Re: Textures with transparency and glBlendFunc

[Frank C. <dev...@ch...>]

On 14-Sep-04, at 2:32 PM, James W. Walker wrote:

> I was puzzled to observe that an object with an ARGB texture did not 
> become invisible, except for specular highlights, when the alpha 
> channel was zero.  Apparently this is because Quesa uses 
> glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) for transparent textures. 
> Is there some good reason for this?

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 ;)

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...

Sorry for the rant :)
Frank.