CHAPTER 8 - Implementation 3

Objectives

Introduce the OpenGL texture functions and options

Basic Strategy

Three steps to applying a texture
- specify the texture
  - read or generate image
  - assign to texture
  - enable texturing
- assign texture coordinates to vertices
  - proper mapping function is left to application
- specify texture parameters
  - wrapping, filtering

Texture Mapping

Texture Example

The texture is a 256 x 256 image that has been mapped to a rectangular polygon which is viewed in perspective

Texture Mapping and the OpenGL Pipeline

Images and geometry flow through separate pipelines that join at the rasterizer

Specify Texture Image

Define a texture image from an array of texels (texture elements) in CPU memory

Glubyte my_texels[512][512];

Define as any other pixel map

Scanned image
Generate by application code

Enable texture mapping

glEnable(GL_TEXTURE_2D)
- OpenGL supports 1-4 dimensional texture maps

Define Image as a Texture

glTexImage2D( target, level, components, w, h, border, format, type, texels );

target: type of texture, e.g. GL_TEXTURE_2D

level: used for mipmapping (discussed later)

components: elements per texel

w, h: width and height of texels in pixels

border: used for smoothing (discussed later)

format and type: describe texels

texels: pointer to texel array

glTexImage2D(GL_TEXTURE_2D, 0, 3, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels);

Converting A Texture Image

OpenGL requires texture dimensions to be powers of 2
If dimensions of image are not powers of 2

gluScaleImage( format, w_in, h_in, type_in, *data_in, w_out, h_out, type_out, *data_out );
- data_in is source image
- data_out is for destination image
Image interpolated and filtered during scaling

Mapping a Texture

Based on parametric texture coordinates
glTexCoord*() specified at each vertex

Typical Code

(see cubeTex.c)

glBegin(GL_POLYGON); glColor3f(r0, g0, b0); glNormal3f(u0, v0, w0); glTexCoord2f(s0, t0); glVertex3f(x0, y0, z0); glColor3f(r1, g1, b1); glNormal3f(u1, v1, w1); glTexCoord2f(s1, t1); glVertex3f(x1, y1, z1); . . glEnd();

Note that we can use vertex arrays to increase efficiency

Interpolation

OpenGL uses bilinear interpolation to find proper texels from specified texture coordinates
Can be distortions

Texture Parameters

OpenGL has a variety of parameters that determine how texture is applied
- Wrapping parameters determine what happens of s and t are outside the (0,1) range
- Filter modes allow us to use area averaging instead of point samples
- Mipmapping allows us to use textures at multiple resolutions
- Environment parameters determine how texture mapping interacts with shading

Wrapping Mode

Clamping: if s, t > 1 use 1, if s, t < 0 use 0
Repeating: use s, t modulo 1

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP )

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP )

or

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT )

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )

Magnification and Minification

More than one texel can cover a pixel (minification) or more than one pixel can cover a texel (magnification)
Can use point sampling (nearest texel) or linear filtering (2 x 2 filter) to obtain texture values

Filter Modes

Modes determined by
- glTexParameteri( target, type, mode )
  
  glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST);
  
  glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_NEAREST);
  
  or
  
  glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_LINEAR);
  
  glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR);
Note that linear filtering requires a border of an extra texel for filtering at edges (border = 1)

Mipmapped Textures

Mipmapping allows for prefiltered texture maps of decreasing resolutions
Lessens interpolation errors for smaller textured objects
Declare mipmap level during texture definition

glTexImage2D( GL_TEXTURE_*D, level, … )

GLU mipmap builder routines will build all the textures from a given image

gluBuild*DMipmaps( … )

using the tga class (given in code examples)

gluBuild2DMipmaps(GL_TEXTURE_2D, myTGAImage.format, myTGAImage.width, myTGAImage.height, myTGAImage.format, GL_UNSIGNED_BYTE, myTGAImage.data);

Example

Texture Functions

Controls how texture is applied

glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param )

GL_TEXTURE_ENV_MODE modes

GL_MODULATE: modulates with computed shade
GL_BLEND: blends with an environmental color
GL_REPLACE: use only texture color

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

Set blend color with GL_TEXTURE_ENV_COLOR

Perspective Correction Hint

Texture coordinate and color interpolation
- either linearly in screen space
- or using depth/perspective values (slower)
Noticeable for polygons “on edge”
- glHint( GL_PERSPECTIVE_CORRECTION_HINT, hint ) where hint is one of
  - GL_DONT_CARE
  - GL_NICEST
  - GL_FASTEST

Generating Texture Coordinates

OpenGL can generate texture coordinates automatically

glTexGen{ifd}[v]()

specify a plane
- generate texture coordinates based upon distance from the plane
generation modes
- GL_OBJECT_LINEAR
- GL_EYE_LINEAR
- GL_SPHERE_MAP (used for environmental maps)

Texture Objects

Texture is part of the OpenGL state
- If we have different textures for different objects, OpenGL will be moving large amounts data from processor memory to texture memory
Recent versions of OpenGL have texture objects
- one image per texture object
- Texture memory can hold multiple texture objects

Applying Textures II

specify textures in texture objects
set texture filter
set texture function
set texture wrap mode
set optional perspective correction hint
bind texture object
enable texturing
supply texture coordinates for vertex
1. coordinates can also be generated

Other Texture Features

Environmental Maps
- Start with image of environment through a wide angle lens
- Can be either a real scanned image or an image created in OpenGL
- Use this texture to generate a spherical map
- Use automatic texture coordinate generation
Multitexturing
- Apply a sequence of textures through cascaded texture units