HP b2600 Reference Manual page 218

G
glGetTexImage
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
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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,
glTexImage1D,
glTexImage2D,
glTexSubImage1D,
glTexSubImage2D,
glTexParameter
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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.
Chapter 7