Re: texture alpha: premultiplied or not?

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

jo...@st... wrote:
> On May 10, 2007, at 23:08 UTC, James Walker wrote:
> 
>> Back in 2004, there was some discussion here about the fact that the 
>> Interactive Renderer essentially assumes that ARGB textures use 
>> premultiplied alpha, and questioning whether it ought to be an
>> option.  Yesterday I changed the OpenGL renderer to assume non
>> -premultiplied  alpha, but now I'm wondering if that was too rash.
> 
> I'd say so.  :)  First, you don't want to break existing software; and
> second, premultiplied alpha can be very useful in making (for example)
> "glow" effects.
> 
>> The QD3D manual doesn't seem to give any hint whether alpha should be
>> premultiplied or not.  I believe there was a claim in 2004 that 
>> QuickDraw 3D did use premultiplied alpha, but I can't confirm that 
>> today.  When I loaded an ARGB texture test in Classic Geom Test, the 
>> result doesn't agree with either of our renderers.
> 
> Hmm.  That's surprising.  It's been a while, of course, but I was sure
> that textures have worked the same way in Quesa as in QD3D from the
> beginning.  Certainly neither I nor my artist partner (Jeff Quan) have
> ever noticed the difference -- and he's even written a tutorial on the
> proper use of premultiplied alpha.  That was for Meshwork users, and
> was years ago, so I'd think he/we would have tested it under QD3D.

OK...

>> Anyway, in the previous discussion, it was suggested that the 
>> premultipled flag could be set with an object property on the
>> renderer,  or a new style object.  It occurs to me now that it might
>> make sense to  set it at the level of individual textures... maybe a
>> new pixel type  constant like kQ3PixelTypeARGB32Premultiplied, maybe
>> an object property  or custom element.
> 
> That'd be OK, but for the sort of thing I've done, it would be
> overkill.  An object property on the renderer would be fine, and a
> style object would be even finer.  I don't see a need to be more
> granular than that.  Most objects either need to glow, or they don't --
> and it's certainly not hard to premultiply some of the textures if
> needed.

Maybe even that's overkill.  Maybe I should just document that 
premultiplied alpha is expected, say next to kQ3PixelTypeARGB32 in the 
header docs.  As for the OpenGL renderer, I didn't like the switching 
back and forth between glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) 
and glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA), but I could go the 
other way: use premultiplied alpha even for vertex colors, and always 
use glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA).

-- 
   James W. Walker, Innoventive Software LLC
   <http://www.frameforge3d.com/>