glGetTexImage

glGetTexImage: return a texture image.

C Specification

void 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: Specifies 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: 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.

glGetTexImage returns a texture image into pixels. target specifies whether the desired texture image is one specified by glTexImage1D (GL_TEXTURE_1D) or by glTexImage2D (GL_TEXTURE_2D). level specifies the level-of-detail number of the desired image. format and type specify the format and type of the desired image array. See the reference pages glTexImage1D and glDrawPixels for a description 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 component zero, alpha set to the value of component one, and green and blue set to 0. 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 1.

To determine the required size of pixels, use glGetTexLevelParameter 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 parameters into account, especially GL_PACK_ALIGNMENT.

Notes

If an error is generated, no change is made to the contents 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 executed between the execution of glBegin and the corresponding 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