[sldev] Alpha Blending Settings Included in Texture Metadata

[Soft]

Afraid not. Those are generated later in the rendering pipeline to
facilitate things like vertex-based lighting. To be useful for
resolving intersections, all the faces would have to be pulled out and
sorted the way objects are sorted, and a lot of work would need to be
done to move the intersections to match intersects.

If you want to know more, google for "deph sorting translucent
polygons" and you'll find dozens of papers and probably a thousand
griping game developers. Also this page that describes the problem
pretty handily. The viewer implements the first few things on the page
exactly. http://www.sjbaker.org/steve/omniv/alpha_sorting.html

The original proposal in this thread for adding clip mapping to faces
makes a lot of sense. That doesn't suffer from the sorting problems
that ranged alpha does, and would probably suit the most common uses
for partial transparency anyway.

On Fri, Jan 16, 2009 at 12:10 PM, Dahlia Trimble
<dahliatrimble at gmail.com> wrote:
> 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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