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From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: Fwd: How to get RGB values from XImage data
Message-ID: <8909011227.AA03855@expire.lcs.mit.edu>
Date: 1 Sep 89 12:27:15 GMT
References: <4981@portia.Stanford.EDU>
Sender: daemon@bloom-beacon.MIT.EDU
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The file format used by xwd is "documented" in the header file
<X11/XWDFile.h> plus the xwd/xwud code.  The contents, in order:
	header
	name
	color data
	image data
The pixel values in the image data index the colormap (described
by the color data) to yield RGB values.  The exact method of indexing
depends on the visual class described in the header, and potentially
on the RGB masks in the header.  However, for a 24-bit DirectColor
image, assuming you have a vanilla MIT R2 or R3 xwd (Ardent may have
fixed this), the color data will not be present in the file.  In
that case, you can just use the RGB masks in the header to directly
decompose each pixel value into RGB subfields.  Other entries in the
header (byte_order, bitmap_unit, bitmap_pad) give the details of how
the pixels are stored in the image data.  Formats are described in
the X protocol document (page 357 in the Digital Press book).