Introduction to Texture Mapping

1. Introduction toTexture Mapping CSE 470/598 Introduction to Computer GraphicsArizona State University Dianne Hansford

2. Texture Mapping Concepts A method to create complexity in an image without the overhead of building large geometric models.

3. Basic Idea • Application of an image onto a model • An image is mapped onto the 2D domain of a 3D model • Correspondence between domain of surface and texture gives method to apply image

4. Texture Form • Textures are almost always rectangularm x n array of pixels called texels(texture elements) • Textures are frequently square and of sizes that are powers of two to support downsize filtering (mipmapping) • It is not necessary to always use the entire texture

5. Texture Coordinates • A texture is usually addressed by values between zero and one • The “addresses” of points on the texture consist of two numbers (s, t) • A vertex can be associated with a point on the texture by giving it one of these texture coordinatesglTexCoord*(s,t);glVertex*(x,y,z);Texture coords part of statelike colors and normals. …although it is really just an array of pixels [0, 1] t [1, 0] [0,0] s [s,t]-space  [u,v]-space of surface  [x,y,z]-space of surface

6. s, t = .33, .33 Pixels & Texture Coordinates [0, 1] t For example: a 32 x 32 pixel image Glubyte mychecker[32][32][3];glEnable(GL_TEXTURE_2D); [1, 0] [0,0] s glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 32, 32, 0, GL_RGB, GL_UNSIGNED_BYTE, mychecker);

7. s, t = .33, .33 Texture Coordinates to Polygons 1,0 1,1 Texture Coordinates 0,0 1,0 • Each vertex of each polygon in assigned a texture coordinate. • OGL finds the appropriate texture coordinate for points “between” vertices during rasterization. – same idea as smooth shading!

8. Example domains of the 3D model Parametric surfaces come with a 2D domain.Meshes: flatten parts to create a 2D domain

9. Textured

10. Modes of Operation • Decal: only the texture color determines the color we see in the frame buffer: GL_DECAL • Modulation: texture color multiplies the color computed for each face (default) • Blend:Similar to modulation but add alpha-blendingglTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE)

11. Programming with Texture Maps • Create texture and load with glTexImage() by eithera. read a jpeg, bmp, …. fileb. define texture within applicationc. copy image from color buffer • Define parameters as to how texture is appliedglTexParameter*()Next slides describe the options here….See Table 9.6 in OGL Red Book • Enable texture mapsglEnable(GL_TEXTURE_2D) • Define texture coordinates for verticesglTexCoord*(s,t); glVertex*(x,y,z);

12. Texture Memory • Texels go into texture memory;depends on implementation – special memory or frame buffer • Transfer of texels from application program to texture memory can be significant if texture large • Texture memory is limited resource –proxy commands to query availability • New graphics cards have MBs of texture memory • Texture objects help to optimize access to textures • In recent years, texturing has moved from software to high performance graphics hardware

13. Texture Objects • When using more than one texture … • Stores texture data and makes it available • Fastest way to apply/bind/reuse textures • Set-up similar to display lists1. Name the texture object2. Bind (create) texture object to texture data/properties3. Prioritize texture object (if maxing out texture memory)4. Bind texture object making data currently available

14. Texture Mapping Issues • What should happen when we zoom in close or zoom out far away? • How do we generate texture coordinates ? • What happens if we use texture coordinates less than zero or greater than one? • Are texture maps only for putting color on objects?

15. Texture to Surface Mapping • Texture map to surface takes place during renderingSimilar to smooth shading method: Triangle rasterized Each pixel mapped back to the texture Use known values at vertices to interpolation over the texture • Each pixel is associated with small region of surface and to a small area of texture. 3 possibilites for association:1) one texel to one pixel (rare) 2) magnification3) minification  Filtering Magnificationone texel to many pixels Minificationmany texels to one pixel

16. Zoom In many pixels correspond to one texel  “blockiness” / jaggies / aliasing solution: apply averaging (magnification filter)

17. Zoom-In: Magnification Filter • Pixel cooresponds to a small portion of one texel • Results in many pixels getting same texel • Without a filtering method, aliasing is common • Magnification filter: smooths transition between pixels pixel Texture space

18. Zoom-In: Magnification Filter Options for smoothing:Simplest: GL_NEAREST Just use the closest texel’s color (default – jaggies common) Also called point sampling Better: GL_LINEAR Weighted average of the four nearest texels Also called bilinear sampling (interpolation) glTexParameter(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); c3 Bilinear interpolationb1 = (s – s0)/(s2 – s0)b2 = (t – t0)/(t2 – t0) c = (1-b2)*( (1-b1)*c0 + b1*c1) + b2*( (1-b1)*c2 + b1*c3) c0 c1 (s0, t0) (s2, t0)

