[sldev] Sculpties: Vacuum-Forming, Skittering, Zone Linking

Dale Mahalko dmahalko at gmail.com
Fri Sep 28 09:27:42 PDT 2007


On 9/28/07, Kent Quirk (Q Linden) <q at lindenlab.com> wrote:
>
> It's just too easy to
> create whacky results by dragging vertices across triangle boundaries
> and the like. That way lies madness.


Where you see madness, I see optimization and untapped power. :-)


One of my goals is to make a single-prim jack-o-lantern sculptie, complete
with a hole in the top, AND the face cutouts in the front.

Tunnels and linked holes are technically impossible in a sculptie but can be
made possible with creative exploitation of texture transparency and the
fact that surface normals are completely invisible if the observer is able
to see the back side of the normal. Here are my example side-views of two
such designs:

http://img519.imageshack.us/my.php?image=abnormalpumpkinsculptierk6.png

As shown in my left figure, this is started by hollowing out a sculptie
pumpkin from the top until a large open space is formed in the interior.
Next create several indentations in the outside-front of the sphere, with
the back end of each indentation flared out and flattened.

Apply a transparent alpha texture at the back of the facehole cutouts (the
light green lines) and a person looking into the face will see through the
transparency, through the back-facing normal of the inside-front surface
immediately behind the faceholes, and all the way to the rear of the pumpkin
where the interior normal is facing the correct direction to be seen by the
observer.

When viewing the inside of the pumpkin, the faceholes will not be visible
because the interior surface normal on the inside-front will hide them from
view . This is handled by making indentations in the interior surface that
align with the faceholes on the outside, and again having a transparency
texture at back of these indentations so the viewer can see through to the
outside the pumpkin



If transparency doesn't work for some reason, it is also possible to make
this work by deeply hollowing out each indentation into the pumpkin, to be
bigger than the actual pumpkin's inside surface yet hidden by the outside
surface, as shown on the right.

The key to making this particular nontransparency illusion work is that the
faceholes need to be long tunnels that prevent the observer from seeing
upwards into the area wihere the facehole normals will block the view
through top opening in the pumpkin, as indicated by the light blue line.



It is potentially possible to make a "spooky" single-prim jack o' lantern
that has one facial expression visible from the outside, and a totally
different one seen from inside, by exploiting the ability to see through the
backside of normals facing the "wrong" way from the viewer..

- Scalar Tardis, aka Dale Mahalko
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