Introduction to Texture Mapping CSE 470/598 Introduction to Computer Graphics Arizona State University Dianne Hansford.

Introduction toTexture Mapping

CSE 470/598

Introduction to Computer GraphicsArizona State University

Dianne Hansford

Texture Mapping Concepts

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

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

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

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 coordinates

glTexCoord*(s,t);glVertex*(x,y,z);

Texture coords part of statelike colors and normals.

s

t

[0,0]

[1, 0]

[0, 1]

[s,t]-space [u,v]-space of surface [x,y,z]-space of surface

…although it is really just an array of pixels

Pixels & Texture Coordinates

s

t

[0,0]

[1, 0]

[0, 1]

s, t = .33, .33s, t = .33, .33

For example: a 32 x 32 pixel image

Glubyte mychecker[32][32][3];

glEnable(GL_TEXTURE_2D);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 32, 32, 0, GL_RGB, GL_UNSIGNED_BYTE, mychecker);

Texture Coordinates to Polygons

1,0

1,1

0,0

1,0

Texture Coordinat

es

Texture Coordinat

es

s, t = .33, .33s, t = .33, .33

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!

Example

domains of the 3D model

Parametric surfaces come with a 2D domain.

Meshes: flatten parts to create a 2D domain

Textured

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-blending

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE)

Programming with Texture Maps

1. Create texture and load with glTexImage() by eithera. read a jpeg, bmp, …. fileb. define texture within applicationc. copy image from color buffer

2. Define parameters as to how texture is appliedglTexParameter*()

Next slides describe the options here….See Table 9.6 in OGL Red Book

3. Enable texture mapsglEnable(GL_TEXTURE_2D)

4. Define texture coordinates for vertices glTexCoord*(s,t); glVertex*(x,y,z);

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

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

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?

Texture to Surface Mapping• Texture map to surface takes place during rendering

Similar to smooth shading method:Triangle rasterizedEach pixel mapped back to the textureUse 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

Magnificationone texel to many pixels

Minificationmany texels to one pixel

Filtering

Zoom In

many pixels correspond to one texel “blockiness” / jaggies / aliasing

solution: apply averaging (magnification filter)

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

Zoom-In: Magnification FilterOptions for smoothing:

Simplest: GL_NEARESTJust use the closest texel’s color (default – jaggies common)Also called point sampling

Better: GL_LINEARWeighted average of the four nearest texelsAlso called bilinear sampling (interpolation)

glTexParameter(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

(s0, t0)

c0 c1

c3

(s2, t0)

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)

Zoom Out: Minification Filter

• One pixel corresponds to many texels• Common with perspective foreshortening

(see example on next slide)

glTexParameter(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

Options:GL_NEARESTGL_LINEARGL_*_MIPMAP_*_ where * = NEAREST OR LINEAR mipmapping

Mipmap Improvements

Images from the flipCode tutorial in the Resources

Perspective foreshorteningand poor texture mappingcauses checkerboard to deform

Mipmaps improve the mapping, returning moreform to the checkerboard

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

Mipmap Representation I

Create several copiesFilter down in sizePre-filter textures = mipmaps

Appropriate sized texture selected based on number of pixels occupied by geometry

Mipmap Representation II

Optimize storage(Schematic of method)

Mipmap Generation

Must provide all sizes of texture from input to 1x1 in powers of 2

gluBuild*DMipmaps() will help with that!

Mipmap Filters

• In order of increasing complexity:– GL_NEAREST_MIPMAP_NEAREST– GL_LINEAR_MIPMAP_NEAREST– GL_NEAREST_MIPMAP_LINEAR– GL_LINEAR_MIPMAP_LINEAR

Intermipmap

Intramipmap

Remember: more complexity = slower rendering

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)

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

clamped repeat

0 1 2 10 2

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

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

texture geometry

Two solutions:1) repeat texture2) use just a portion of the texture(to match the aspect ratio)

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 plane

Plane 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*(…)

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

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

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 applications

Medical: 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

More Mapping Methods

• Height maps• Normal maps / Bump maps• Shadow maps • Environment maps • ….

We’ll discuss these another day!

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

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/