I think the OP was having trouble cropping the rotated image because it was...erm...rotated.

Here's what I am planning to use.. This is to get the coordinates of the corners of the cropping box..

#!/usr/bin/perl -w
use strict;
use Math::Complex;

# box1 is the image 
my @box1 = (-150,-150,150,-150,150,150,-150,150);

#box 2 is the cropping box
my @box2 = (-300,-75,0,-75,0,75,-300,75);

my @box2r = save_rotate_points(undef,45,@box2);
print "Rotate BOX 2\n";
foreach my $x (@box2r) {
    print $x , "\n";
}


my ($x1,$y1,$x2,$y2) = get_rotation_box(undef,@box2r);
my @points = ($x1,$y1,$x2,$y1,$x2,$y2,$x1,$y2);
print "Bounding Box:\n";
foreach my $x (@points) {
    print $x , "\n";
}

sub save_rotate_points {
    my ($widget,$angle,@points) = @_;
    my $count = @points;
    my (@x, @y);
    $angle = $angle * pi() / 180;
    for(my $i = 0; $i < $count ; $i+=2){
        my $xp = ($points[$i] * cos($angle)) - ($points[$i+1] * sin($a
+ngle));
        my $yp = ($points[$i] * sin($angle)) + ($points[$i+1] * cos($a
+ngle));
        push @x, $xp;
        push @y, $yp;
    }
    @points = ($x[0],$y[0],$x[1],$y[1],$x[2],$y[2],$x[3],$y[3]);
    return @points;
}

sub get_center_rectangle {
    my ($widget,@points) = @_;
    my $deltax = ($points[0]+$points[2]+$points[4]+$points[6])/4;
    my $deltay = ($points[1]+$points[3]+$points[5]+$points[7])/4;
    return $deltax,$deltay;
}

sub get_rotation_box {
    my ($widget,@points) = @_;
    # get smallest value
    my $i = 0;
    my ($smallx,$smally,$bigx,$bigy) = (0,0,0,0);
    foreach my $x (@points) {
        if($i == 0){
            if($x <= $smallx){
                $smallx = $x;
            }
            if($x >= $bigx){
                $bigx = $x;
            }
        }else{
            if($x <= $smally){
                $smally = $x;
            }
            if($x >= $bigy){
                $bigy = $x;
            }
            $i = -1;
        }
        $i++;
    }
    return $smallx,$smally,$bigx,$bigy;
}

sub move_points {
    my ($widget,$mx,$my,@points) = @_;
    my $i = 0;
    my @new_points;
    print $mx," ",$my,"\n";
    foreach my $x (@points) {
        if($i == 0){
            print $x , "+" , $mx , "\n";
            $x+=$mx;
        }else{
            $x+=$my;
            $i = -1;
        }
        push @new_points, $x;
        $i++;
    }
    return @new_points;
}
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