Texture Mapping in OpenGL

Texture Mapping in OpenGL
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Steps in the Initialization Process

Name the texture
Bind the texture to its name
Specify the parameters to use
Specify an application mode (either replace or modulate)
Create the texture object
Enable texture mapping

Steps in the Rendering Process

Select a texture
Specify a mapping from the texture to pixels

Naming a Texture

The Function:
- void glGenTextures(GLsizei n, GLuint* names)
Purpose:
- Returns n unused "names" (actually integers)
- The "names" need not be contiguous

Naming a Texture (cont.)

An Example

openglexamples/textures/image.c (Fragment: name)

Binding a Texture to a Name

The Function:
- void glBindTexture(GLenum target, GLuint name)
The Target:
- Either GL_TEXTURE_1D, GL_TEXTURE_2D, GL_TEXTURE_3D or GL_TEXTURE_CUBE_MAP

Binding a Texture to a Name (cont.)

An Example

openglexamples/textures/image.c (Fragment: bind)

Specifying Parameters

The Function:
- void glTexParameter*(GLenum target, GLenum name, TYPE value)
- where the target is either GL_TEXTURE_1D, GL_TEXTURE_2D, GL_TEXTURE_3D or GL_TEXTURE_CUBE_MAP
Wrapping:
- Name: GL_TEXTURE_WRAP_S or GL_TEXTURE_WRAP_T (the \(s\) and \(t\) directions)
- Value: GL_CLAMP, GL_REPEAT, (GL_CLAMP_TO_EDGE, GL_CLAMP_TO_BORDER, GL_MIRRORED_REPEAT)
Border Color:
- Name: GL_TEXTURE_BORDER_COLOR
- Value: RGBA
Magnification and Minification:
- Name: GL_TEXTURE_MAG_FILTER and GL_TEXTURE_MIN_FILTER
- Value: GL_NEAREST and GL_LINEAR (and others)

Specifying Parameters (cont.)

An Example

openglexamples/textures/image.c (Fragment: parameters)

Specifying the Application Mode

The Function:
- void glTexEnvf(GLenum target, Glenum name, TYPE value)
- where target is GL_TEXTURE_ENV and name is GL_TEXTURE_ENV_MODE
Values:
- GL_REPLACE, GL_ADD, GL_COMBINE, or GL_BLEND
- The last three are different ways of combing source and destination pixels

Specifying the Application Mode (cont.)

An Example

openglexamples/textures/image.c (Fragment: mode)

Creating the Texture Object

The Function:
- void glTexImage2D(GLenum target, GLint level, GLint internalFormat, GLsizei width, GLsizei height, GLint borderWidth, GLenum format, Glenum type const GLvoid* texels)
- where target is GL_TEXTURE_2D (or one of several other values)
Level:
- 0 when one resolution is supplied
Internal Format:
- 1..4 to indicate which components are included or one of many symbolic constants
Format:
- GL_RGB, GL_RGBA, GL_COLOR_INDEX (or others)
Deleting:
- Use glDeleteTextures()

Creating the Texture Object (cont.)

An Example

openglexamples/textures/image.c (Fragment: create)

Enabling Texture Mapping

The Function:
- void glEnable(GLenum capability)
Capabilities:
- GL_TEXTURE_2D, GL_TEXTURE_3D, (or one of several others)

Enabling Texture Mapping (cont.)

An Example

openglexamples/textures/image.c (Fragment: enable)

Rendering

Selecting a Texture:
- Use glBindTexture() again
Mapping to Texels:
- glTexCoord*(TYPE coords)

Rendering (cont.)

An Example

openglexamples/textures/image.c (Fragment: render)

Reading Images

An Observation:
- This is not OpenGL's responsibility
Utilities:
- GLUT has functions for reading OBJ model files which includes a function glmReadPPM() that reads .ppm files
- SDL has a function SDL_LoadBMP() for reading .bmp files
- Many other utilities exist

Reading Images (cont.)

An Example

openglexamples/textures/image.c (Fragment: image)

Using Multiple Textures

Using Multiple Textures (cont.)

Initialization

openglexamples/textures/multiple.c (Fragment: init)

Rendering

openglexamples/textures/multiple.c (Fragment: render)

Manipulating Textures

Scaling:
- gluScaleImage()
Copying:
- glCopyTexImage2D()
Parts:
- glTexSubImage2D()

Non-2D Textures

One-Dimensional:
- Essentially a 2D texture with a height of 1
Three-Dimensional:
- Different grids at different depths
- Mostly used for medical imaging

Mipmaps

Initializing:
- Use glTexParameter*() with a GL_TEXTURE_MIN_FILTER of either GL_NEAREST_MIMPAP_NEAREST, GL_NEAREST_MIMPAP_LINEAR, GL_LINEAR_MIMPAP_NEAREST or GL_LINEAR_MIMPAP_LINEAR
Example:
- See the "Red Book"

Texture Mapping onto a Sphere

We Need a Quadric:
- A quadric (sometimes called a quadric surface) is the locus of zeros of a quadratic polynomial (i.e., letting \(\bs{Q}\) be an \(n \times n\) matrix, \(\bs{r}\) be an \(n\)-dimensional point, and \(s\) be a scalar, the set of \(\bs{p}\) that satisfy \(\bs{p}^T \bs{Q} \bs{p} + \bs{p}^T r + s = 0\)
We Need an Appropriate Texture:
- A cylindrical projection

Texture Mapping onto a Sphere (cont.)

The Display Callback

openglexamples/textures/sphere.c (Fragment: display)

Texture Mapping onto other Shapes

We Need the Shape:
- Teapots are always good!
We Need an Appropriate Texture:
- Something irregular works best

Texture Mapping onto other Shapes (cont.)

The Display Callback

openglexamples/textures/teapot.c (Fragment: display)

There's Always More to Learn