Re: [isugamedev-devel] Alpha blending

[Lou H. <lh...@ia...>]

Ok, but how do I sort the polygons if OpenGL draws them immediately when I
call glVertex?  The only way for me to sort polygons from back to front is
if I know the transformed coordinates, but I don't know that until I call
glVertex.  Unless there's some other feature of OpenGL that I still don't
know about (which there probably is).

- Lou

----- Original Message -----
From: "Chad Austin" <ae...@ae...>
To: "Lou Herard" <lh...@ia...>
Cc: "gdd" <isu...@li...>
Sent: Wednesday, April 03, 2002 2:42 AM
Subject: Re: [isugamedev-devel] Alpha blending


> Your problem is that, with alpha blending, you can't use the z-buffer.
:(  When
> you draw your first polygon, you end up putting initial values in the
z-buffer.
> When you try to draw the polygons behind the first polygon, they don't get
drawn
> at all because the z-buffer says those pixels don't need to be touched
anymore.
>
> The solution to your problem kinda sucks...  when you draw alpha-blended
> polygons, you have to disable the z-buffer and manually sort your polygons
from
> back to front.  After drawing every solid polygon in the scene, you draw
the
> alpha-blended ones from front to back.
>
> A simple way to do this with your cube is just take the midpoint of each
face
> and sort by its distance from the viewer (or z value, if you're in view
> coordinates).
>
> If you have static, non-intersecting shapes, you can build a BSP tree.
BSP
> trees let you quickly sort polygons from front-to-back, which makes
drawing the
> translucent polys a simple tree traversal.  You can pick up a book on
graphics
> (Realtime Rendering is the one I recommend) for more information.
>
> >     Ok, now I'm kinda fooling around with transparency (alpha blending)
and I
> > can't get past this nagging characteristic of my program.  Maybe someone
knows
> > how to fix this:  I started with a box with an open top and open bottom.
All
> > of the four sides have alpha values of 0.5.  When I rotated that box
rotates,
> > though, the first polygon drawn is always opaque.  So, I modified the
box so
> > that there is a bottom side (also with an alpha value of 0.5), and now
all
> > four of the walls are translucent, but the bottom is completely opaque,
> > despite the alpha values that I specified.  Is this an OpenGL thing, or
am I
> > missing something?  I attached my code so you can see exactly what's
> > happening. (The program actually draws two boxes, but I'm only worried
about
> > the one that has five sides)
> >
> > - Lou
> >
> >                         Name: Create_Window.cpp
> >    Create_Window.cpp    Type: Plain Text (text/plain)
> >                     Encoding: quoted-printable
>