On Thu, Mar 03, 2005 at 08:10:33PM +0100, Roland Scheidegger wrote: > Ville Syrjälä wrote: > >>>Like I said before only the RGB components are blended. You can > >>>choose to write 0, 1, As, 1-As, Ad or 1-Ad to the destination > >>>alpha ([EMAIL PROTECTED] register). Currently the > >>>driver seems to write 0. It would probably be a better idea to > >>>write 1 instead. > >> > >>Sorry, but I just can't see that in the driver. And there's no > >>ALPHA_DST_SEL bit, at least not in the mach64 reg file I have... > > > > > >I was just looking at the specs :) They are named MACH64_ALPHA_DST_* > >in mach64_reg.h. The driver doesn't explicitly specify any value > >which means 0 gets written. > > > >I actually just stumbled on this issue a few days ago with the mach64 > > DirectFB driver. My plan for the DirectFB driver is simply to allow > >ZERO + ZERO/ONE/SRCALPHASAT blend functions for destinations with > >alpha. It's a rather serious limitation but I think it's better than > >incorrect rendering. The issue is even worse on older Rage chips > >since they can only write 0 to the destination alpha. But that is not > >an issue for the DRI driver since those chips aren't supported. > I think now I understand. It has alpha channel and all, but it simply > won't perform the blending equation on the alpha channel, instead simply > writing zero, one, source alpha, 1 - minus source alpha, dst alpha, or 1 > - dst alpha.
Exactly. > Actually the driver does not write 0, it writes the source > alpha value (MACH64_ALPHA_DST_SRCALPHA) as far as I can tell. Ok I missed that with my grepping. I was probably looking for ALPHA_DST_SEL myself too :) > Actually, this design means it would have some very limited support for > blend_func_separate :-). I had the same though :) > Looks like a stupid design limitation to me (what would it cost to > implement that additional blend adder to the 3 you need anyway?), but ah > well. Maybe this wasn't required by DirectX 1.0 ;-). Mach64 chips have quite a few stupid alpha limitations. Most likely the nastiest one being that texture alpha can't be modulated. And then there's the whole fog vs. alpha thing. > In practice though, this might just work quite often, the alpha-blended > alpha values are probably not required a lot? Not with X. With DirectFB they are needed every time you render to an ARGB window and then expect to display the window alpha blended on screen. > >>Regardless if it can actually blend alpha values or not, there > >>would be some half-way useful alpha values probably in the buffer. > > > > > >Either you get the correct results or the wrong results. Not sure if > > there are any really useful things you can do with incorrect values. > I meant that you might just get the correct alpha values sometimes > (depending on the blend func that should be true I guess). Right. Though there are only a few combinations that give correct results. -- Ville Syrjälä [EMAIL PROTECTED] http://www.sci.fi/~syrjala/ ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://ads.osdn.com/?ad_ide95&alloc_id396&op=click -- _______________________________________________ Dri-devel mailing list Dri-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/dri-devel
