glGetTexImage function

Article
12/11/2020

The glGetTexImage function returns a texture image.

Syntax

void WINAPI glGetTexImage(
   GLenum target,
   GLint  level,
   GLenum format,
   GLenum type,
   GLvoid *pixels
);

Parameters

target: Specifies which texture is to be obtained. GL_TEXTURE_1D and GL_TEXTURE_2D are accepted.
level: The level-of-detail number of the desired image. Level 0 is the base image level. Level n is the nth mipmap reduction image.
format: 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, GL_BGR_EXT, GL_BGRA_EXT, and GL_LUMINANCE_ALPHA.
type: 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.

Return value

This function does not return a value.

Error codes

The following error codes can be retrieved by the glGetError function.

Name	Meaning
GL_INVALID_ENUM	target, format, or type was not an accepted value.
GL_INVALID_VALUE	level is less than zero or greater than log2 (max), where max is the returned value of GL_MAX_TEXTURE_SIZE.
GL_INVALID_OPERATION	The function was called between a call to glBegin and the corresponding call to glEnd .

Remarks

The glGetTexImage function returns a texture image into pixels. The target parameter specifies whether the desired texture image is one specified by glTexImage1D(GL_TEXTURE_1D) or by glTexImage2D(GL_TEXTURE_2D). The level parameter specifies the level-of-detail number of the desired image. The format and type parameters specify the format and type of the desired image array. For a description of the acceptable values for the format and type parameters, respectively, see glTexImage1D and glDrawPixels.

Operation of glGetTexImage is best understood by considering 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 zero, width set to the width of the texture image (including border if one was specified), and height set to one for 1-D images, or to the height of the texture image (including border, if one was specified) for 2-D 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, and green, blue, and alpha set to zero.

Two-component textures are treated as RGBA buffers, with red set to the value of component zero, alpha set to the value of component one, and green and blue set to zero. Finally, three-component textures are treated as RGBA buffers with red set to component zero, green set to component one, blue set to component two, and alpha set to zero.

To determine the required size of pixels, use glGetTexLevelParameter to ascertain the dimensions of the internal texture image, and 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 parameters into account, especially GL_PACK_ALIGNMENT.

If an error is generated, no change is made to the contents of pixels.

The following functions retrieve information related to glGetTexImage:

glGet with argument GL_PACK_ALIGNMENT and others

glGetTexLevelParameter with argument GL_TEXTURE_WIDTH

glGetTexLevelParameter with argument GL_TEXTURE_HEIGHT

glGetTexLevelParameter with argument GL_TEXTURE_BORDER

glGetTexLevelParameter with argument GL_TEXTURE_COMPONENTS

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Header	Gl.h
Library	Opengl32.lib
DLL	Opengl32.dll