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.