Really depends on the compilers, what you can do, and how constrained you
are. Fortran programs can generally be optimised more aggressively: the
compiler has more information and the programmer is more restricted. But
that doesn't mean it will happen. Some compilers are a Fortran or C parser
in front of the same code generator, and they take no advantage of the
difference. Some compilers are just plain dumb.

C++ can have additional performance problems because many decisions must
be left till execution.

If you're comfortable with Fortran and you don't foresee any maintenance
problems, why not use it? At worst you'll break even.

--
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A lot depends on which language you have experience with (I wouldn't
want to start a major project for someone else in a completely new
language).  Also, what packages are already available to save you
work?  I guess there are good FFT routines already written for
both C++ and Fortran.  Fortran is designed with number-crunching in mind
so it makes sense to use that for the calculations if you are happy with
it.

For the GUI, there is a lot more choice.  Does "important" mean that the
the interface is very complex, or that it has to look pretty?  If you
have not used C++ before (especially if you have not used an object
oriented language and/or C before) then any GUI is going to be
restricted to whatever the "wizard" can generate, without a lot of work
and learning.

There are many languages designed to provide interfaces to
computation-intensive code - Tcl, for example, or Python.  These have
graphics toolkits (Tcl is used usually with Tk, python can be used with
Tk or with a range of other packages).  Again, it depends a lot on what
you are used to.  The advantage of these languages over C++ is that they
are simpler and often interpreted (speed is not so important, while
development time is quicker).

But again, if you have not used any of these then you are better going
with something you do know, if you can interface it to Fortran and it
has a GUI toolkit.  GUI programming, except for simple cases, is very
challenging (although this may be a simple case if you are just
displaying results).

Alternatively, how about something like IDL (a maths package/array
manipulation language) or Mathematica?  They already integrate
calculations and GUI, and their internal routines are as fast as
anything you are likely to write (the advantage of using Fortran or C++
is for complex programs with many different calculations - if you are
just doing FFTs, then the FFT routine in a maths package should be just
as fast).

Andrew
PS For more general information on languages, and pointers to more
information on the languages I mention above, look at
http://www.andrewcooke.free-online.co.uk/andrew/lang.html

PPS Are you *sure* that software like this does not already exist?  From
my own experience I know that astronomers have a wide variety of image
processing software, for example.

in the long term, however, you may want to avoid putting a lot
of effort into something that is platform-dependent. in which
case, a Fortran/Java solution would probably be better.

--
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"There are lies, damned lies, and statistics" -- Benjamin Disraeli
"...and benchmarks" -- Garry Hodgson
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China Beach Blue a crit dans le message ...

Please note I am not referenig to the  choice of programming language....

I use Compaq Visual Fortran 6.1 (It is not Digital Visual Fortran anymore)
for very demanding floating point calculations. (I am  mostly programming in
Fortran 95 and sometimes Fortran77; occasionally I mix my programmes with
C++ and visual Basic ).  I am more than satisfied with CVF6.1. It comes with
two excellent set of libraries (Professional version): IMSL and Compaq
Extended Math Library (FFT...). The later one is new and I find it "damn
good" with an excellent documentation.  They have Array Visualizer (for data
visualizing) and lot more (great support team, discussion forum...)

check CVF site at http://www.digital.com/fortran/
for an independent win32 Fortran compiler comparison check:
http://www.polyhedron.com/

and for more feedback from other user post  your message in

(people in comp.lang.fortran are mostly scientists and engineers, I am sure
you will find some body working in image processing...)

Nobat