CS 431/636 Advanced Rendering Techniques

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1 CS 431/636 Advanced Rendering Techniques Dr. David Breen Matheson 308 Thursday 6PM 8:50PM Presentation 7 5/23/06

2 Questions from Last Time? Hall Shading Model Shadows Reflections Refractions

3 Slide Credits Jonathan Cohen - Johns Hopkins University Shayan Sarkar - Carnegie Mellon University Leonard McMillan, Jovan Popovic, Fredo Durand - MIT Steve Marschner - Cornell University David Luebke - University of Virginia Pat Hanrahan - Stanford University

4 2D Texture Mapping

5 The Quest for Visual Realism

6 Photo-textures The concept is very simple!

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Adding Texture Mapping to Illumination Texture mapping can be used to alter some

We can simply use the texture as the final color for the pixel, or we can just

12 Adding Texture Mapping to Illumination Texture mapping can be used to alter some or all of the constants in the illumination equation. We can simply use the texture as the final color for the pixel, or we can just use it as diffuse color, or we can use the texture to alter the normal, or... the possibilities are endless! Phong's Illumination Model Constant Diffuse Color Diffuse Texture Color Texture used as Label Texture used as Diffuse Color Lecture 15 Slide Fall 2002

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21 Difficulties with texture mapping Another difficulty is how to map a 2D image onto an arbitrary 3D model Ex: If you wanted to map onto a sphere, you can t do it without distorting the texture 03/14/ Graphics I 15

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33 Difficulties with texture mapping Interested in mapping from screen to texture coordinates 03/14/ Graphics I 8

34 Difficulties with texture mapping Aliasing issues, it is especially apparent with regular textures 03/14/ Graphics I 9

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39 Antialiasing and Texture Mapping l Texture mapping is uniquely harder Coherent textures present pathological artifacts Correct filter shape changes l Texture mapping is uniquely easier Textures are known ahead of time They can thus be prefiltered David Luebke 1/19/99

40 Antialiasing and Texture Mapping l More on texture problems Coherent texture frequencies become infinitely high with increasing distance n Ex: checkerboard receding to horizon Unfiltered textures lead to compression n Ex: pixel covers entire checkerboard n Unfiltered mapping: pixel is black or white n Moving checkerboard Ÿ flashing pixels David Luebke 1/19/99

41 Antialiasing and Texture Mapping l Problem: a square pixel on screen becomes a curvilinear quadrilateral in texture map (see W&W, p 140) l The coverage and area of this shape change as a function of the mapping l Most texture antialiasing algorithms approximate this shape somehow David Luebke 1/19/99

42 l Mip-mapping Antialiasing and Texture Mapping MIP = Multim in Parvo (many things in a small place) Ignores shape change of inverse pixel But allows size to vary l Idea: store texture as a pyramid of progressively lower-resolution images, filtered down from original David Luebke 1/19/99

43 Antialiasing: : Mip-Mapping Depth of Mip-Map David Luebke 1/19/99

44 Antialiasing: Mip Mapping l Distant textures use higher levels of the mipmap l Thus, the texture map is prefiltered l Thus, reduced aliasing! David Luebke 1/19/99

45 Antialiasing: Mip Mapping l Which level of mip-map to use? Think of mip-map as 3-D pyramid Index into mip-map with 3 coordinates: u, v, d (depth) l Q: What does d correspond to in the mip-map? l A: size of the filter David Luebke 1/19/99

46 Antialiasing: Mip Mapping l The size of the filter (i.e., d in the mip-map) depends on the pixel coverage area in the texture map In general, treat d as a continuous value Blend between nearest mip-map level using tri-linear interpolation David Luebke 1/19/99

47 Antialiasing: Mip Mapping l Q: What s s wrong with the mip-map approach to prefiltering texture? l A: Assumes pixel maps to square in texture space l More sophisticated inverse pixel filters (see F&vD p 828): Summed area tables Elliptical weighted average filtering David Luebke 1/19/99

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54 Texture and Materials Slide 3 of 28 1 of 2 11/13/03 3:46 PM

Texture and Materials http://graphics.stanford.

