linux-fbdev-devel

Re: [Linux-fbdev-devel] Alpha blending

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

On Mon, 2 Apr 2001, Scott A McConnell wrote:
> I assume the Transp is for Alpha blending.
> It appears that the  API can support Alpha blending at the pixel level.
> 
> What about support at the rectangular level?
> 
> Not that they are an authority.... ;-)
> 
> http://www.pcwebopedia.com/TERM/a/alpha_channel.html
> 
> "Typically, you wouldn't define the alpha channel on a pixel-by-pixel
> basis, but rather per object. Different parts of the object would have
> different levels of transparency depending on how much you wanted the
> background to show through. This allows you to
> create rectangular objects that appear as if they are irregular in shape
> -- you define the rectangular edges as transparent so that the
> background shows through. This is especially important for animation,
> where the background changes from one frame to the next. "

That's something different. `transp' in fbdev is about transparency of the
screen (for a video underlay or so), not about transparency of objects to be
blended together on the screen.

> Wouldn't we want an Alpha/Transp value to be passed to the three
> accelerated functions?

No, we don't want transparant text consoles :-)

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

Re: [Linux-fbdev-devel] Alpha blending

[Sven L. <lu...@dp...>]

On Tue, Apr 03, 2001 at 08:17:41AM +0200, Geert Uytterhoeven wrote:
> On Mon, 2 Apr 2001, Scott A McConnell wrote:
> > I assume the Transp is for Alpha blending.
> > It appears that the  API can support Alpha blending at the pixel level.
> > 
> > What about support at the rectangular level?
> > 
> > Not that they are an authority.... ;-)
> > 
> > http://www.pcwebopedia.com/TERM/a/alpha_channel.html
> > 
> > "Typically, you wouldn't define the alpha channel on a pixel-by-pixel
> > basis, but rather per object. Different parts of the object would have
> > different levels of transparency depending on how much you wanted the
> > background to show through. This allows you to
> > create rectangular objects that appear as if they are irregular in shape
> > -- you define the rectangular edges as transparent so that the
> > background shows through. This is especially important for animation,
> > where the background changes from one frame to the next. "
> 
> That's something different. `transp' in fbdev is about transparency of the
> screen (for a video underlay or so), not about transparency of objects to be
> blended together on the screen.

What about anti-aliasing ?

> > Wouldn't we want an Alpha/Transp value to be passed to the three
> > accelerated functions?
> 
> No, we don't want transparant text consoles :-)

Why not, it could be fun, with a big pengouin display behind ?

Alternatively, we could have a framework for userland set themable display
engines, or something such.

(not really serious here, so don't take it so)

Friendly,

Sven Luther

Re: [Linux-fbdev-devel] Alpha blending

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

On Tue, 3 Apr 2001, Sven LUTHER wrote:
> On Tue, Apr 03, 2001 at 08:17:41AM +0200, Geert Uytterhoeven wrote:
> > On Mon, 2 Apr 2001, Scott A McConnell wrote:
> > > I assume the Transp is for Alpha blending.
> > > It appears that the  API can support Alpha blending at the pixel level.
> > > 
> > > What about support at the rectangular level?
> > > 
> > > Not that they are an authority.... ;-)
> > > 
> > > http://www.pcwebopedia.com/TERM/a/alpha_channel.html
> > > 
> > > "Typically, you wouldn't define the alpha channel on a pixel-by-pixel
> > > basis, but rather per object. Different parts of the object would have
> > > different levels of transparency depending on how much you wanted the
> > > background to show through. This allows you to
> > > create rectangular objects that appear as if they are irregular in shape
> > > -- you define the rectangular edges as transparent so that the
> > > background shows through. This is especially important for animation,
> > > where the background changes from one frame to the next. "
> > 
> > That's something different. `transp' in fbdev is about transparency of the
> > screen (for a video underlay or so), not about transparency of objects to be
> > blended together on the screen.
> 
> What about anti-aliasing ?

Anti-aliasing has nothing to do with transparency on non-transparent text
consoles.

> > > Wouldn't we want an Alpha/Transp value to be passed to the three
> > > accelerated functions?
> > 
> > No, we don't want transparant text consoles :-)
> 
> Why not, it could be fun, with a big pengouin display behind ?
> 
> Alternatively, we could have a framework for userland set themable display
> engines, or something such.
> 
> (not really serious here, so don't take it so)

Sure! And use the YUV-layer on ATI RAGE 3D for a transparent image of `big
brother^H^H^H^H^H^H^Hpenguin Linus' watching you work from the right top corner
of the screen :-)

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

Re: [Linux-fbdev-devel] Alpha blending

[Sven L. <lu...@dp...>]

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).

> > > > Wouldn't we want an Alpha/Transp value to be passed to the three
> > > > accelerated functions?
> > > 
> > > No, we don't want transparant text consoles :-)
> > 
> > Why not, it could be fun, with a big pengouin display behind ?
> > 
> > Alternatively, we could have a framework for userland set themable display
> > engines, or something such.
> > 
> > (not really serious here, so don't take it so)
> 
> Sure! And use the YUV-layer on ATI RAGE 3D for a transparent image of `big
> brother^H^H^H^H^H^H^Hpenguin Linus' watching you work from the right top corner
> of the screen :-)

Yes, something such, ...

maybe even a live webcame feed of linus or something such ?

:)))

Friendly,

Sven Luther

Re: [Linux-fbdev-devel] Alpha blending

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

On Tue, 3 Apr 2001, Sven LUTHER wrote:
> On Tue, Apr 03, 2001 at 10:05:01AM +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.

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

Re: [Linux-fbdev-devel] Alpha blending

[Sven L. <lu...@dp...>]

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 ?

Friendly,

Sven Luther

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