1991-2006
Silicon Graphics, Inc.
glTexParameter
3G

# 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 &af; log 2 &af; max &af; w h + 1 mipmap levels. The first mipmap level is the original texture, with dimensions w × h . Each subsequent mipmap level has dimensions max &af; 1 floor &af; w 2 i × max &af; 1 floor &af; 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 &af; log 2 &af; max &af; 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 &af; s , where frac &af; 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/.