Frederic Bouvier [EMAIL PROTECTED] said:
I am using Blender to model an Airbus A320 and I would like to
make the windows transparent. I added alpha to my texture but
the problem is that there is a backface culling so I don't
see the interior of the plane through the window but what is
behind the plane. Should I have to add an object in the interior
with the normal reversed or is there a way to avoid backface
culling when texturing with the UV editor ?
Well I'm not a blender user, but I can probably help some. Eventually the
file needs to be ported to ac3d format which I am familiar with. Backface
culling IIRC from the open gl book refers to whether the backside of the
polygons are shown or not. Generally, for overall appearance, you don't want
the backside showing. It tends to screw up the shading. Most of the models
are only using it for flat polygons that are not part of a closed object. For
example, fuselage and wing surfaces don't show backfaces, but the flaps on the
A-4 model are just two dimensional rectangles that do show backfaces.
For aircraft you probably don't want transparent windows unless you are
building an interior for the model. If you are doing an interior, the
interior walls of the aircraft should sit just barely inside the exterior
walls, both with culled backfaces (the back side of the polygons face each
other and you would get flickering from z-fighting in addition to shading
problems). The interior and exterior can actually have identical geometry,
with the exterior polys facing out and the interior polys facing in, but that
usually doesn't look very good, because real walls have depth.
The only remaining issue is related to alpha sorting. At the very least the
polygons with transparent textures should be listed after the polygons that
you want to be able to see through the transparencies. For example if you
want to be able to see the interior from outside, then the exterior polygons
need to be after the interior ones. Same with wings, engines, horizontal
stabilizers, or anything that should show through the windows when viewed from
the side.
The reason for this is polys listed last are rendered first (reverse order).
It's like a stack of paper, so the last things in are on top of the pile and
get handled first. When the rendering processes a transparent polygon, it
won't remember objects that are already done and should show through, it only
knows how to cull what is covered by a polygon. It will only properly render
the polygons that follow behind transparencies (lower in the stack, first in
the ac3d file). I'm not sure what is required in Blender to accomplish this
sorting, but one option is to load the resulting ac3d file into an editor and
cut and paste the windows (or the fuselage with windows) down to the bottom of
the file.
BTW, the A320 model is looking really great. Can't wait to try it!
Best,
Jim
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