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Environment Mapping Allows for world to be reflected on an object without modeling the physics Map the world surrounding an object onto a cube Project that cube onto the object During the shading

57 Environment Mapping Allows for world to be reflected on an object without modeling the physics Map the world surrounding an object onto a cube Project that cube onto the object During the shading calculation: Bounce a ray from the viewer off the object (at point P) Intersect the ray with the environment map (the cube), at point E Get the environment map s color at E and illuminate P as if there were a virtual light source at position E You see an image of the environment reflected on shiny surfaces 03/14/ Graphics I 32

Environment Maps If, instead of using the ray from the surface point to the projected texture's center, we used the direction of the reflected ray to index a texture map.

58 Environment Maps If, instead of using the ray from the surface point to the projected texture's center, we used the direction of the reflected ray to index a texture map. We can simulate reflections. This approach is not completely accurate. It assumes that all reflected rays begin from the same point. Lecture 15 Slide Fall 2002

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60 What's the Best Chart? Lecture 15 Slide Fall 2002

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64 Reflection Mapping Lecture 15 Slide Fall 2002

65 Bump Mapping How do you make a surface look rough? Option 1: model the surface with many small polygons Option 2: perturb the normal vectors before the shading calculation the surface doesn t actually change, but shading makes it look that way bump map fakes small displacements above or below the true surface can use texture-mapping for this For the math behind it all look at Angel /14/ Graphics I 30

Bump Mapping Textures can be used to alter the surface normal of an object. This does not change the actual shape of the surface -- we are only shading it as if it were a different shape!

66 Bump Mapping Textures can be used to alter the surface normal of an object. This does not change the actual shape of the surface -- we are only shading it as if it were a different shape! This technique is called bump mapping. The texture map is treated as a single-valued height function. The value of the function is not actually used, just its partial derivatives. The partial derivatives tell how to alter the true surface normal at each point on the surface to make the object appear as if it were deformed by the height function. Since the actual shape of the object does not change, the silhouette edge of the object will not change. Bump Mapping also assumes that the Illumination model is applied at every pixel (as in Phong Shading or ray tracing). Swirly Bump Map Sphere w/diffuse Texture Sphere w/diffuse Texture & Bump Map Lecture 15 Slide Fall 2002

67 Bump mapping If Pu and Pv are orthogonal and N is normalized: v P = [ x( u, v), y( u, v), z( u, v)] v v v Normal N = P u P v v v v P = P + B( u, v) N T Initial point Simulated elevated point after bump N v P r P r' D v v N D v P v u P v v v v v v N ª N + BuPu + BvPv v D B u B v Variation of normal in u direction = = B( s - D, t) - B( s + D, t) 2D B( s, t - D) - B( s, t + D) 2D Variation of normal in v direction Compute bump map partials by numerical differentiation Lecture 15 Slide Fall 2002

68 More Bump Map Examples Bump Map Cylinder w/diffuse Texture Map Cylinder w/texture Map & Bump Map Lecture 15 Slide Fall 2002

69 One More Bump Map Example Notice that the shadow boundaries remain unchanged. Lecture 15 Slide Fall 2002

70 Bump Mapping In CG, we can perturb the normal vector without having to make any actual change to the shape. What would be the problems with bump mapping? Original model (5M) Simplified (500) Simple model with bump map 03/14/ Graphics I 31

71 Displacement Mapping We use the texture map to actually move the surface point. This is called displacement mapping. How is this fundamentally different than bump mapping? The geometry must be displaced before visibility is determined. Is this easily done in the graphics pipeline? In a ray-tracer? Lecture 15 Slide Fall 2002

72 Stochastic Microgeometry for Displacement Mapping Reconstruct a stochastic distribution over a mesh

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76 The Best of All Worlds All these texture mapping modes are great! The problem is, no one of them does everything well. Suppose we allowed several textures to be applied to each primitive during rasterization. Lecture 15 Slide Fall 2002

77 Wrap Up Discuss status/problems/issues with this week s programming assignment

Texture-Mapping Tricks. How Bad Does it Look? We've Seen this Sort of Thing Before. Sampling Texture Maps

Texture-Mapping Tricks. How Bad Does it Look? We've Seen this Sort of Thing Before. Sampling Texture Maps Texture-Mapping Tricks Filtering Textures Textures and Shading Bump Mapping Solid Textures How Bad Does it Look? Let's take a look at what oversampling looks like: Click and drag the texture to rotate