Re: [Algorithms] Alpha Channel

[gl <gl...@nt...>]

Yes, I did mean 1555.  Neurons misfiring...
--
gl

----- Original Message -----
From: "Scott Justin Shumaker" <sjs...@um...>
To: <gda...@li...>
Sent: Sunday, August 27, 2000 12:50 AM
Subject: Re: [Algorithms] Alpha Channel


> A bit OT, but oh well:
> The 5551 format is not available in D3D.  Only the 1555 format (alpha in
> the high bit) is.   This is equivalent to the OpenGL 1_5_5_5_REV packed
> pixel format exposed in OpenGL 1.2.
>
> Direct3D only exposes the REV packed pixel formats (they don't call them
> reversed, of course), while OpenGL prefers the regular formats but does
> expose the REV formats in 1.2.
>
> You have to be very careful that you understand the formats correctly.
> The packed formats look the same on all machine architectures (i.e.,
> 1_5_5_5_REV always has the alpha in the high bit), but OpenGL formats like
> GL_UNSIGNED_BYTE - GL_RGBA have components in different places depending
> on the endianness of the host machine.
>
> Even more OT:
>
> Color keying can actually be a win if you have a lot of 2d code
> interspersed with 3d code.  Because of texture limitations, if you have a
> 2d-windowing system that need to run in a 3d window you'll probably end up
> compositing it in system memory, than slicing it into small tiles, each
> which is uploaded as a texture.  On the other hand, you can probably do a
> 2d blt to the render target on most D3D hardware for 2d stuff, and this is
> far faster.  (By way of comparison, the software driver at my employer
> runs the 2d stuff 3x faster than a Geforce2 without doing 2d blits).
>
>  --
> Scott Shumaker
> sjs...@um...
>
> On Sat, 26 Aug 2000, gl wrote:
>
> >
> > Take a look at the 'D3DFrame' texture loader code (part of the SDK) - it
> > does exactly that from alpha files.
> >
> > If you can use 16bit, choose the 5551 format if available, as 1 bit of
alpha
> > is all you need.  BTW, you are really describing colour keying, but
don't
> > use it, as it's legacy and can cause all kinds of problems when combined
> > with alpha blending (search the archives for details).
> > --
> > gl
> >
> > ----- Original Message -----
> > From: "Pai-Hung Chen" <pa...@ac...>
> > To: <gda...@li...>
> > Sent: Saturday, August 26, 2000 9:08 PM
> > Subject: [Algorithms] Alpha Channel
> >
> >
> > > Hi,
> > >
> > > I want to use billboard in my program but cannot find a good way to
create
> > > 32-bit bitmap with alpha channel of appropriate values.  Basically I
want
> > to
> > > create a bitmap of tree with leaves in various green colors (with
alpha =
> > > 255) and all non-green colors transparent (alpha = 0) so that I can
use it
> > > as the texture for my billboarded trees.  It would be nice if I can
> > specify
> > > a non-green color C and make the alpha of all the pixels of color C in
the
> > > bitmap to 0.  Is there any program capable of doing that with 32-bit
> > bitmap
> > > with alpha channel?  This may be off-topic and you could send your
advise
> > to
> > > me off-line.
> > >
> > > Thanks in advance,
> > >
> > > Pai-Hung Chen
> > >
> > >
> > > _______________________________________________
> > > GDAlgorithms-list mailing list
> > > GDA...@li...
> > > http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list
> > >
> >
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> >
>
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