glCopyTexSubImage2D

glCopyTexSubImage2D: man3/glCopyTexSubImage2D.xml

glCopyTexSubImage2D replaces a rectangular portion of a two-dimensional texture image or cube-map 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 glReadPixels 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, or parameters of the specified texture array or to texel values outside the specified subregion.

glPixelStore modes affect texture images.