When render targets are used, they output to the render target inverted
due to the way that opengl works. This fixes that issue by inverting
the projection matrix so that it renders the image upside down and
inverting the front face from counterclockwise to clockwise.
Just for a quick background: D3D's fmod intrinsic is very imprecise.
Naturally floating points aren't precise at all, and when the numbers
you're dealing with become very large, it can often be off by 0.1 or
more.
However, apparently 0.1 isn't enough of an offset to ensure a proper
value when using the fmod intrinsic and then flooring the value. 0.2
seems to fix the issue and make the image display properly.
jp9000
BtbN