[sldev] Alpha Blending Settings Included in Texture Metadata

Dahlia Trimble dahliatrimble at gmail.com
Fri Jan 16 10:10:05 PST 2009


Depending on my setting for object mesh detail and my camera distance, I've
seen as many as 18 polygons on a single flat surface of a cube when viewed
in wireframe mode. Could these provide the necessary polygons to support the
"naive solution"?

On Fri, Jan 16, 2009 at 7:42 AM, Soft <soft at lindenlab.com> wrote:

> On Fri, Jan 16, 2009 at 8:50 AM, Carlo Wood <carlo at alinoe.com> wrote:
> > Would this solve the simple problems that I ran into?
> > For example:
> >
> > A house (with four walls), each wall is nearly opaque, but
> > they contain an alpha channel (for a few very small windows).
> > If you stand outside and rotate around a bit, you see walls
> > that are *behind* the wall that you're standing in front of
> > first, as if big parts of the wall that your standing in front
> > of don't exist.
> >
> > Or, I have a house where a part has floor-to-ceiling windows,
> > with a stripe (in the texture) every 2 meters or so to 'support'
> > the glass. If I put an Xmas tree in my livingroom that is
> > partly transparent, at some angles (most of the angles actually)
> > you see those stripes in full through the tree - as if the tree
> > wasn't there at all.
> >
> > I've never understood why this ordering bug exists; because
> > I can't think of a reason that makes it hard to get it right :p.
> > Can someone explain what the problem/reason is for these "features"?
>
> Yes, clip mapping would solve that problem at the expense of not being
> able to have partially transparent pixels.
>
> Efficiently sorting polys with alpha surfaces is actually an unsolved
> problem in computer graphics. To understand why, make two walls out of
> glass in SL and then arrange them so they intersect to form an X when
> seen from overhead. Now, look at the pair from the north, south, east
> or west side, and ask yourself which wall is in front. The answer is
> neither... or both.
>
> One naive solution is dividing objects in half wherever two planes
> intersect, but even a few dozen prims could degenerate into tens of
> millions of new surfaces with this approach. That X would turn two
> surfaces into four. Try a tic-tac-toe board, #, and four surfaces
> becomes twelve. Add just two more lines and the twelve becomes twenty
> four. Follow that trend and imagine what it does to memory and
> performance.
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