Re: texture alpha: premultiplied or not?

[James W. W. <os...@jw...>]

On May 11, 2007, at 4:27 AM, Roger Holmes wrote:

>
> On 11 May, 2007, at 00:08, 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.
>>
>> 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.
>
> You mean the interactive renderers in Quesa and QuckDraw3D are
> different?

Right.  At least on the one machine where I made the comparison.

>>
>> 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.

>
>>
>> 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.
>
> Sounds like a good idea to me, data can be imported into my programs
> so if you are going to support two types of alpha data, then the type
> needs to be bound to the data, how else could I render an image with
> a mixture of pre-multiplied and non pre-multiplied textures on many
> objects, pixmap markers, foreground and background images.
>
> When was the concept of pre-multiplied alpha invented? After
> QuickDraw3D?

I think the idea of premultiplied alpha goes back to about 1984.   
When did QD3D come out, about 1994?