glCopyTexSubImage2D -	copy a two-dimensional texture
subimage

glCopyTexSubImage2D
	glCopyTexSubImage2D(target, level, xoffset, yoffset, x, y, width, height) -> None

void glCopyTexSubImage2D( GLenum target,
                          GLint level,
                          GLint xoffset,
                          GLint yoffset,
                          GLint x,
                          GLint y,
                          GLsizei width,
                          GLsizei height )

target   Specifies the target	texture.  Must be
         GL_TEXTURE_2D

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

xoffset  Specifies a texel offset in the x direction within
         the texture array.

yoffset  Specifies a texel offset in the y direction within
         the texture array.

x, y	   Specify the window coordinates of the lower left
         corner of the rectangular region of pixels to be
         copied.

width	   Specifies the width of the texture subimage.

height   Specifies the height	of the texture subimage.

glCopyTexSubImage2D replaces a rectangular portion of	a
two-dimensional texture image	with pixels from the current
GL_READ_BUFFER (rather than from main	memory,	as is the case
for glTexSubImage2D).

The screen-aligned pixel rectangle with lower	left corner at
(x, y) and with width	width and height height	replaces the
portion of the texture array with x indices xoffset through
xoffset + width - 1, inclusive, and y	indices	yoffset
through yoffset + height - 1,	inclusive, at the mipmap level
specified by level.

The pixels in	the rectangle are processed exactly as if
glCopyPixels had been	called,	but the	process	stops just
before final conversion.  At this point, all pixel component
values are clamped to	the range [0, 1] and then converted to
the texture's	internal format	for storage in the texel
array.

The destination rectangle in the texture array may not
include any texels outside the texture array as it was
originally specified.	 It is not an error to specify a
subtexture with zero width or	height,	but such a
specification	has no effect.

If any of the	pixels within the specified rectangle of the
current GL_READ_BUFFER are outside the read window
associated with the current rendering	context, then the
values obtained for those pixels are undefined.

No change is made to the internalformat, width, height, or
border parameters of the specified texture array or to texel
values outside the specified subregion.

glCopyTexSubImage2D is available only	if the GL version is
1.1 or greater.

Texturing has	no effect in color index mode.

glPixelStore and glPixelTransfer modes affect	texture	images
in exactly the way they affect glDrawPixels.

GL_INVALID_ENUM is generated if target is not	GL_TEXTURE_2D.

GL_INVALID_OPERATION is generated if the texture array has
not been defined by a	previous glTexImage2D or
glCopyTexImage2D operation.

GL_INVALID_VALUE is generated	if level is less than 0.

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

GL_INVALID_VALUE is generated	if x <	-b or if y <  -b,
where	b is the border	width of the texture array.

GL_INVALID_VALUE is generated	if xoffset < -b,
(xoffset + width) > (w - b), yoffset <  -b, or
(yoffset + height) > (h - b),	where w	is the
GL_TEXTURE_WIDTH, h is the  GL_TEXTURE_HEIGHT, and b is the
GL_TEXTURE_BORDER of the texture image being modified.  Note
that w and h include twice the border	width.
GL_INVALID_OPERATION is generated if glCopyTexSubImage2D is
executed between the execution of glBegin and	the
corresponding	execution of glEnd.

glGetTexImage
glIsEnabled with argument GL_TEXTURE_2D

glCopyPixels,	glCopyTexImage1D, glCopyTexImage2D,
glCopyTexSubImage1D, glPixelStore, glPixelTransfer,
glTexEnv, glTexGen, glTexImage1D, glTexImage2D,
glTexParameter, glTexSubImage1D, glTexSubImage2D