Re: The big divide - graphics contexts and window servers

[Jeff Teunissen]

"Philippe C.D. Robert" wrote:
> 
> On Mon, 18 Mar 2002, Jeff Teunissen wrote:
> > Pete French wrote:
> > > > OpenGL is designed for 3D, and it's not happy with 2D. Most GL
> > > > implementations don't accelerate 2D operations beyond throwing a
> > > > quad on the screen, and some even leave it incomplete.
> > >
> > > Are you sure about this ? I thought there wernt concepts of '2D' and
> > > '3D' in OpenGL - you just provide a transformation matrix, and if
> > > that happens to be 'flat' so that the points get mapped onto one
> > > plane then you have 2D and the library knowns nothing about it.
> >
> > Of course the library knows everything about it, the library is what
> > has to draw it!  The difference is there are fast paths and slow paths
> > in any GL pipeline, and "2D" stuff is usually slow-pathed because
> > there's usually not enough of it to matter.
> 
> What do you mean by 2D stuff exactly? Could you give an example maybe?
> 
> My guess is that 2D does not exist at all, there is no special path for
> 2D rendering in commodity drivers. Thus 2D is as slow or fast as 3D. But
> of course this is just a guess, I did not looked at sources to proof it.

I'll give a simple example.

Say I have two quads on the screen, one taking up the whole screen (the
background) and another in front of it (the window). The background has alpha
1.0, the window has alpha 0.7. Both are textured; the background has a typical
desktop image on it, and the window has a texture containing text on it.

In some systems, the window will look like it has 1.0 alpha and will keep
doing that unless alpha goes below 0.5, at which point it disappears. In
others, it will actually work, but it'll be very slow. If I add some
transformation to the window so that it is no longer billboarded, the speed
comes back.

"Commodity drivers" includes a LOT of stuff, including the old 3Dfx Voodoo
cards, which eventually did get a passable GL (as opposed to their MiniGL). It
was never good, though.

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