Re: [isugamedev-devel] Alpha blending

[Chad A. <ae...@ae...>]

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
> 
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