glTexParameter

NAME

glTexParameterf, glTexParameteri, glTexParameterfv, glTexParameteriv -- set texture parameters

C SPECIFICATION

void glTexParameterf(GLenum target,
                     GLenum pname,
                     GLfloat param)
void glTexParameteri(GLenum target,
                     GLenum pname,
                     GLint param)

PARAMETERS

target: Specifies the target texture, which must be either GL_TEXTURE_1D, or GL_TEXTURE_2D.
pname: Specifies the symbolic name of a single-valued texture parameter. pname can be one of the following: GL_TEXTURE_MIN_FILTER, GL_TEXTURE_MAG_FILTER, GL_TEXTURE_WRAP_S, or GL_TEXTURE_WRAP_T.
param: Specifies the value of pname.

C SPECIFICATION

void glTexParameterfv(GLenum target,
                      GLenum pname,
                      const GLfloat *params)
void glTexParameteriv(GLenum target,
                      GLenum pname,
                      const GLint *params)

PARAMETERS

target: Specifies the target texture, which must be either GL_TEXTURE_1D, or GL_TEXTURE_2D.
pname: Specifies the symbolic name of a texture parameter. pname can be one of the following: GL_TEXTURE_MIN_FILTER, GL_TEXTURE_MAG_FILTER, GL_TEXTURE_WRAP_S, GL_TEXTURE_WRAP_T, or GL_TEXTURE_BORDER_COLOR.
params: Specifies a pointer to an array where the value or values of pname are stored.

DESCRIPTION

Texture mapping is a technique that applies an image onto an object's surface as if the image were a decal or cellophane shrink-wrap. The image is created in texture space, with an (s, t) coordinate system. A texture is a one-, or two-dimensional image and a set of parameters that determine how samples are derived from the image.

glTexParameter assigns the value or values in params to the texture parameter specified as pname. target defines the target texture, either GL_TEXTURE_1D, or GL_TEXTURE_2D. The following symbols are accepted in pname:

GL_TEXTURE_MIN_FILTER

The texture minifying function is used whenever the pixel being textured maps to an area greater than one texture element. There are six defined minifying functions. Two of them use the nearest one or nearest four texture elements to compute the texture value. The other four use mipmaps.

A mipmap is an ordered set of arrays representing the same image at progressively lower resolutions. If the texture has dimensions 2^n * 2^m there are max(n, m) + 1 mipmaps. The first mipmap is the original texture, with dimensions 2^n * 2^m. Each subsequent mipmap has dimensions 2^(k - 1) * 2^(l - 1) where 2^k * 2^l are the dimensions of the previous mipmap, until either k = 0 or l = 0. At that point, subsequent mipmaps have dimension 1 * 2^(l - 1) or 2^(k - 1) * 1 until the final mipmap, which has dimension 1 * 1. Mipmaps are defined using glTexImage1D or glTexImage2D or with the level-of-detail argument indicating the order of the mipmaps. Level 0 is the original texture; level max(n, m) is the final 1 * 1 mipmap.

params supplies a function for minifying the texture as one of the following:

GL_NEAREST: Returns the value of the texture element that is nearest (in Manhattan distance) to the center of the pixel being textured.
GL_LINEAR: Returns the weighted average of the four texture elements that are closest to the center of the pixel being textured. These can include border texture elements, depending on the values of GL_TEXTURE_WRAP_S and GL_TEXTURE_WRAP_T, and on the exact mapping.
GL_NEAREST_MIPMAP_NEAREST: Chooses the mipmap that most closely matches the size of the pixel being textured and uses the GL_NEAREST criterion (the texture element nearest to the center of the pixel) to produce a texture value.
GL_LINEAR_MIPMAP_NEAREST: Chooses the mipmap that most closely matches the size of the pixel being textured and uses the GL_LINEAR criterion (a weighted average of the four texture elements that are closest to the center of the pixel) to produce a texture value.
GL_NEAREST_MIPMAP_LINEAR: Chooses the two mipmaps that most closely match the size of the pixel being textured and uses the GL_NEAREST criterion (the texture element nearest to the center of the pixel) to produce a texture value from each mipmap. The final texture value is a weighted average of those two values.
GL_LINEAR_MIPMAP_LINEAR: Chooses the two mipmaps that most closely match the size of the pixel being textured and uses the GL_LINEAR criterion (a weighted average of the four texture elements that are closest to the center of the pixel) to produce a texture value from each mipmap. The final texture value is a weighted average of those two values.

