PGPLOT::Device - autogenerate PGPLOT device names


version 0.09


  use PGPLOT::Device;

  $device = PGPLOT::Device->new( $spec );
  $device = PGPLOT::Device->new( \%specs );

  # straight PGPLOT
  pgbegin( 0, $device, 1, 1);

  # PDL
  $win = PDL::Graphics::PGPLOT::Window->new({ Device => $device} );


It is sometimes surprisingly difficult to create an appropriate PGPLOT device. Coding for both interactive and hardcopy devices can lead to code which repeatedly has to check the device type to generate the correct device name. If an application outputs multiple plots, it needs to meld unique names (usually based upon the output format) to the user's choice of output device. The user should be given some flexibility in specifying a device or hardcopy filename output specification without making life difficult for the developer.

This module tries to help reduce the agony. It does this by creating an object which will resolve to a legal PGPLOT device specification. The object can handle auto-incrementing of interactive window ids, interpolation of variables into file names, automatic generation of output suffices for hardcopy devices, etc.

Here's the general scheme:

  • The application creates the object, using the user's PGPLOT device specification to initialize it.

  • Before creating a new plot, the application specifies the output filename it would like to have. The filename may use interpolated variables. This is ignored if the device is interactive, as it is meaningless in that context

  • Each time that the object value is retrieved using the next() method, the internal window id is incremented, any variables in the filename are interpolated, and the result is returned.

Interactive devices

Currently, the /xs and /xw devices are recognized as being interactive. PGPLOT allows more than one such window to be displayed; this is accomplished by preceding the device name with an integer id, e.g. 2/xs. If a program generates several independent plots, it can either prompt between overwriting plots in a single window, or it may choose to use multiple plotting windows. This module assists in the latter case by implementing auto-increment of the window id. The device specification syntax is extended to +N/xs where N is an integer indicating the initial window id.

Hardcopy devices

Hardcopy device specifications (i.e. not /xs or /xw) are specified as filename/device. The filename is optional, and will automatically be given the extension appropriate to the output file format. If a filename is specified in the specification passed to the new method, it cannot be overridden. This allows the user to specify a single output file for all hardcopy plots. This works well for PostScript, which can handle multiple pages per file, but for the PNG device, this results in multiple output files with numbered suffices. It's not pretty! This module needs to be extended so it knows if a single output file can handle more than one page.

Variables may be interpolated into the filenames using the ${variable} syntax (curly brackets are required). Note that only simple scalars may be interpolated (not hash or array elements). The values may be formatted using sprintf by appending the format, i.e. ${variable:format}. Variables which are available to be interpolated are either those declared using our, or those passed into the class constructor.

The internal counter which tracks the number of times the device object has been used is available as ${devn}.



  $dev = PGPLOT::Device->new( $spec, \%opts );

This constructs a new object. $spec is the PGPLOT device specification, with the following allowed representations:


This results in the default PGPLOT behavior for the device.


N is an integer. This resolves to a constant output device. Usually device is /xw or /xs.


N is an integer. This will create a device which autoincrements. Usually device is /xw or /xs.


filename is an output file name. Its format is as described in "Hardcopy devices". An extension will be automatically added, if required.

The %opts hash is available to pass other options to the constructor. These are:


This is a hashref containing values to be interpolated into filenames. PGPLOT::Device dereferences the hashref at interpolation time, so will track any changes made by the application. For example:

  my %vars;
  $dev = PGPLOT::Device->new( "foo${a}${b}/ps",
                              { vars => \%vars } );

  $vars{a} = 3;
  $vars{b} = 4;

  print $dev->next, "\n";

will result in Additionally, if the values are scalar references, they will be dereferenced. This way the application is not forced to use a hash for its internal use:

  my ( $a, $b );
  my %vars = ( a => \$a, b => \$b )

  $dev = PGPLOT::Device->new( "foo${a}${b}/ps",
                               { vars => \%vars } );

  $a = 3;
  $b = 4;
  print $dev->next, "\n";

will also result in


  $dev->override( $filename, \%opts );

This method is used to override the initial values of $filename passed to the new() method for non-interactive devices. This allows the user control over the interactive device, but gives the application more control over hardcopy destinations.

