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Surprising Odds and Ends

Hi Folks,

I've been working on the 2nd Edition of my book for
the past couple of weeks. As always when I try to write
about something in IDL that I think I know something about,
I'm learning all kinds of new things. I thought I might
share a couple of the stranger things with you. There is
no purpose, really, except to get you ready for the IDL
Expert Programmers Association exam that will be given
early next month to Ben and a couple of other lucky

1. The units for the TICKLEN keyword are in normalized
   coordinates relative to the window size. The units
   for the XTICKLEN and YTICKLEN keywords are in normalized
   units with respect to the TICKLEN keyword. This makes
   sense, I suppose, but it is completely contrary to the
   IDL documentation for XTICKLEN and provided many confusing
   moments for me. (I actually think I saw this documented
   somewhere, but I can't find it now when I am looking in
   all the obvious places.)

2. The HISTOGRAM function documentation states that it
   will use the minimum and maximum value of the data
   to calculate image histograms. But this appears to
   be untrue. At least with byte images, the minimum
   and maximum values of the histogram are always 0
   and 255, no matter what values are in the image.

3. The HEAP_GC command (which, heaven forbid, you *really*
   shouldn't be using anyway) is dependent on program
   level. For example, in Cleanup routines I want to 
   destroy pointers in my info structure. But if the
   program crashes in an event handler, the info structure
   is undefined in the Cleanup routine. In such a case
   I might want to clean up the pointers by issuing a HEAP_GC
   command. But I could never get this to work. Today I
   found out why. The heap apparently exists *only*
   at the main IDL level. If you try to call Heap_GC from
   some other level (e.g., inside a Cleanup routine) the
   command appears to work, but nothing really happens. This
   command can *only* be used from the IDL command line.

4. Another way of doing this:
      ptrToUndefinedVar = Ptr_New(/Allocate_Heap)

   is to do this:
      ptrToUndefinedVar = Ptr_New(xx)

   where XX is an undefined variable. Neat! It makes storing
   the extra keyword collect via keyword inheritance MUCH easier.

      ptrToExtraKeywords = Ptr_New(extra)

   And I always have a valid pointer that can be de-referenced.
   (I'm sure the pointer gurus already knew this, but it takes
   me a bit longer sometimes.)

5. PRINTER offsets are calculated from the edge of the printable
   portion of the page, rather than from the page edge, as they
   are for PostScript files. Each offset point is different for
   each printer. Thus, you have to program in printer-specific
   fudge factors if you want centered output. Ouch!

Enough for now. I'm leaning about 10 new things a day, so I'll
probably have more before the exam.



David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155