As more texture elements are sampled in the minification process, fewer aliasing artifacts will be apparent. While the GL_NEAREST and GL_LINEAR minification functions can be faster than the other four, they sample only one or four texture elements to determine the texture value of the pixel being rendered and can produce moire patterns or ragged transitions. The default value of GL_TEXTURE_MIN_FILTER is GL_NEAREST_MIPMAP_LINEAR.

GL_TEXTURE_MAG_FILTER

The texture magnification function is used when the pixel being textured maps to an area less than or equal to one texture element. It sets the texture magnification function to any of the following:

GL_NEAREST: Returns the value of the texture element that is nearest (in Manhattan distance) to the center of the pixel being textured.
GL_LINEAR: Returns the weighted average of the four texture elements that are closest to the center of the pixel being textured. These can include border texture elements, depending on the values of GL_TEXTURE_WRAP_S and GL_TEXTURE_WRAP_T, and on the exact mapping.

GL_NEAREST is generally faster than GL_LINEAR, but it can produce textured images with sharper edges because the transition between texture elements is not as smooth. The default value of GL_TEXTURE_MAG_FILTER is GL_LINEAR.

GL_TEXTURE_WRAP_S

Sets the wrap parameter for texture coordinate s to either GL_CLAMP or GL_REPEAT. GL_CLAMP causes s coordinates to be clamped to the range [0, 1] and is useful for preventing wrapping artifacts when mapping a single image onto an object. GL_REPEAT causes the integer part of the s coordinate to be ignored; the GL uses only the fractional part, thereby creating a repeating pattern. Border texture elements are accessed only if wrapping is set to GL_CLAMP. Initially, GL_TEXTURE_WRAP_S is set to GL_REPEAT.

GL_TEXTURE_WRAP_T

Sets the wrap parameter for texture coordinate t to either GL_CLAMP or GL_REPEAT. See the discussion under GL_TEXTURE_WRAP_S. Initially, GL_TEXTURE_WRAP_T is set to GL_REPEAT.

GL_TEXTURE_BORDER_COLOR

Sets a border color. params contains four values that comprise the RGBA color of the texture border. Integer color components are interpreted linearly such that the most positive integer maps to 1.0, and the most negative integer maps to -1.0. The values are clamped to the range [0, 1] when they are specified. Initially, the border color is (0, 0, 0, 0).

NOTES

Suppose texturing is enabled (by calling glEnable with argument GL_TEXTURE_1D, or GL_TEXTURE_2D) and GL_TEXTURE_MIN_FILTER is set to one of the functions that requires a mipmap. If either the dimensions of the texture images currently defined (with previous calls to glTexImage1D, or glTexImage2D) do not follow the proper sequence for mipmaps (described above), or there are fewer texture images defined than are needed, or the set of texture images have differing numbers of texture components, then it is as if texture mapping were disabled.

Linear filtering accesses the four nearest texture elements only in 2D textures. In 1D textures, linear filtering accesses the two nearest texture elements.

ERRORS

GL_INVALID_ENUM is generated when target or pname is not one of the accepted defined values, or when params should have a defined constant value (based on the value of pname) and does not.

GL_INVALID_OPERATION is generated if glTexParameter is executed between the execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED GETS

glGetTexParameter
glGetTexLevelParameter

glTexParameter

NAME

C SPECIFICATION

PARAMETERS

C SPECIFICATION

PARAMETERS

DESCRIPTION

NOTES

ERRORS

ASSOCIATED GETS

SEE ALSO