RE: [GD-General] software for artwork

SourceForge Headquarters 225 Broadway Suite 1600 San Diego, CA 92101 +1 (858) 422-6466

The premultiply is pretty simple.

The standard blend equation is this:

result = texture.C * texture.A + fb.C * (1-texture.A)

...which means that when texture.A is close to zero, the values of texture.C
really aren't very exciting - they produce only small changes. Perceptually,
the only thing that matters is (texture.C*texture.A). So do the multiply:

texture2.C = texture.C * texture.A
texture2.A = texture.A

Then palettise.

Many (myself included) then say you should keep it in this form right to the
actual blend, so you do a ONE:INVSRCALPHA blend instead of the more normal
SRCALPHA:INVSRCALPHA blend. But the PS2 can't actually do this blend (doh!),
so you need to convert back to the original form once you've palettised and
do:

texture3.C = texture2.C / texture2.A
texture3.A = texture2.A

(cope with the singularities at texture2.A==0 by making texture3.C anything
you like, usually black).

...and there you go. The pallising was done in a much more
perceptually-significant colourspace, so it should be much higher quality.

(note that the PS2 can do ONE:SRCALPHA blending, so you could just invert
the alpha channel, but then the rest of the blend pipeline is a pain if you
want to fade stuff in and out and so on).

Tom Forsyth - Muckyfoot bloke and Microsoft MVP.

This email is the product of your deranged imagination,
and does not in any way imply existence of the author.

> -----Original Message-----
> From: Lionel Fumery [mailto:li...@mi...]
> Sent: 13 November 2002 22:13
> To: gam...@li...
> Subject: Re: [GD-General] software for artwork
> 
> 
> Hello,
> 
> I have currently exactly this kind of stuff to write : 
> something to convert
> 32bits pictures into 8bits with palette ARGB for the 
> PlayStation 2. I never
> really though to this question, so I though it would be easy 
> to do (simple
> octree in 4D space, or something like that).
> 
> Could you give me more details about the alpha channel 
> multiplication ?
> A quick search with Google shows me a lot of answers about 
> the Kohonon nets.
> But is there somewhere a not-too-bad free lib with source 
> that could help me
> ? (in fact, I didn't give a lot of time in my schedule to do 
> that task :))
> 
> Thank you for any advice,
> 
> Lionel.
> 
> ----- Original Message -----
> From: "Tom Forsyth" <to...@mu...>
> To: <gam...@li...>
> Sent: Wednesday, November 13, 2002 3:30 PM
> Subject: RE: [GD-General] software for artwork
> 
> 
> > Yep - we premultiply, palettise, and then un-premultiply 
> again. Works
> well.
> > We've also spent quite a lot of effort finding good palettisers.
> > Kohonon-network based ones are good, and weused that for 
> the last project.
> > The new one is a mad mix of various perceptual techniques 
> and stuff (we
> have
> > a chap fascinated by this stuff), works in an arbitrary number of
> > dimensions, but is very very slow. We plan to use the old 
> kohonen one for
> > quick previews, and then overnight crunch away at the 
> textures with the
> > slow-but-superb one.
> >
> > But we never found any decent out-of-the-box ones.
> >
> >
> > Tom Forsyth - Muckyfoot bloke and Microsoft MVP.
> >
> > This email is the product of your deranged imagination,
> > and does not in any way imply existence of the author.
> >
> > > -----Original Message-----
> > > From: Javier Arevalo [mailto:ja...@py...]
> > > Sent: 13 November 2002 09:11
> > > To: gam...@li...
> > > Subject: Re: [GD-General] software for artwork
> > >
> > >
> > > Mickael Pointier <mpo...@ed...> wrote:
> > >
> > > > Standard quantization algorithms create artefacts, but thats not
> > > > visually to disturbing when applied to color components,
> > > surprisingly
> > > > if you extend these algorithm to 4d, the final result is really,
> > > > really crappy...
> > > >
> > > > I think the reasons are multiples, the first beeing 
> that when A tend
> > > > toward 0, you do not really care about the true values of RGB
> > > > components (they are almost invisible at this point), 
> so a lot of
> > > > colors are used in semi translucent tints, that could 
> have been used
> > > > in nearly opaque areas.
> > >
> > > Yeah that's one of the places where premultiplied alpha
> > > textures help...
> > >
> > >   Javier Arevalo
> > >   Pyro Studios
> >
> >
> >
> >
> > -------------------------------------------------------
> > This sf.net email is sponsored by: Are you worried about
> > your web server security? Click here for a FREE Thawte
> > Apache SSL Guide and answer your Apache SSL security
> > needs: http://www.gothawte.com/rd523.html
> > _______________________________________________
> > Gamedevlists-general mailing list
> > Gam...@li...
> > https://lists.sourceforge.net/lists/listinfo/gamedevlists-general
> > Archives:
> > http://sourceforge.net/mailarchive/forum.php?forum_id=557
> >
> 
> 
> 
> -------------------------------------------------------
> This sf.net email is sponsored by: Are you worried about 
> your web server security? Click here for a FREE Thawte 
> Apache SSL Guide and answer your Apache SSL security 
> needs: http://www.gothawte.com/rd523.html
> _______________________________________________
> Gamedevlists-general mailing list
> Gam...@li...
> https://lists.sourceforge.net/lists/listinfo/gamedevlists-general
> Archives:
> http://sourceforge.net/mailarchive/forum.php?forum_id=557
> 