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#!/usr/bin/perl -w

use Gimp;
use Gimp::Fu;
use strict;
use warnings;

podregister {
  my @layers = $image->get_layers;
  my $n = @layers or die "At least one layer is required\n";
  my ($i, $I, $p, $P, $d); # declare here so seen by eval below
  my $func = eval "sub{\nno strict;\n#line 0 \"expression\"\n$expr\n}";
  die "Syntax error in expression '$expr': $@\n" if $@;
  # now apply the expr
  eval { $image->undo_group_start };
  # 5.004 has problems with overlaying functions
  Gimp->progress_init("Applying Expression...",-1);
  for $i (0..$n-1) {
    $I = $n-$i-1;
    $p = @layers > 1 ? $i/$#layers : 0;
    $P = 1-$p;
    $d = $layers[$i];
    eval { &$func };
    die "Evaluation error in frame $i: $@\n" if $@;
    Gimp->progress_update(($i+1)/$n);
  }
  eval { $image->undo_group_end };
};

exit main;
__END__

=head1 NAME

layer_apply - Apply a perl expression to every layer of the image

=head1 SYNOPSIS

<Image>/Filters/Generic/Apply Perl Expression...

=head1 DESCRIPTION

Runs a perl expression for each layer of an image. The perl expression
can use the following variables:

=over 4

=item $image

The image.

=item $d ("drawable")

The current drawable (currently always a layer).

=item $i, $I ("index")

The index of the current layer. C<$i> counts from 0 to $n-1, i.e. from top
to the bottom of the stack with layer #0 being the top of the stack.

C<$I> counts "backwards", i.e. from $n-1 to 0, so layer 0 is at the
bottom.

=item $p, $P ("percent")

Similar to $i, but counts from 0 to 1 (inclusive). $p is equivalent to
C<$i/($n-1)> and $P is equivalent to C<$I/($n-1)>.

=item $n ("number of frames")

The number of layers in the image.

=item $d2 ("drawable")

The drawable2 argument, entirely optional and can be used for anything you
like.