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From: anderson@osc.edu (Steve Anderson)
Newsgroups: comp.graphics.visualization
Subject: Re: apE: image depth and colormaps
Summary: correct, 8-bits is only for display
Message-ID: <1449@oscsunb.osc.edu>
Date: 5 Mar 91 15:33:17 GMT
References: <1991Mar4.185030@cs.Buffalo.EDU>
Organization: The Ohio Supercomputer Graphics Project
Lines: 49

In article <1991Mar4.185030@cs.Buffalo.EDU>, okeefe@cs.Buffalo.EDU (Paul O'Keefe) writes:
> 
> 
> I need some help understanding the relationship between
> image depth and colormaps in apE.
> 
> I using apE on a SparcStation with an 8-bit color X11 server.
> This machine also has a Taac board which has 24-bit color.
> I was hoping to export apE images as 24-bit Sun rasterfiles
> then import them into Taac format and see them in true color.
> 
> However, the apE manual says that all colormaps have 245 entries.
> 
> Question: how does this jive?
> 
> Is the image computed in true-color and the 8-bit colormap is just
> a subset used for display within apE?
> 
> Paul O'Keefe
> okeefe@cs.buffalo.edu

correct. Images are (can be) computed and maintained in 24-bits, with
dithering happening at display time to the "best representation"
on the hardware.

apE colormaps are commonly used when you want to limit the number of
colors; applying a colormap when computing the image, instead of
using full-color. Images rendered using colormaps can be manipulated
and displayed much faster (obviously). 

apE colormaps are 245 elements because they had to be slightly smaller
than 256 to allow usefullness in window systems which reserve several
slots. We also keep the interface colors outside of the 245, so there
is less flashing of windows as the colormaps come in and out of
the system.

For dithering, there is a "default colormap" which has a dither palette
in the 245 colormap elements.

--steve




-- 
 H. Stephen Anderson			 | email: anderson@osgp.osc.edu
 The Ohio Supercomputer Graphics Project | Phone: (614) 292-3274
 1224 Kinnear Road                       |
 Columbus, Ohio 43212                    |