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CS361

Week 8 - Monday. CS361. Last time. What did we talk about last time? Image texturing Magnification Minification Mipmapping Summed area tables. Questions?. Assignment 3. Project 3. Mipmap Review. Mipmapping in action.

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CS361

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  1. Week 8 - Monday CS361

  2. Last time • What did we talk about last time? • Image texturing • Magnification • Minification • Mipmapping • Summed area tables

  3. Questions?

  4. Assignment 3

  5. Project 3

  6. Mipmap Review

  7. Mipmapping in action • 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

  8. Trilinear filtering • One way to improve quality is to interpolate between u and vtexels 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

  9. Summed-area table • 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

  10. Anisotropic filtering • 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

  11. Other Texturing Issues

  12. Volume textures • 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

  13. Cube maps • 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

  14. Texture caching • 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

  15. Texture compression • 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 XNA projects have much larger .xnb 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

  16. More texture compression • 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

  17. Procedural texturing • 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)

  18. Texture animation • 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

  19. Material Mapping • 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

  20. Alpha Mapping • 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

  21. Student Lecture: Bump Mapping

  22. Bump Mapping

  23. Bump mapping • 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

  24. Macro, meso, and micro • 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

  25. Blinn's methods • James Blinn proposed the offset vector bumpmap or offset map • Stores bu and bv 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

  26. Normal maps • 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

  27. Parallax mapping • 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:

  28. Parallax mapping continued • 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:

  29. Relief mapping • 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

  30. Heightfield texturing • 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

  31. Upcoming

  32. Next time… • Radiometry • Photometry • Colorimetry • BRDFs

  33. Reminders • Start reading Chapter 7 • Start working on Project 3 • Even though it's not assigned yet • Assignment 3 is due on Friday

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