GLGETTEXIMAGE() MachTen Programmer’s Manual GLGETTEXIMAGE()

NAME
glGetTexImage - return a texture image

C SPECIFICATION
void glGetTexImage( GLenum target,
GLint level,
GLenum format,
GLenum type,
GLvoid *pixels )

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PARAMETERS
target Specifies which texture is to be obtained.
GL_TEXTURE_1D and GL_TEXTURE_2D are accepted.

level Specifies the level-of-detail number of the
desired image. Level 0 is the base image level.
Level $n$ is the $n$th mipmap reduction image.

format Specifies a pixel format for the returned data.
The supported formats are GL_RED, GL_GREEN,
GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, GL_LUMINANCE,
and GL_LUMINANCE_ALPHA.

type Specifies a pixel type for the returned data. The
supported types are GL_UNSIGNED_BYTE, GL_BYTE,
GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT,
GL_INT, and GL_FLOAT.

pixels Returns the texture image. Should be a pointer to
an array of the type specified by type.

DESCRIPTION
glGetTexImage returns a texture image into pixels. target
specifies whether the desired texture image is one speci-
fied by glTexImage1D (GL_TEXTURE_1D) or by glTexImage2D
(GL_TEXTURE_2D). level specifies the level-of-detail num-
ber of the desired image. format and type specify the
format and type of the desired image array. See the ref-
erence pages glTexImage1D and glDrawPixels for a descrip-
tion of the acceptable values for the format and type
parameters, respectively.

To understand the operation of glGetTexImage, consider the
selected internal four-component texture image to be an
RGBA color buffer the size of the image. The semantics of
glGetTexImage are then identical to those of glReadPixels
called with the same format and type, with x and y set to
0, width set to the width of the texture image (including
border if one was specified), and height set to 1 for 1D
images, or to the height of the texture image (including
border if one was specified) for 2D images. Because the
internal texture image is an RGBA image, pixel formats
GL_COLOR_INDEX, GL_STENCIL_INDEX, and GL_DEPTH_COMPONENT
are not accepted, and pixel type GL_BITMAP is not
accepted.

If the selected texture image does not contain four
components, the following mappings are applied. Single-
component textures are treated as RGBA buffers with red
set to the single-component value, green set to 0, blue
set to 0, and alpha set to 1. Two-component textures are
treated as RGBA buffers with red set to the value of com-
ponent zero, alpha set to the value of component one, and
green and blue set to 0. Finally, three-component tex-
tures are treated as RGBA buffers with red set to compo-
nent zero, green set to component one, blue set to compo-
nent two, and alpha set to 1.

To determine the required size of pixels, use glGet-
TexLevelParameter to determine the dimensions of the
internal texture image, then scale the required number of
pixels by the storage required for each pixel, based on
format and type. Be sure to take the pixel storage param-
eters into account, especially GL_PACK_ALIGNMENT.

NOTES
If an error is generated, no change is made to the con-
tents of pixels.

ERRORS
GL_INVALID_ENUM is generated if target, format, or type is
not an accepted value.

GL_INVALID_VALUE is generated if level is less than 0.

GL_INVALID_VALUE may be generated if level is greater than
$log sub 2 max$, where $max$ is the returned value of
GL_MAX_TEXTURE_SIZE.

GL_INVALID_OPERATION is generated if glGetTexImage is exe-
cuted between the execution of glBegin and the correspond-
ing execution of glEnd.

ASSOCIATED GETS
glGetTexLevelParameter with argument GL_TEXTURE_WIDTH
glGetTexLevelParameter with argument GL_TEXTURE_HEIGHT
glGetTexLevelParameter with argument GL_TEXTURE_BORDER
glGetTexLevelParameter with argument GL_TEXTURE_COMPONENTS
glGet with arguments GL_PACK_ALIGNMENT and others

SEE ALSO
glDrawPixels, glReadPixels, glTexEnv, glTexGen, glTexIm-
age1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D,
glTexParameter

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