Name
glTexParameter — set texture parameters
C Specification
void glTexParameterf( |
GLenum target, |
GLenum pname, | |
GLfloat stop ) ; |
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
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/.