Re: texture alpha: premultiplied or not?

[<jo...@st...>]

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.

> 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.

Best,
- Joe

--
Joe Strout -- jo...@st...
Strout Custom Solutions