Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!jarthur!usc!snorkelwacker!bloom-beacon!kelly.UUCP!marvin
From: marvin@kelly.UUCP (Kyle Marvin)
Newsgroups: comp.windows.x
Subject: Re:  Colormap Size of DirectColor Visual
Message-ID: <9005291438.AA16727@kelly.com>
Date: 29 May 90 14:38:47 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 28

>  Xdpyinfo reports that the DirectColor visual on a Sun has 8 colormap 
>  entries.  Yet I can store 256 colors with XStoreColors.  Can someone 
>  explain this "apparent" discrepency?

Xlib documentation says the following about the Visual structure:

"The map_entries member defines the number of available colormap entries
 in a newly created colormap.  For DirectColor or TrueColor, this is the
 size of an individual pixel subfield."

In other words, a 3/3/2 DirectColor visual (as on the Sun server) will
advertise 8 (2^^3) entries since that is the number of discrete values
for a given color channel.

A common application bug I've seen is to use the XDisplayCells (which
uses the map_entries member) function to determine the total number of 
colormap entries.  If the default visual on the server is
TrueColor or DirectColor, the application thinks that the number of
colormap entries is much smaller than the actual number that exist.
Try running "xmag" or "xshowcmap" with a server started with the
"-cc 5" option and see what happens.  A better approach might be to
use 2 raised to the XDisplayPlanes() power to determine the total
colormap size, although there are no guarantees that this many cells
are actually *writable* by the application.

Kyle W. Marvin
Visual Information Technologies, Inc. (VITec)
UUCP: uunet!convex!vitsun!marvin