19. Zoom Out: Minification Filter • One pixel corresponds to many texels • Common with perspective foreshortening(see example on next slide) Options: GL_NEAREST GL_LINEARGL_*_MIPMAP_*_ where * = NEAREST OR LINEAR  mipmapping glTexParameter(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

20. Mipmap Improvements Perspective foreshorteningand poor texture mappingcauses checkerboard to deform Mipmaps improve the mapping, returning moreform to the checkerboard Images from the flipCode tutorial in the Resources

21. Better Min Filter: Mipmaps • “mip” stands for multum in parvo, or “many things in a small place” • Basic idea: Create many textures of decreasing size and use one of these subtextures when appropriate

22. Mipmap Representation I Create several copiesFilter down in sizePre-filter textures = mipmaps Appropriate sized texture selected based on number of pixels occupied by geometry

23. Mipmap Representation II Optimize storage(Schematic of method)

24. Mipmap Generation Must provide all sizes of texture from input to 1x1 in powers of 2 gluBuild*DMipmaps() will help with that!

25. Intermipmap Intramipmap Mipmap Filters • In order of increasing complexity: • GL_NEAREST_MIPMAP_NEAREST • GL_LINEAR_MIPMAP_NEAREST • GL_NEAREST_MIPMAP_LINEAR • GL_LINEAR_MIPMAP_LINEAR Remember: more complexity = slower rendering

26. Filtering in Summary • Zoom-in calls for mag filter • Zoom-out calls for min filter • More advanced filters require more time/computation but produce better results • Mipmapping is an advanced min filter • Caution: requesting mipmapping without pre-defining mipmaps will turn off texturing; (see Filtering in OGL)

27. Wrapping Modes: Repeat or Clamp • Can assign texture coords outside of [0,1] and have them either clamp or repeat the texture map • Repeat Issue: Making the texture borders match-up 2 1 2 0 1 0 repeat clamped

28. Borders and Sizing • Linear filtering needs an extra row/column of texels at the border • Solution: Add a border to your textureglTexParamter3fv(GL_TEXTURE_2D, GL_TEXTURE_BORDER_COLOR, color) • Some help copying texel values from original texture: glTexSubImage2D() • Size of texture: must be 2^n in width and heightgluScaleImage() can be used to convert image to acceptable dimension

29. Assigning Texture Coordinates • Parametric surfaces make assignment easy • However, distortion of texture will occur • Can minimize distortions by preserving aspect ratio of texture and geometry Two solutions:1) repeat texture2) use just a portion of the texture(to match the aspect ratio) geometry texture

30. Auto Texture Coordinate Generation • GLU Quadrics: use gluQuadricTexture() to enable auto generation of textures • OGL determines texture coordinates based on the distance of a vertex from a planePlane can be defined in object or eye coordinatesobject: texture remains fixed wrt objecteye: texture is dynamic, for example contour lines • OGL determines texture coordinates based on surface normal direction – environment mapping (see “More Texture Mapping” slides)glEnable(GL_TEXTURE_GEN_S)glTexGen*(…)

31. Problem with Perspective • Shape distortions caused by perspective maps not handled by simple interpolation of textures • More advanced methodglHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST) • Also: GL_FASTEST • (Other Hints for antialiasing of lines and polygons.) • More advanced method - slower

32. 1D Textures • Similar to 2D texture but height = 1 • Can create pattern of colors for line segments or curves • Contouring • Can use for Cel shading from intel

33. 3D Textures • 3D texture elements are called voxels(volume elements) • Embed an object in the voxels to determine color at vertices • Commonly used in medical or geoscience applicationsMedical: CT or MRI layered 3D dataGeoscience: rock strata or gas measurements • Caution: Texture memory can run out fast! • Example: Uni Hamburg’s Virtual Mummy http://www.uke.uni-hamburg.de/zentren/experimentelle_medizin/informatik/forschung/mumie/index.en.html

34. More Mapping Methods • Height maps • Normal maps / Bump maps • Shadow maps • Environment maps • ….We’ll discuss these another day!

35. Resources I • Chapter 9 of the text • Overview, pp 360-369 • Assigning Texture Coords, pp 414-417 • As much of the rest of the chapter as you can handle

36. Resources II • Fun Reading • From the SIGGRAPH tutorial pages: http://www.siggraph.org/education/materials/HyperGraph/mapping/r_wolfe/r_wolfe_mapping_1.htm • Texture Mapping - http://www.geocities.com/SiliconValley/2151/tmap.html • Advanced OpenGL Texture Mapping - http://www.flipcode.com/tutorials/tut_atmap.shtml • Texture Mapping as a Fundamental Drawing Primativehttp://www.sgi.com/misc/grafica/texmap/