Texture Mapping

Texture mapping is the process of taking an image, presented typically as a 2-dimensional array of RGB values and mapping it onto a polygon. Setting up texture mapping involves the following steps: define a texture by specifying the image and its format (through glTexImage2d(), specify how object vertices correspond to points in the texture, and finally enable texture mapping. First, the texture must be input or generated by the program. OpenGL provides a wide variety of other features, but we will only summarize a few here, which are sufficient for handling a single 2-dimensional texture. To handle multiple textures, see the commands glGenTextures() and glBindTexture().

glTexImage2D(GLenum target, int level, int internalFormat, int width, int height, int border, GLenum format, GLenum type, void *pixels):

This converts a texture stored in the array pixels into an internal format and stored in pixels. The first argument is typically GL_TEXTURE_2D. The other arguments specify various elements of how the mapping is to be performed. See the reference manual for complete information.

glTexEnvf(GLenum target, GLenum pname, GLfloat param):

Specifies texture mapping environment parameters. The target must be GL_TEXTURE_ENV. The pname must be GL_TEXTURE_ENV_MODE. This determines how a color from the texture image is to be merged with an existing color on the surface of the polygon. The param may be any of the following:

• GL_MODULATE: multiply color components together.
• GL_BLEND: linearly blend color components.
• GL_DECAL: use the texture color.
• GL_REPLACE: use the texture color.

There are subtle differences between GL_DECAL and GL_REPLACE when different formats are used or when the A component of the RGBA color is not 1. See the reference manual. The default is GL_MODULATE.

glTexParameterf(GLenum target, GLenum pname, GLfloat param):

glTexParameterfv(GLenum target, GLenum pname, const GLfloat *params):

Specify how texture interpolation is to be performed. The first version is for defining scalar parameters, and the second is for vector parameters. Assuming 2-dimensional textures, the target is GL_TEXTURE_2D, the pname is either:

• GL_TEXTURE_MAG_FILTER: magnification filter.
• GL_TEXTURE_MIN_FILTER: minification filter.

Magnification is used when a pixel of the texture is smaller than the corresponding pixel of the screen onto which it is mapped and minification applies in the opposite case. Typical values are either

• GL_NEAREST: take the nearest texture pixel.
• GL_LINEAR: take the weighted average of the 4 surrounding texture pixels.

This procedure may also be invoked to specify other properties of texture mapping.

glTexCoord*(...):

Specifies the texture coordinates of subsequently defined vertices for texture mapping. For a standard 2-dimensional textures, the texture coordinates are a pair (s,t) in the interval . The texture coordinate specifies the point on the image that are to be mapped to this vertex. OpenGL interpolates the mapping of intermediate points of the polygon.

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Last updated: Sat, Feb 8, 2003.