Re: [Linux-fbdev-devel] Alpha blending

[Geert U. <ge...@li...>]

On Tue, 3 Apr 2001, Sven LUTHER wrote:
> On Tue, Apr 03, 2001 at 10:24:25AM +0200, Geert Uytterhoeven wrote:
> > On Tue, 3 Apr 2001, Sven LUTHER wrote:
> > > On Tue, Apr 03, 2001 at 10:05:01AM +0200, Geert Uytterhoeven wrote:
> > > > On Tue, 3 Apr 2001, Sven LUTHER wrote:
> > > > > What about anti-aliasing ?
> > > > 
> > > > Anti-aliasing has nothing to do with transparency on non-transparent text
> > > > consoles.
> > > 
> > > Err, it was my understanding that you use the alpha channel to do
> > > anti-aliasing, even if in the end you don't support transparency, but then i
> > > may be wrong, (mmm, maybe i am).
> > 
> > Yes, for an object that still has to be blended with a background image.
> > 
> > But since a text console has a solid background, you can calculate the
> > resulting RGB values directly:
> > 
> >     [rgb]_res = [rgb] * a + [rgb]_bg * (1-a)
> > 
> > This is indeed equivalent with alpha blending a pixel with transparency a to a
> > solid background pixel.
> 
> Ok, i see that, but what about hardware supported anti-aliasing/alpha
> blending. will providing the value to the chip and let it do stuff not be
> faster (especially as some more advanced chip don't know about 24bpp, treating
> it as 32bpp and ignore the 8bit of the alpha channel ?

For anti-aliased fonts, you either have to use larger fonts (to be downscaled
by hardware), or store pre-anti-aliased fonts at the normal size, but including
an alpha value. For an 8-bit alpha value, this increases storage size by a
factor of 8, compared to monochrome bitmaps.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@li...

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds