Re: Textures with transparency and glBlendFunc

[James W. W. <ja...@wr...>]

Frank Condello <dev...@ch...> wrote:

>This blendfunc is used to mimic QD3D's blending. To get a
>transparent pixel you need to have an alpha of zero as well as RGB
>values that add up to zero (premultiplied alpha). The idea is that
>you can get additive blending and normal alpha blending by changing
>the art, rather than doing it in code. This is one legacy I'd rather
>do without to be honest...
>
>This is the source of many problems, including what you observed
>above (i.e. you can't fade out an ARGB texture reasonably in code).
>It also causes the problems with ARGB textures in fog - not that fog
>works on _any_ transparent triangles at the moment ;)

OK, thanks for the explanation.

>IMHO, Quesa really needs an API that allows you to either explicitly
>set the blend function for the underling 3D API, or have blend
>function "aliases" that are translated for each API - kinda moot at
>this point since OpenGL is the only API involved. Either that, or a
>new type of Shader that does something similar - I'd love to see a
>"MultipassShader" object that can reference one or more texture
>shaders and apply them in layers with different blend functions and
>UV transformations - that would at least allow Quesa to compete with
>other scene graphs on the rendering side of things. The
>one-texture-per-triangle limitation grows stale real fast...

In the near term, would anyone object if I added a global option for 
blending function, such as an object property on the renderer?  If it 
is OK, should the property specify 4 values, namely the source and 
destination blend factors for textures and for vertex colors, or just 
a single boolean flag indicating whether texture alpha is 
premultiplied?
-- 
   James W. Walker, ScriptPerfection Enterprises, Inc.
   <http://www.write-brain.com/>