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Published byJanel Bond Modified over 9 years ago
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Week 8 - Monday
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What did we talk about last time? Workday Before that: Image texturing ▪ Magnification ▪ Minification Mipmapping Summed area tables
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Typically a chain of mipmaps is created, each half the size of the previous That's why cards like square power of 2 textures Often the filtered version is made with a box filter, but better filters exist The trick is figuring out which mipmap level to use The level d can be computed based on the change in u relative to a change in x
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One way to improve quality is to interpolate between u and v texels from the nearest two d levels Picking d can be affected by a level of detail bias term which may vary for the kind of texture being used
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Sometimes we are magnifying in one axis of the texture and minifying in the other Summed area tables are another method to reduce the resulting overblurring It sums up the relevant pixels values in the texture It works by precomputing all possible rectangles
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Summed area tables work poorly for non-rectangular projections into texture space Modern hardware uses unconstrained anisotropic filtering The shorter side of the projected area determines d, the mipmap index The longer side of the projected area is a line of anisotropy Multiple samples are taken along this line Memory requirements are no greater than regular mipmapping
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Image textures are the most common, but 3D volume textures can be used These textures store data in a (u, v, w) coordinate space Even volume textures can be mipmapped Quadrilinear interpolation! In practice, volume textures are usually used for fog, smoke, or explosions 3D effects that are inconsistent over the volume
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A cube map is a kind of texture map with 6 faces Cube maps are used to texture surfaces based on direction They are commonly used in environment mapping A ray is made from the center of the cube out to the surface The component with the largest magnitude selects which of the 6 faces The other components are used for (u,v) coordinates Cube maps can cause awkward seams when jumping between faces
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You will never need to worry about this in this class, but texture memory space is a huge problem There are many different caching strategies, similar ones used for RAM: Least Recently Used (LRU): Swap out the least recently used texture, very commonly used Most Recently Used (MRU): Swap out the most recently used texture, use only during thrashing Prefetching can be useful to maintain consistent frame rates
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JPEG and PNG are common compression techniques for regular images In graphics hardware, these are too complicated to be decoded on the fly That's why the finished SharpDX projects have pre- processed.tkb files Most DirectX texture compression divides textures into 4 x 4 tiles Two 16-bit RGB values are recorded for each tile Each texel uses 2 bits to select one of the two colors or two interpolated values between them
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Ericsson texture compression (ETC) is used in OpenGL It breaks texels into 2 x 4 blocks with a single color It uses per-pixel luminance information to add detail to the blocks Normal maps (normals stored as textures) allow for interesting compression approaches Only x and y components are needed since the z component can be calculated The x and y can then be stored using the BC5 format for two channels of color data
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A procedural texture is made by computing a function of u and v instead of looking up a texel in an image Noise functions are often used to give an appearance of randomness Volume textures can be generated on the fly Values can be returned based on distance to certain feature points (redder colors near heat, for example)
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Textures don't have to be static The application can alter them over time Alternatively, u and v values can be remapped to make the texture appear to move Matrix transformations can be used for zoom, rotation, shearing, etc. Video textures can be used to play back a movie in a texture Blending between textures can allow an object to transform like a chameleon
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The lighting we have discussed is based on material properties Diffuse color Specular color Smoothness coefficient m A texture can be used to modify these values on a per-pixel basis A normal image texture can be considered a diffuse color map One that affects specular colors is a specular color map (usually grayscale) One that affects m is a gloss map
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Alpha values allow for interesting effects Decaling is when you apply a texture that is mostly transparent to a (usually already textured) surface Cutouts can be used to give the impression of a much more complex underlying polygon 1-bit alpha doesn't require sorting Cutouts are not always convincing from every angle
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Bump mapping refers to a wide range of techniques designed to increase small scale detail Most bump mapping is implemented per- pixel in the pixel shader 3D effects of bump mapping are greater than textures alone, but less than full geometry
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Macro-geometry is made up of vertices and triangles Limbs and head of a body Micro-geometry are characteristics shaded in the pixel shader, often with texture maps Smoothness (specular color and m parameter) based on microscopic smoothness of a material Meso-geometry is the stuff in between that is too complex for macro-geometry but large enough to change over several pixels Wrinkles Folds Seams Bump mapping techniques are primarily concerned with mesoscale effects
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James Blinn proposed the offset vector bump map or offset map Stores b u and b v values at each texel, giving the amount that the normal should be changed at that point Another method is a heightfield, a grayscale image that gives the varying heights of a surface Normal changes can be computed from the heightfield
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The results are the same, but these kinds of deformations are usually stored in normal maps Normal maps give the full 3- component normal change Normal maps can be in world space (uncommon) Only usable if the object never moves Or object space Requires the object only to undergo rigid body transforms Or tangent space Relative to the surface, can assume positive z Lighting and the surface have to be in the same space to do shading Filtering normal maps is tricky
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Bump mapping doesn't change what can be seen, just the normal High enough bumps should block each other Parallax mapping approximates the part of the image you should see by moving from the height back to the view vector and taking the value at that point The final point used is:
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At shallow viewing angles, the previous approximation can look bad A small change results in a big texture change To improve the situation, the offset is limited (by not scaling by the z component) It flattens the bumpiness at shallow angles, but it doesn't look crazy New equation:
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The weakness of parallax mapping is that it can't tell where it first intersects the heightfield Samples are made along the view vector into the heightfield Three different research groups proposed the idea at the same time, all with slightly different techniques for doing the sampling There is still active research here Polygon boundaries are still flat in most models
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Yet another possibility is to change vertex position based on texture values Called displacement mapping With the geometry shader, new vertices can be created on the fly Occlusion, self-shadowing, and realistic outlines are possible and fast Unfortunately, collision detection becomes more difficult
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Radiometry Photometry Colorimetry BRDFs
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Start reading Chapter 7 Finish Project 2 Due on Friday
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