When you render to a texture the origin is the origin.
That is, the origin in window space is mapped to the origin in texture space, and the positive X and Y directions are aligned with the positive S and T directions.
There is a single set of (texture coordinates, window coordinates), or if you want to take a step back, a single set of (texture coordinates, normalized device coordinates) to map the texture space origin to the window space origin.
From: Florian Bösch [mailto:email@example.com]
On Fri, Dec 7, 2012 at 4:31 PM, Georg Kolling <Georg.Kolling@imgtec.com> wrote:
When you render to a texture the origin is bottm/left not top/left. If you're serious about origin preservation such as not to end up with cases where there is no right UV set anymore or you render things upside down variably, you upload with origin bottom/left not top/left. Alternatively you fix glViewport, glScissors, FBOs, to have origin top/left such that you'll not have to render things upside down and can apply one set of UV coords regardless.