Re: 8-bit alpha in 32-bit frame-buffer?????

[Habil Silva <habil.silva@netc.pt>]

 I've read somewhere that you can even use them

----- Original Message -----

From: Tom.P.Monkish (aka Monkfish)

To: gameprogrammer@gameprogrammer.com

Sent: Wednesday, April 12, 2000 7:05 PM

Subject: 8-bit alpha in 32-bit frame-buffer?????

>> Are the 8-bits of alpha in a 32-bit frame-buffer used by game developers for anything?

 

32-bit video modes are the same as 24-bit video modes. So why do they exist? Simple: they're faster, because it's easier for the CPU to send out or read in 4 bytes (dwords) each time, instead of 3 bytes (read one word, than one byte). So think of the alpha byte as a padding byte - standard (without hardware acceleration) VESA modes don't use them for anything! Of course, they waste more memory.

 

 

>> If they are used, then how are their absence dealt with in a 16-bit (565) display?

 

Lets just say, that in 16-bit modes, there's no support for alpha channels, the obvious advantage is that they're faster - each pixel only uses 2 bytes. 15-bit color modes (not used anymore?) are ARGB, with one alpha bit, and 5 bits for each color (1555). Just like 32-bit modes, the

padding bit isn't used.

 

 

>> Do most modern games simply ignore this storage facility?

>> Can they be used to do sort-independent translucency?

>> How can this be achieved when combining alpha info with an opaque texture completely obliterates the alpha channel information?

 

I think that the 3D 'alpha' acceleration features of modern cards do use the information stored in the alpha channel, but I'm not sure. What I do know, is that most cards don't mind what value the alpha channel is given (usually zero) and that you can use it however you wish.

 

 

Thanks :-)

Tom

 

 

No problem,

Habil