Path: utzoo!attcan!uunet!cs.utexas.edu!usc!ginosko!xanth!mcnc!decvax!savax!thompson
From: thompson@savax.UUCP (thompson mark)
Newsgroups: comp.sys.amiga.tech
Subject: Re: The Future of HAM
Keywords: Overlay bit, alpha channel
Message-ID: <1061@savax.UUCP>
Date: 14 Aug 89 23:48:58 GMT
References: <1579@psueea.UUCP> <19144@usc.edu> <507@tardis.Tymnet.COM>
Reply-To: thompson@savax.UUCP (thompson mark)
Organization: Disturbing Products, Inc.
Lines: 37

In article <507@tardis.Tymnet.COM> jms@tardis.Tymnet.COM (Joe Smith) writes:
>In article <19144@usc.edu> addison@pollux.usc.edu (Richard Addison) writes:
><And, since it would be nice to have a real alpha channel, how about another
><HAM mode with 7 planes, and 4 bits per color component with the following
><encoding for the top 3 bits to determine to use of the lower 4 bits:
>  [diagram deleted]
><Note that this, too, uses 16 bits per color register (4 red, 4 green, 4 blue,
><and 4 alpha).  Of course, these 4 alpha bits could be available in other
><video modes.
>
>What's an alpha channel?  Tell us more.
>-- 
>Joe Smith (408)922-6220 | SMTP: JMS@F74.TYMNET.COM or jms@tymix.tymnet.com

I would absolutely love to see some sort of alpha channel scheme included
into the Amiga. It is essentially a value that describes each pixels effective
coverage or opacity. It is truly invaluable for rendering antialiased images
over another image be it graphically generated or genlocked video. This is
otherwise known as compositing. Here is what it will do for you.....
If you have some whiz-bang character generator (like broadcast titler) and
you put up some antialiased fonts over a black background, they look great.
But once you replace that plain background with an actual image, you'll notice
a jagged dark shadow around your fonts. This is because when the antialising
was performed, edge pixels had to be blended to some predefined static
background color. If instead of changing the color of the edge pixel we 
reduced its alpha (proportionally to the amount the pixel is actually
covered by the edge), then the fonts could be dynamically blended with any
backgound image using something like:

new pixel = background pixel color * (1-alpha) + font pixel color * alpha

This allows smooth compositing with no visible edge boundries.
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|      Mark Thompson                                                     |
|      decvax!savax!thompson       Designing high performance graphics   |
|      (603)885-9583               silicon today for a better tomorrow.  |
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