Name

glTexParameter — 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 of the active texture unit, which must be either GL_TEXTURE_2D or GL_TEXTURE_CUBE_MAP.

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 of the active texture unit, which must be either GL_TEXTURE_2D or GL_TEXTURE_CUBE_MAP.

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, or GL_TEXTURE_WRAP_T.

params

Specifies a pointer to an array where the value of pname is 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 two-dimensional or cube-mapped 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 of the active texture unit, either GL_TEXTURE_2D or GL_TEXTURE_CUBE_MAP. 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 w × h , there are floor ⁡ log 2 ⁡ max ⁡ w h + 1 mipmap levels. The first mipmap level is the original texture, with dimensions w × h . Each subsequent mipmap level has dimensions max ⁡ 1 floor ⁡ w 2 i × max ⁡ 1 floor ⁡ h 2 i , where i is the mipmap level, until the final mipmap is reached, which has dimension 1 × 1 .

To define the mipmap levels, call glTexImage2D, glCompressedTexImage2D, or glCopyTexImage2D with the level argument indicating the order of the mipmaps. Level 0 is the original texture; level floor ⁡ log 2 ⁡ max ⁡ w h 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.

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 initial 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 either GL_NEAREST or GL_LINEAR (see below). 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 initial value of GL_TEXTURE_MAG_FILTER is GL_LINEAR.

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.

GL_TEXTURE_WRAP_S

Sets the wrap parameter for texture coordinate s to either GL_CLAMP_TO_EDGE, GL_MIRRORED_REPEAT, or GL_REPEAT. GL_CLAMP_TO_EDGE causes s coordinates to be clamped to the range 1 2N 1 - 1 2N , where N is the size of the texture in the direction of clamping. 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. GL_MIRRORED_REPEAT causes the s coordinate to be set to the fractional part of the texture coordinate if the integer part of s is even; if the integer part of s is odd, then the s texture coordinate is set to 1 - frac ⁡ s , where frac ⁡ s represents the fractional part of s. 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_TO_EDGE, GL_MIRRORED_REPEAT, or GL_REPEAT. See the discussion under GL_TEXTURE_WRAP_S. Initially, GL_TEXTURE_WRAP_T is set to GL_REPEAT.

Notes

Suppose that a texture is accessed from a fragment shader or vertex shader and has set GL_TEXTURE_MIN_FILTER to one of the functions that requires mipmaps. If either the dimensions of the texture images currently defined (with previous calls to glTexImage2D, glCompressedTexImage2D, or glCopyTexImage2D) 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 were defined with different formats or types, then the texture image unit will return (R, G, B, A) = (0, 0, 0, 1).

Similarly, if the width or height of a texture image are not powers of two and either the GL_TEXTURE_MIN_FILTER is set to one of the functions that requires mipmaps or the GL_TEXTURE_WRAP_S or GL_TEXTURE_WRAP_T is not set to GL_CLAMP_TO_EDGE, then the texture image unit will return (R, G, B, A) = (0, 0, 0, 1).

glTexParameter specifies the texture parameters for the texture bound to the active texture unit, specified by calling glActiveTexture.

Errors

GL_INVALID_ENUM is generated if target or pname is not one of the accepted defined values.

GL_INVALID_ENUM is generated if params should have a defined symbolic constant value (based on the value of pname) and does not.

Associated Gets

glGetTexParameter

See Also

glActiveTexture, glBindTexture, glCopyTexImage2D, glCopyTexSubImage2D, glPixelStorei, glTexImage2D, glTexSubImage2D

Copyright

Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.