On Tuesday 01 February 2005 16:23, Timothy Miller wrote:
> Nicolai Haehnle wrote:
> > There's really nothing wrong with perspective correction in the software
> > model as far as I can see, the problem lies with Z as I explained in the
> > other thread ("Depth buffer"). Nobody has taken up the challenge of
> > figuring out how to do orthographic projection correctly with the
current
> > software model (I'm saying it can't be done, because where OpenGL
specifies
> > two independent variables, the software model only has one).
>
> Are you talking about Z and W or are you talking about W and Q?
>
> Z and W can be easily interconverted. Plus we can do correct
> perspective correction (is this what you're calling 'orthographic'?)
> It's projective textures that we can't do (because we don't have Q).Orthographic means that there is no perspective foreshortening (sp?). Everything that is parallel in world coordinates will also be parallel on the screen, and all lines that have the same length in world coordinates will also have the same length in screen coordinates. This mode of projection is essential for everything related to 2D graphics, and it is often a useful mode to work in in modelling programs - think of building the model of a house which has many parallel planes (unless you're Hundertwasser, that is). Since parallels and lengths are retained in the projection, there is no perspective correction going on (actually, the perspective correction just vanishes because both the OpenGL W and the W in the software model end up as 1) - but you obviously still want to use a depth buffer. This is where your assumption that Z and W can be easily interconverted using a fixed formula simply breaks down. cu, Nicolai
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