glGetTexImage

NAME

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.

DESCRIPTION

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. Please see the reference pages glTexImage1D and glDrawPixels for a description of the acceptable values for the format and type parameters, respectively.

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, 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 zero or greater than ld max, where max is the returned value of GL_MAX_TEXTURE_SIZE.

GL_INVALID_OPERATION is generated if glGetTexImage is called between a call to glBegin and the corresponding call to 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