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From: smb@data.com (Steven M. Boker)
Newsgroups: comp.sys.next
Subject: Compositing question
Message-ID: <1991Mar9.000952.903@data.com>
Date: 9 Mar 91 00:09:52 GMT
Distribution: na
Organization: Data Transforms, Inc.
Lines: 37

I have been putting a little time into updating Dave Joerg's MovieApp
and I've come across a compositing situation that I can't seem to think
my way around.  Essentially what I want to do is to overlay multiple
NXImage's with the visual effect of them being on tracing paper.  The
top sheet is fully visible, the next sheet is gray, the next sheet below
that is lighter gray and so on.  I have tried the dissolve:topoint: method
in NXImage, but there is a lower limit to the grayscale that is produced.

In other words, if I have a 1.0 gray pixel and dissolve at 50% with a
0.0 gray pixel, the result should be a .5 gray pixel.  If I repeat the
operation on the resulting pixel recursively, the pixel should approach
0.0 as a limit.  However, the pixel's gray stops before 0.0 (or vice-versa
before 1.0).

If you are wondering what all of this is good for, I've been animating
data sets.  They look like flocks of birds, really.  I would like to be
able to create "vapor trails" behind each bird.  These vapor trails should
decay to white as a limit, but don't.

I've tried a large number of compositing combinations and transparency
combinations and have continued to run into the same phenomenum.  Am I
missing something obvious?  

Thanks.  

ps.  I should be able to upload the 2.0 version of MovieApp to the archives
     within a couple of days.  I'll post when its ready.  The 68040 is
     a big win here, but be sure you have plenty of memory for the
     bitmap caching.

Steve.

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