texture mapping why texture mapping?

Transcription

1 texture mapping computer graphics texture mapping 2009 fabio pellacini 1 why texture mapping? objects have spatially varying details represent as geometry: correct, but very expensive computer graphics texture mapping 2009 fabio pellacini 2

2 why texture mapping? use simple geometry store varying properties in images map to objects [Wolfe / SG97 Slide set] computer graphics texture mapping 2009 fabio pellacini 3 why texture mapping? produces compelling results [Jeremy Birn] computer graphics texture mapping 2009 fabio pellacini 4

3 why texture mapping? easily change object appearance [Praun et al., 2001] computer graphics texture mapping 2009 fabio pellacini 5 mapping function surfaces are 2d domains determine a function that maps them to images Mapping Function Surface Image computer graphics texture mapping 2009 fabio pellacini 6

4 mapping functions projections maps 3d surface points to 2d image coordinates 3 2 : R [0,1] different types of projections f often corresponding to simple shapes useful for simple object [Wolfe / SG97 Slide set] computer graphics texture mapping 2009 fabio pellacini 7 projections planar computer graphics texture mapping 2009 fabio pellacini 8

5 projections cubical computer graphics texture mapping 2009 fabio pellacini 9 projections cylindrical computer graphics texture mapping 2009 fabio pellacini 10

6 projections spherical computer graphics texture mapping 2009 fabio pellacini 11 projections planar projection along xy plane of size (w,h) use affine transform to orient the plane differently spherical projection of unit sphere consider point in spherical coordinates cylindrical projection of unit cylinder of height h consider point in cylindrical coordinates treat caps separately f ( p) = ( p / w, p / h) x y f ( p) = ( φ, θ ) f ( p) = ( φ, p / h) y computer graphics texture mapping 2009 fabio pellacini 12

7 looking up texture values normal: do not repeat texture clamp image coordinates to [0,1] then lookup tiled: repeat texture multiple times take mod of image coordinates then lookup normal tiled computer graphics texture mapping 2009 fabio pellacini 13 texture mapping artifacts tiling textures might introduce seems discontinuities in the mapping function change texture to be tileable when possible computer graphics texture mapping 2009 fabio pellacini 14

8 texture mapping artifacts mapping textures will introduce distortions unavoidable artifacts local scale and rotation differences distorted undistorted computer graphics texture mapping 2009 fabio pellacini 15 mapping function explicit coordinates store texture coordinates on control points interpolate as any other parameter follow interpolation rule defined by surface type parametric surfaces: can use parameters directly known as UV mapping computer graphics texture mapping 2009 fabio pellacini 16

9 uv mapping subdivision surfaces level 0 level 1 level 2 computer graphics texture mapping 2009 fabio pellacini 17 uv mapping vs. projection parameterization projection computer graphics texture mapping 2009 fabio pellacini 18

10 uv mapping parametric surfaces [Wolfe / SG97 Slide set] computer graphics texture mapping 2009 fabio pellacini 19 uv mapping polygon meshes computer graphics texture mapping 2009 fabio pellacini 20

11 uv mapping polygon meshes [Piponi et al., 2000] computer graphics texture mapping 2009 fabio pellacini 21 uv mapping polygon meshes break up model intro single texture [ Discreet] computer graphics texture mapping 2009 fabio pellacini 22

12 interpolating uv coordinates on meshes pay attention when rasterizing triangles for raytracing just use baricentric coordinates texture linear interp. perspective interp. [MIT OpenCourseware] used also for colors computer graphics texture mapping 2009 fabio pellacini 23 painting textures on models if painting is required, paint directly on surfaces system determines inverse mapping to update image seems/distortions present, but user does not know computer graphics texture mapping 2009 fabio pellacini 24

13 texture magnification [MIT OpenCourseware] linearly interpolate closest pixels in texture texture rendered image computer graphics texture mapping 2009 fabio pellacini 25 texture minification [MIT OpenCourseware] compute average of texture pixels projected onto each view pixels texture computer graphics texture mapping rendered image 2009 fabio pellacini 26