Note that $filename may include a PGPLOT device specification, which will override any specified earlier, but this is frowned upon.

It takes the same options as does the new() method.


  $devn = $dev->devn;
  $dev->devn( $new_value);

This is an accessor which retrieves and/or sets the device number for interactive devices.


  if ( $device->ask ) { .. }

This is true if the device is interactive and constant, so that new plots erase old plots. This can be used with the pgask() PGPLOT subroutine to ensure that the user will see all of the plots. See "EXAMPLES".


  $dev_str = $dev->next;

This method is the basis for the automatic updating of the device specification when the object is used as a string. If desired it may be used directly. It will return the next device specification. It increments the device number.


  $dev_str = $dev->current;

This returns the device string which would be generated in the current environment. It does not alter the environment.


  $dev_str = $dev->current;

This returns the last generated device string. It does not alter the environment.


  if ( $dev->is_const ) { ... }

This method returns true if the device specification does not interpolate any variables or device numbers.


  if ( $dev->would_change ) { ... }

This method returns true if the last generated device specification would differ from one generated with the current environment. It returns true if no device specification has yet been generated.

It does not change the current environment.


  if ( $dev->is_interactive ) { ... }

This method returns true if the device is an interactive device.


  if ( $dev->is_ephemeral ) { ... }

This method returns true if the plot display will disappear if the device is closed (e.g., the /xw device ).


  • Here's the prototypical example. The application outputs multiple plots and the user is allowed to specify an output device. The device is initialized directly from the user's input:

      $device = PGPLOT::Device->new( $user_device_spec );

    Before each call to pgbegin or PDL::G::P::Window-new>, indicate via the override method the new hardcopy filename, without any suffix. The filename will be ignored if the user has specified an interactive device:

      $device->override( 'out_${theta:%05.2f}' );

    Use next() to retrieve the value:

      pgbegin( 0, $device->next, 1, );
      $win = PDL::Graphics::PGPLOT::Window->new({ Device => $device->next} );
  • The application outputs multiple plots, and the user should be able to decide whether a single interactive device window should be used, or whether multiple ones should be used. In the first instance, the user specifies the device as /xs, in the second +/xs or +1/xs:

      $device = PGPLOT::Device->new( $user_device_spec );
      $device->override( 'hardcopy-${vara}-${varb}' );
      $win = PDL::Graphics::PGPLOT::Window->new({ Device => $device->next} );
      [... generate plot 1 ... ]
      # do this after generating the plot, because Window
      # be constant, and that'll confuse is_const()
      pgask( $device->ask );
      # next plot.
      if ( $device->would_change )
        $win = PDL::Graphics::PGPLOT::Window->new({ Device => $device->next} );
      # etc.
      # make sure that the user is prompted before the device is closed
      # if the device will disappear.
      pgask( 1 ) if $device->ephemeral;

    Note that would_change() will return true if no specification has yet been generated. This allows one to simplify coding if plots are generated within loops:

      my $win;
      my %vars;
      my $device = PGPLOT::Device->new( $user_device_spec );
      $device->override( 'file-${a}-${b}', { vars => \%vars } );
      my $not_first = 0;
      for my $plot ( @plots )
        $vars{a} = $plot->{a};
        $vars{b} = $plot->{b};
        # prompt user before displaying second and subsequent plots if
        # a new plot will erase the previous one
        pgask( $param{device}->ask ) if $not_first++;
        if ( $device->would_change )
          $win->close if defined $win;
          $win = PDL::Graphics::PGPLOT::Window->new({ Device => $device->next} );
        [... plot stuff ...]
      if ( defined $win )
       # make sure that the plot stays up until the user is done with it
       pgask(1) if $device->ephemeral;


Please report any bugs or feature requests on the bugtracker website or by email to

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.


The development version is on github at and may be cloned from git://


Please see those modules/websites for more information related to this module.


Diab Jerius <>


This software is Copyright (c) 2017 by Smithsonian Astrophysical Observatory.

This is free software, licensed under:

  The GNU General Public License, Version 3, June 2007