Reading. 11. Texture Mapping. Texture mapping. Non-parametric texture mapping. Required. Watt, intro to Chapter 8 and intros to 8.1, 8.4, 8.6, 8.8.

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1 Reading Reqired Watt, intro to Chapter 8 and intros to 8.1, 8.4, 8.6, 8.8. Optional 11. Textre Mapping Watt, the rest of Chapter 8 Woo, Neider, & Davis, Chapter 9 James F. Blinn and Martin E. Newell. Textre and reflection in compter generated images. Commnications of the ACM 19(10): , October Textre mapping Non-parametric textre mapping With non-parametric textre mapping : Textre mapping (Woo et al., fig. 9-1) Textre mapping allows yo to take a simple polygon and give it the appearance of something mch more complex. De to Ed Catmll, PhD thesis, 1974 Refined by Blinn & Newell, 1976 Textre mapping ensres that all the right things happen as a textred polygon is transformed and rendered. Textre size and orientation are fixed They are nrelated to size and orientation of polygon Gives cookie-ctter effect 3 4

2 Parametric textre mapping Implementing textre mapping A textre lives in it own abstract image coordinates paramaterized by (,v) in the range ([0..1], [0..1]): v It can be wrapped arond many different srfaces: With parametric textre mapping, textre size and orientation are tied to the polygon. Idea: = x/w = φ/2π v = θ/π Separate textre space and screen space Textre the polygon as before, bt in textre space Deform (render) the textred polygon into screen space = φ/2π A textre can modlate jst abot any parameter diffse color, speclar color, speclar exponent, Compting (,v) textre coordinates in a ray tracer is fairly straightforward. Note: if the srface moves/deforms, the textre goes with it. 5 6 textre image coords The textre is sally stored as an image. Ths, we need to convert from abstract textre coordinates: (,v) in the range ([0..1], [0..1]) Textre resampling To get the in between vales, we need to resample the textre. v tex = v h tex to textre image coordinates: b x (a, b) ( tex,v tex ) in the range ([0.. w tex ], [0.. h tex ]) v tex = v h tex v Q b Q (y Q, φ Q ) a textre pixel coords v tex = v w tex A common choice is bilinear interpolation: ( x y) T( ab, ) = T i+, j+ y Close-p Ray intersection Q = φ/2π abstract textre coords a textre pixel coords v tex = v w tex = T[ i, j] + T[ i+ 1, j] + Q: What do yo do when the textre sample yo need lands between textre pixels? T[ i, j+ 1] + T[ i+ 1, j+ 1] 7 8

3 Textre mapping and the z-bffer Solid textres Textre-mapping can also be handled in z-bffer algorithms. Method: Q: What kinds of artifacts might yo see from sing a marble veneer instead of real marble? y Scan conversion is done in screen space, as sal Each pixel is colored according to the textre Textre coordinates are fond by Gorad-style interpolation z One soltion is to se solid textres: x Use model-space coordinates to index into a 3D textre Like carving the object from the material One difficlty of solid textring is coming p with the textres. Note: Mapping is more complicated if yo want to do perspective right! 9 10 Solid textres (cont'd) Displacement mapping Here's an example for a vase ct from a solid marble textre: Textres can be sed for more than jst color. In displacement mapping, a textre is sed to pertrb the srface geometry itself: Q() N() = normal[q()] Q() = Q() + d()n() d() These displacements animate with the srface Solid marble textre by Ken Perlin, (Foley, IV-21) Q: Do yo have to do hidden srface calclations on Q? 11 12

4 Bmp mapping Displacement vs. bmp mapping In bmp mapping, a textre is sed to pertrb the normal: Inpt textre Use the original, simpler geometry, Q(), for hidden srfaces Use the normal from the displacement map for shading: N = normal[q()] Rendered as displacement map over a rectanglar srface Q() Q: What artifacts in the images wold reveal that bmp mapping is a fake? Displacement vs. bmp mapping (cont'd) Environment mapping In environment mapping (also known as reflection mapping), a textre is sed to model an object's environment: Original rendering Rendering with bmp map wrapped arond a cylinder Bmp map and rendering by Wyvern Aldinger Rays are bonced off objects into environment Color of the environment sed to determine color of the illmination Really, a simplified form of ray tracing Environment mapping works well when there is jst a single object or in conjnction with ray tracing Under simplifying assmptions, environment mapping can be implemented in hardware. With a ray tracer, the concept is easily extended to handle refraction as well as reflection

5 Combining textre maps Combining textre maps (cont'd) Using textre maps in combination gives even better effects, as Yong Sherlock Holmes demonstrated Environment- mapped mirror reflection Phong lighting with diffse textre Bmp mapping + Glossy reflection Combine textres and add dirt Constrction of the glass knight, (Foley, IV-24) Rivet stains + Shinier reflections Close-p Constrction of the glass knight, (Foley, IV-24) Smmary What to take home from this lectre: The meaning of the boldfaced terms. Familiarity with the varios kinds of textre mapping, inclding their strengths and limitations. 19