Re: texture alpha: premultiplied or not?

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

Roger Holmes wrote:

>>>> If you'd like to check it yourself, my test model is here:
>>>> <ftp://ftp.frameforge3d.com/misc/texture-argbT.3dmf.zip>.  It
>>>> contains 2
>>>> flat TriMeshes.  One is opaque and gray, another has an ARGB texture
>>>> whose background has alpha = 0, color = white, intended for use as
>>>> non-premultiplied.  The opaque TriMesh looks transparent in Classic,
>>>> which I don't understand at all.
>>> Er, maybe I'm being thick (as usual), but alpha==0 means transparent,
>>> alpha ==1.0 (or 0xFF) means opaque. What's to not understand?
>> The fact that this particular TriMesh had no transparency color
>> attribute and no texture, but looked translucent in Classic.
> 
> I have opened your model on my latest product using Quesa and on an  
> old G4 running OS9 and QuickDraw3D.
> 
> On the OLD machine with QuickDraw3D I see two surfaces, an opaque  
> grey one and in front of it a patterned one. With the interactive  
> renderer there is no transparency in either, but with the Microspot  
> Renderer the white parts of the patterned surface do not appear i.e.  
> they are transparent. Possibly the NuBus accelerator card and/or  
> LightWorks supported transparency.

That suggests that the Microspot renderer assumes non-premultiplied 
alpha.  You see, the texture in my test model was set up for 
non-premultiplied alpha... the transparent areas have alpha = 0 and 
color = white.  Therefore, a renderer that assumes non-premultiplied 
alpha should show it as completely tranparent, while a renderer that 
assumes premultiplied alpha should show it as translucent white.

> With Quesa I do not see the grey surface at all, it does not even  
> affect the bounding box, which it would if it were transparent. Maybe  
> my program deletes it for some reason, it does go through and deletes  
> certain objects just after reading a file for backward compatibility  
> with old version which left invisible junk in the file, like empty  
> groups.

My bad.  I had converted the model to ASCII form with Anatas, and 
somehow the presence of name elements prevented Quesa from loading it 
correctly.  I have re-uploaded the file without the name elements.

> The interactive renderer again shows the white pixels and  
> once again with the Microspot renderer they are transparent.

If you look closely, they are not completely opaque white in the 
interactive renderer, they are translucent.

> For this file it seems that the interactive renderer is compatible  
> across QD3D and Quesa, as is the Microspot renderer (not surprisingly  
> as its the same source code).

What's the same source code as what?
-- 
   James W. Walker, Innoventive Software LLC
   <http://www.frameforge3d.com/>