On 09/15/2011 03:59 PM, Mik Wells wrote:
Hi,
It isn't the window system (e.g. Microsoft Windows) which uses the Y coordinate
as the vertical axis of the screen. It's OpenGL. (On my windowing system Y is
down).
That is accounted for in the viewport transform, not inside OpenGL itself.
Worrying about what OpenGL implicitly calls "up" is not useful
It may not be useful at the app level, but it's essentail if you're trying to
calculate the up-vector.
The up-vector is a relative concept. It doesn't mean anything on its own.
Invertible or not, it's still just another matrix
The projection matrix has a very different role to the ModelView matrix, which
is one reason why it has its own matrix stack.
That doesn't mean it isn't just another matrix. It's certainly useful
and advisable to separate them conceptually, but nothing forces you to
do so.
In OpenGL 3.1+ (core profile) there isn't even a glMatrixMode() anymore
(or a glMatrix-anything for that matter). You just do whatever you want
in your vertex shaders. If you wanted to, you could calculate a
separate projection for each vertex and then move them all around in
clip space. It would be ugly, sure, but there's nothing stopping you.
--"J"
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