14 texture minification remember point-sampling introduces artifacts need average of texture below a pixel [MIT OpenCourseware] computer graphics texture mapping 2009 fabio pellacini 27 mip-mapping approximate algorithm for computing filters store texture at different resolution look up the appropriate image based on its projected size [MIT OpenCourseware] computer graphics texture mapping 2009 fabio pellacini 28

15 3d solid texturing define a 3D field of values, indexed using P in-memory array: too much memory procedurally: hard to define often add noisy-like details on 2d images [Wolfe / SG97 Slide set] computer graphics texture mapping 2009 fabio pellacini 29 types of mapping computer graphics texture mapping 2009 fabio pellacini 30

16 texture mapping material parameters diffuse coefficient computer graphics texture mapping 2009 fabio pellacini 31 texture mapping material parameters specular coefficient computer graphics texture mapping 2009 fabio pellacini 32

17 displacement mapping variations of surface positions, thus normals requires fine tessellation of object geometry computer graphics texture mapping 2009 fabio pellacini 33 displacement mapping update position by displacing points along normal Pd = P + hn recompute normals by evaluating derivatives no closed form solution: do it numerically N d Pd u P v d ΔPd Δu ΔP Δv d computer graphics texture mapping 2009 fabio pellacini 34

18 computer graphics texture mapping 2009 fabio pellacini 35 bump mapping variation of surface normals apply normal perturbation without updating positions computer graphics texture mapping 2009 fabio pellacini 36 bump mapping simple example: bump mapping xy plane z y x N P P ), ( ), ( ), ( ), ( v u h v u v u h v u v u d + + = = + = y x z z y z x P P N v h u h v h u h v u d d d = = + + =

19 bump vs. displacement mapping bump map displacement map computer graphics texture mapping 2009 fabio pellacini 37 bump vs. displacement mapping bump map displacement map computer graphics texture mapping 2009 fabio pellacini 38

20 combining maps types combine multiple maps to achieve realistic effects computer graphics texture mapping 2009 fabio pellacini 39 lighting effects using texture mapping computer graphics texture mapping 2009 fabio pellacini 40

21 shadow mapping graphics pipeline does not allow shadow queries we can use texturing and a multipass algorithm [NVIDIA/Everitt et al.] project a color texture project a depth texture computer graphics texture mapping 2009 fabio pellacini 41 shadow mapping algorithm pass 1: render scene from light view pass 1: copy depth buffer in a new texture pass 2: render scene from camera view pass 2: transform each pixel to light space pass 2: compare value to depth buffer pass 2: if current < buffer depth then shadow computer graphics texture mapping 2009 fabio pellacini 42

22 shadow mapping algorithm [NVIDIA/Everitt et al.] camera view light view shadow buffer computer graphics texture mapping 2009 fabio pellacini 43 shadow mapping algorithm [NVIDIA/Everitt et al.] camera view light distance projected shadow buffer computer graphics texture mapping 2009 fabio pellacini 44

23 shadow mapping limitations not enough resolution: blocky shadows pixels in shadow buffer too large when projected [Fernando et al., 2002] biasing: surfaces shadow themselves remember the epsilon in raytracing made much worst by resolution limitation computer graphics texture mapping 2009 fabio pellacini 45 environment mapping graphics pipeline does not allow reflections we can use texturing and a multipass algorithm [Wolfe / SG97 Slide set] computer graphics texture mapping 2009 fabio pellacini 46

24 environment mapping algorithm pass 1: render scene 6 times from object center pass 1: store images onto a cube pass 2: render scene from the camera view pass 2: use cube projection to look up values variation of this works also for refraction computer graphics texture mapping 2009 fabio pellacini 47 environment map limitations incorrect reflections objects in incorrect positions: better for distant objs rays go through objects inefficient: need one map for each object computer graphics texture mapping 2009 fabio pellacini 48

25 light effects take home message pipeline not suitable for lighting computations algorithms are complex to implement and not robust lots of tricks and special cases but fast interactive graphics: use pipeline algorithms high-quality graphics: use pipeline for view, raytracing for lighting computer graphics texture mapping 2009 fabio pellacini 49 texturing demos OpenGL tutor: texture.exe NVidia samples: bumpy_shiny_patch.exe hw_shadowmap_simple.exe simple_soft_shadows.exe computer graphics texture mapping 2009 fabio pellacini 50