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Texture

Texture. Visual detail without geometry. Texture Mapping. desire for heightened realism. Texture Mapping. Means of adding visual detail to a scene without adding geometric detail

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Texture

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  1. Texture Visual detail without geometry

  2. Texture Mapping • desire for heightened realism

  3. Texture Mapping • Means of adding visual detail to a scene without adding geometric detail • For each point on the surface, there is a corresponding point in a texture image: fetch surface characteristics from the texture • Characteristics can be color, lighting parameters, surface normal, displacement

  4. Texture Definitions • Regular speech: conventional definition – visual and tactile properties of a surface • Graphics: operational definition – any image used in texture mapping • Vision: mathematical definition – a signal with the property of 'stationarity' (any part is like another part)

  5. Stationarity

  6. Stationarity

  7. Stationarity

  8. Texture Mapping • Cover a surface with an image • Each point has a texture coordinate (u,v) • Typically, the texture coordinates are given at vertices and interpolated like other parameters

  9. Texture Mapping • Simplest object: cylinder (sides only) • u coordinate is angle (normalized to 0..1) • v coordinate is height (normalized ditto) • "label on a can of soup" • Not all shapes can be parameterized so easily

  10. Texture Mapping • Map a 2D surface (in 3D) to a plane • On the one hand – no problem! The plane is big enough • On the other hand -- we want to satisfy constraints: • no distortion of areas • no distortion of angles • no seams • Impossible!

  11. Solutions • Remove one or more of the constraints: • Allow angles to be distorted • Allow areas to be distorted • Allow seams • Change the problem • Make a mapping from a surface to a surface • Make a mapping from a volume to a surface

  12. Relaxing Constraints • Usual solution – just put up with distortion • A little bit of distortion is not too visible • Allow distortion, but use it: the planar texture is pre-distorted

  13. Relaxing Constraints • Permit seams • Instead of a single "chart", we have a whole "atlas" – the surface is locally flat, so this can work • Need to hide the seams • easy if texture has lots of high-frequency detail • align seams with model features (e.g., hairline) • put seams in rarely-seen locations

  14. Solid Texture • Instead of a 2D plane, use a 3D volume of texture • Mapping is trivial • Object appears to be carved from textured material • Drawback: way more texture needed

  15. Solid Texture

  16. Traditional Texture Mapping • Specify (u,v) coordinates for vertices • (u,v) in ({0,1},{0,1}) – canonical values • Need to specify various characteristics of using the texture • what to do with coordinates outside range • how to combine texture and lighting • mip mapping

  17. Boundary Conditions • One-dimensional texture for simplicity • What does T(1.1) evaluate to? • Three options: • clamp: T(t) = T(1), t>1 • mirror: T(t) = T(1-t), 2>t>1 • repeat: T(t) = T(floor(t)) • May introduce visible discontinuities

  18. Combining Texture and Lighting • Texture supplies a color (value in texture map) • Illumination calculation also supplies a color (calculated by local lighting) • Combining them: • use texture only ("decal") • use constant fraction of texture • modulate (multiply values together)

  19. Bump Mapping • Jim Blinn, 1981 • Modify normal direction according to texture • No extra geometry, illusion of high geometric complexity • N = Ninterpolated* (1-b) + b * Ntexture • and renormalize

  20. Normal Maps • No underlying normal to perturb – entire normal comes from texture • No 3D geometry needed! Can give illusion of geometry on sprites (just use fake normal in lighting calculation)

  21. Magnification and Minification • Texture rarely appears onscreen at the stored resolution • If it appears smaller: minification filtering • If it appears larger: magnification filtering • Filtering options: • nearest neighbour • linear interpolation • cubic interpolation (smoother, more expensive)

  22. MIP mapping • "multum in parvo", "many [things] in a small place" • Store multiple prefiltered scales of texture • Prefiltering done ahead of time, with great care, reducing aliasing and preserving important features • Interpolate between mipmap levels instead of filtering

  23. the TextureSampler • In HLSL, reading a value from a texture is done with the TextureSampler • auxiliary function defined in shader • specifies what interpolation to use • given coordinates, returns texture value (color) • Shader interprets texture • used for color, normal (bump mapping), position (displacement mapping) , ...

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