[gnuastro-devel] [task #14146] Select pixels that are blank

[Mohammad Akhlaghi]

URL:
  <http://savannah.gnu.org/task/?14146>

                 Summary: Select pixels that are blank
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Mon 05 Sep 2016 11:30:05 PM JST
         Should Start On: Mon 05 Sep 2016 12:00:00 AM JST
   Should be Finished on: Mon 05 Sep 2016 12:00:00 AM JST
                Category: Arithmetic
                Priority: 5 - Normal
              Item Group: Enhancement
                  Status: Postponed
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00

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Details:

With the new equal operator in Arithmetic, we can choose certain pixels in the
image, except blank pixels (NaN in float types and fixed constants for other
types). 

The problem with blank values is that in floating points, the IEEE NaN
standard defines this value as failing any conditional operator, so NaN is not
equal to its self and the only reliable way we can check for it is the `isnan'
function. For integer types the blank value is a constant (defined by the type
in Gnuastro's `lib/gnuastro/fits.h'. But we don't want the users to have to
check that value manually.

One solution to these problems might be this: we add a unique type of operand
(not a file name or number) called `blank' (the name can be changed). Then
when the equal operator confronts this operand (either as the first popped
operand or the second), it will use `isnan', not the equal operator.
Internally (for the time being, see task #13869), in Arithmetic, everything is
in double type, so the single `isnan' is enough. Later it can be expanded to
accommodate all types.




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