Subject: Mac and FITS

From: Carl A. Dobson <[email-masked]>
Date: Fri, 25 Jun 93 10:17:34 PDT

I've seen three programs on the Mac which display images
produced here upside down.  The images look fine in saoimage and IDL
on a Sun.  Have you seen this problem?  Is it possible that it is
a problem with the FITS files, or is this a known problem on the Mac?
I'll be happy to send you a file and/or a Mac program if it will help
diagnose the situation.  The Mac programs I used were NIH Image 1.23
with FITS reader (distributed by SAO), MAIA (I don't remember the
version number), and CDBrowser (again, I don't know the version number).

	I've also seen the same problem with a program called fitstopgm
on the Sun (a free program by Jef Poskanzer).

Thanks,
Carl Dobson

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From: dwells (Don Wells)
Date: Fri, 25 Jun 93 13:31:22 EDT

Carl,

Upside-down display is a standard, classical, perennial problem, one
that is not peculiar to the Mac. It is a question of conventions,
left-handed vs right-handed, top-first vs bottom-first. TV monitors
have always painted left-to-right with the first pixel in the
upper-left corner, a left-handed coordinate system.  High school trig
courses teach us to think about right-handed coordinate systems with
the origin toward the lower-left corner. Most -- but not all --
programmers of astronomical image processing systems have chosen to
follow the convention of their high school trig courses rather than
the convention used by TV hardware.  On the whole I still think that
this is the best decision, but certainly reasonable people can differ
about this matter.  In any case, if your FITS headers contain proper
WCS information there will be no confusion about the relative
locations of pixels on the sky.

May I put your message in the Mac directory on fits.cv.nrao.edu?

-Don