courtesy of John hart used to save some of the filtering work needed during texture minification

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**Texture Filtering Methods**

Nearest Neighbor interpolation Bilinear Interpolation Trilinear Interpolation Anisotropic filtering Precomputed rectangular(trapezoidal) maps <anisotropic filtering)

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example No mip mapping vs with mip mapping

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example Bilinear mip mapping vs trilinear mip mapping

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**Specifying texture coordinates**

Texture coordinates needed at every vertex Hard to specify by hand Difficult to wrap a 2D texture around a 3D object

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Planar mapping Compute texture coordinates at each vertex by projecting the map coordinates onto the model

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Cylindrical Mapping

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Spherical Mapping

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Cube Mapping

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**Modelling Surface Properties**

Can use a texture to supply any parameter of the illumination model ambient, diffuse, and specularcolor Specular exponent roughness

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**Environment Maps Use texture to represent reflected color**

Texture indexed by reflection vector Approximation works when objects are far away from the reflective object

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**Spatially variant resolution**

Environment Maps Using a spherical environment map Spatially variant resolution

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Environment Maps Using a cubical environment map

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Environment Mapping Environment mapping produces reflections on shiny objects Texture is transferred in the direction of the reflected ray from the environment map onto the object Reflected ray: R=2(N·V)N-V Environment Map Viewer Reflected ray Object

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Approximations Made The map should contain a view of the world with the point of interest on the object as the eye We can’t store a separate map for each point, so one map is used with the eye at the center of the object Introduces distortions in the reflection, but the eye doesn’t notice Distortions are minimized for a small object in a large room The mapping can be computed at each pixel, or only at the vertices

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Example

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Refraction Maps Use texture to represent refraction

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**Use texture to represent opacity**

Opacity Maps Use texture to represent opacity RGB channels Use the alpha channel to make portions of the texture transparent. Cheaper than explicit modelling alpha channels

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Illumination Maps Use texture to represent illumination footprint

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Illumination Maps Quake light maps

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**Does not change silhouette edges**

Bump Mapping Use texture to perturb normals Textures are treated as height field creates a bump-like effect + = original surface bump map modified surface Does not change silhouette edges

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Bump mapping

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**Normal Mapping (bump mapping)**

Replace normals

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Bump Mapping Many textures are the result of small perturbations in the surface geometry Modeling these changes would result in an explosion in the number of geometric primitives. Bump mapping attempts to alter the lighting across a polygon to provide the illusion of texture.

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**Bump mapping only affects the normals,**

Displacement Mapping Use texture to displace the surface geometry + = Bump mapping only affects the normals, Displacement mapping changes the entire surface (including the silhouette)

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Displacement Mapping

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**Can simulate an object carved from a material**

3D Textures Use a 3D mapping Marble or wood The object is “carved”out of the solid texture Can simulate an object carved from a material

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Texture Synthesis Texture Synthesis

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**Transparency Alpha Blending**

a: foreground object , b: background object, o: output Alpha : opacity , C : color

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