6 texture mapping computer graphics

6 texture mapping computer graphics

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  3/12/2019 1 SBE 306B -Computer Systems III Texture Mapping Dr. Ayman Eldeib Systems & Biomedical Engineering Department Spring 2019 Computer Graphics Texture Mapping Texture Mapping geometry image x y z screen
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  3/12/2019 2 Computer Graphics TextureMapping Why Texture Mapping?  Scenes created with diffuse lighting look convincingly three-dimensional, but are flat, chalky, and “cartoonish”  Phong lighting lets us simulate materials like plastic and (to a lesser extent) metal, but scenes still seem very cartoonish and unreal  Big problem: polygons are too coarse-grained to usefully model fine surface detail  Solution: texture mapping Computer Graphics Texture Mapping  Adding surface detail helps keep CG images from looking so simple  Explicitly modeling this detail in geometry can be very expensive Zebra stripes, wood grain, writing on a whiteboard  Texture mapping pastes images onto the surfaces in the scene, adding realistic fine detail without exploding the geometry Cont. Why Texture Mapping?
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  3/12/2019 3 Computer Graphics TextureMapping Texture Mapping and OpenGL Pipeline  Images and geometry flow through separate pipelines that join at the rasterizer  “complex” textures do not affect geometric complexity vertices geometry pipeline image pixel pipeline Rasterizer (screen) vertices image Computer Graphics Texture Mapping Texture Mapping  Map variations in 2D image to 3D object 3D object 2D image
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  3/12/2019 4 Computer Graphics TextureMapping  Adds Visual Detail Texture Mapping Cont. Computer Graphics Texture Mapping Spring 2011 Computer Graphics Texture Mapping  Bump Mapping  Texture = change in surface normal! Sphere w/ diffuse texture Swirly bump map Sphere w/ diffuse texture and swirly bump map Texture Mapping Cont.
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  3/12/2019 5 Computer Graphics TextureMapping  Displacement Mapping Cont. Texture Mapping Computer Graphics Texture Mapping  In short: it is impractical to explicitly model fine surface detail with geometry  Solution: use images to capture the “texture” of surfaces  Texture maps can modulate many factors that affect the rendering of a surface such as: Color or reflectance (diffuse, ambient, specular) Transparency (smoke effects) Cont. Texture Mapping
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  3/12/2019 6 Computer Graphics TextureMapping Texture Mapping Polygonal Models  To each vertex (x,y,z in object coordinates), must associate 2D texture coordinates (u,v)  So texture fits “nicely” over object Computer Graphics Texture Mapping geometry + = image texture map Texture Mapping
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  3/12/2019 7 Computer Graphics TextureMapping Texture Mapping Option: Varieties of Projections Computer Graphics Texture Mapping Map Shape  Map shapes correspond to various projections Planar, Cylindrical, Spherical  First, map (square) texture to basic map shape  Then, map basic map shape to object Or vice versa: Object to map shape, map shape to square  Usually, this is straightforward Maps from square to cylinder, plane, sphere well defined Maps from object to these are simply spherical, cylindrical, cartesian coordinate systems
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  3/12/2019 8 Computer Graphics TextureMapping Planar Mapping  Drop z coord  (u,v) = (x,y) Computer Graphics Texture Mapping Cylindrical Mapping
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  3/12/2019 9 Computer Graphics TextureMapping Spherical Mapping Computer Graphics Texture Mapping Cube Mapping
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  3/12/2019 10 Computer Graphics TextureMapping Cube Mapping Cont. Computer Graphics Texture Mapping  When scan converting a polygon (triangle) must map each pixel to texture image  In reality, each pixel in raster image does not map to a pixel in texture image  Actually, each pixel maps to a region in texture - Scale |PIX| < |TEX| oEasy: interpolate (bilinear) between texel values |PIX| > |TEX| oHard: average the contribution from multiple texels |PIX| ~ |TEX| oStill need interpolation! Interpolation of Texture Coordinates
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  3/12/2019 11 Computer Graphics TextureMapping  Interpolation  Geometrically correct interpolation  Nearest neighbor  Bilinear  Bicubic  Scale interpolate Interpolation of Texture Coordinates Cont. Computer Graphics Texture Mapping Texture Mapping  Interpolation methods Geometrically correct interpolation o Must interpolate in world space not screen space Nearest neighbor o Take value of nearest pixel Bilinear o Linearly average four nearest pixels based on distance Bicubic o Weighted average of surrounding pixels
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  3/12/2019 12 Computer Graphics TextureMapping Texture Mapping Fundamentals  A texture is typically a 2-D image Image elements are called texels A texture is typically a 2-D array of texels Value stored at a texel affects surface appearance in some way The mapping of the texture to the surface determines the correspondence, i.e., how the texture lies on the surface o Mapping a texture to a triangle is easy. o Mapping a texture to an arbitrary 3-D shape is more complicated. Computer Graphics Texture Mapping Texture Mapping Rendering  Rendering uses the mapping: Find the visible surface at a pixel Find the point on that surface corresponding to that pixel Find the point in the texture corresponding to that point on the surface Use the parameters associated with that point on the texture to shade the pixel  Using triangulated meshes reduces the problem to mapping a portion of the image to each triangle
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  3/12/2019 13 Computer Graphics TextureMapping  We typically parameterize the texture as a function in (u, v) For simplicity, normalize u & v  Associate each triangle with a texture  Give each vertex of the triangle a texture coordinate (u, v)  For other points on the triangle, interpolate texture coordinate from the vertices Much like interpolating color or depth Cont. Texture Mapping Rendering Computer Graphics Texture Mapping MIP-maps  For a texture of 2n x 2n pixels, compute n-1 textures, each at ½ the resolution of previous  This multiresolution texture is called a MIP-map Original Texture Lower Resolution Versions
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  3/12/2019 14 Computer Graphics TextureMapping  Keep textures pre-filtered at multiple resolutions For each pixel, linearly interpolate between two closest levels Fast, easy for hardware MIP-maps Computer Graphics Texture Mapping User Generated Mappings  For complex 3-D objects, mapping textures is still something of an art…so we often let the user do it
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  3/12/2019 15 Computer Graphics TextureMapping Applying Texture using OpenGL  specify texture read or generate image assign to texture  assign texture coordinates to vertices  specify texture parameters wrapping, filtering Three Steps Computer Graphics Texture Mapping Specify Texture Image  Define a texture image from an array of texels in CPU memory dimensions of image must be powers of 2 glTexImage2D( target, level, components, w, h, border, format, type, *texels ); glTexCoord*() specified at each vertex  Texel colors are processed by pixel pipeline
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  3/12/2019 16 Computer Graphics TextureMapping Texture Application Methods  Filter Modes  minification or magnification  special mipmap minification filters  Wrap Modes  clamping or repeating e.g. glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )  Texture Functions  how to mix primitive’s color with texture’s color  blend, modulate or replace texels glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param ) GL_TEXTURE_ENV_MODE modes: GL_MODULATE, GL_BLEND, GL_REPLACE g{tÇ~ lÉâg{tÇ~ lÉâg{tÇ~ lÉâg{tÇ~ lÉâ