[Algorithms] screenspace xy -> uv

[Roy J. <sag...@st...>]

Hi guys,

I'm using dx8 and I'm trying to render an object which has the
texture-coordinates setup in such a way that the texture-coordinates of the
object correspond to the screenspace x/y coordinates. So basically, if the
object would be a sphere and I was rendering on an empty screen, you'd see
the sphere as an (ahem) 'circular viewing hole' to the texture. A bit like
when I would use a stencil-buffer to only allow pixels to be set where the
sphere was. (Btw, the reason I'm doing all this is because I want to take a
render-to-texture surface and map it on to the object.)

Um, anyhow, I'm having trouble doing this. The problem is perspective
correction. When I have a simple square with constant z facing the camera
everything's ok, but when I take one of the square's vertices and change the
z the texture looks warped.

How do I fix this? In dx8 I can't turn off perspective correction anymore,
so
is there a way to do it nicely? Can it be done using vertex-shaders?

Thanks in advance for any help,
Roy.