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Date: 1 Jun 90 19:08:29 GMT
From: convex!ucscc.UCSC.EDU!chucko  (30502000)
Organization: University of California, Santa Cruz  CATS
Subject: Pixmap vs. Image
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To: xpert@expo.lcs.mit.edu

Summary:
To use Pixmap or Image structures when pixel computations
are performed often.

I am in the design stage of an X application that involves
quite a bit of image manipulation.  Images come to me as 16-bit,
and will usually be treated as 8-bit by my application.  I need
to keep around the 16-bit data in case I need to get to the
real (unquantized) data.  In order to "blend" or "overlay" 2 or
more images and display them in an 8-bit deep window I need to
do computations on corresponding pixels in each image to create
the blended image (the computation is similar to finding the pixel
value when using XStandardColormaps).

My understanding, so far, is that images (Ximage structure) reside
on the client and shipping them to my window can take lots of time
(many <<dozens of?>> seconds) for large images.  Pixmaps, however,
reside on the workstation (server) and can be loaded into the
window much faster.  Here's the question:

If I have to do these computations (an add and multiply) on each pixel,
and load the result into a window.  First, where do the computations
take place (server or client)?  Second, if they take place on the
client, is it faster to use Images which are already there, or use
Pixmaps which seems to imply a round trip for each pixel?  Which is
faster, which is easier, which is politically correct????

Your help is much appreciated.

Reply to the newsgroup if you think it's of interest to others,
otherwise direct and perhaps I'll summarize.

Chuck Stein                         chucko@ucscc.ucsc.edu
Mirror Imaging / NOARL West

{ I do represent the opinion of my employer. }