Save GooCanvas2 to PNG file - perl

Question

After drawing with GooCanvas2, I'm trying to take a 'screenshot' of the canvas and save it to a .PNG file.
This script provides a very nice example using Gtk2/GooCanvas, but having converted that script to Gtk3/GooCanvas2, I get an error that I don't understand:
Write PNG...
*** unhandled exception in callback:
*** `need' is not a valid cairo_status_t value; valid values are: success, no-memory, invalid-restore, invalid-pop-group, no-current-point, invalid-matrix, invalid-status, null-pointer, invalid-string, invalid-path-data, read-error, write-error, surface-finished, surface-type-mismatch, pattern-type-mismatch, invalid-content, invalid-format, invalid-visual, file-not-found, invalid-dash, invalid-dsc-comment, invalid-index, clip-not-representable, temp-file-error, invalid-stride, font-type-mismatch, user-font-immutable, user-font-error, negative-count, invalid-clusters, invalid-slant, invalid-weight at goopng2.pl line 90.
*** ignoring at /usr/share/perl5/Gtk3.pm line 546.
The error is generated by Gtk3::Gdk::PixbufLoader->write(). I have not modified that function at all:
$surface->write_to_png_stream (sub {
my ($closure, $data) = #_;
$loader->write($data);
});
And this is the converted script:
#!/usr/bin/perl -w
use strict;
use warnings;
use GooCanvas2;
use Gtk3 '-init';
use Glib qw(TRUE FALSE);
my $window = Gtk3::Window->new('toplevel');
$window->signal_connect('delete_event' => sub { Gtk3->main_quit; });
$window->set_default_size(640, 600);
my $vbox = Gtk3::VBox->new;
$vbox->set_border_width(4);
$vbox->show;
$window->add($vbox);
my $swin = Gtk3::ScrolledWindow->new;
$swin->set_shadow_type('in');
$vbox->pack_start($swin, 1, 1, 0);
my $canvas = GooCanvas2::Canvas->new();
$canvas->set_size_request(600, 450);
$canvas->set_bounds(0, 0, 1000, 1000);
$swin->add($canvas);
my $root = $canvas->get_root_item();
my $rect = GooCanvas2::CanvasRect->new(
parent => $root,
'x' => 100,
'y' => 100,
'width' => 400,
'height' => 400,
'line-width' => 10,
'radius-x' => 20,
'radius-y' => 10,
'stroke-color' => 'yellow',
'fill-color' => 'red'
);
my $text = GooCanvas2::CanvasText->new(
'parent' => $root,
'text' => "Hello World",
'x' => 300,
'y' => 300,
'width' => -1,
'anchor' => 'center',
'font' => 'Sans 24',
);
$text->rotate(45, 300, 300);
# Create PNG
my $sb = Gtk3::Button->new_with_label('Write PNG and JPG');
$vbox->pack_start($sb, FALSE, FALSE, 0);
$sb->show;
$sb->signal_connect("clicked", \&write_png_clicked, $canvas);
$window->show_all();
Gtk3->main;
sub write_png_clicked {
my ($but, $canvas) = #_;
print "Write PNG...\n";
my $surface = Cairo::ImageSurface->create ('rgb24', 1000, 1000);
# also argb32 is available
# my $surface = Cairo::ImageSurface->create ('argb32', 1000, 1000);
my $cr = Cairo::Context->create($surface);
# make a background rectangle filled white so saved file looks same as screen
# otherwise a black background may appear, it's like pdf, if it isn't
# drawn , it will be a black background, It won't automagically pick up
# a white background on a canvas
$cr->rectangle( 0, 0, 1000, 1000 );
$cr->set_source_rgb( 1, 1, 1 );
$cr->fill;
$canvas->render($cr, undef, 1);
# this works, but see below for way to use pixbuf and jpg
# my $status = $surface->write_to_png ("$0.png");
# print "$status\n";
my $loader = Gtk3::Gdk::PixbufLoader->new;
$surface->write_to_png_stream (sub {
my ($closure, $data) = #_;
$loader->write($data);
});
$loader->close;
my $pixbuf = $loader->get_pixbuf;
print $pixbuf->get_bits_per_sample(),"\n";
print $pixbuf->get_colorspace(),"\n";
$pixbuf->save ("$0.png", 'png');
print "done png\n";
$pixbuf->save ("$0.jpg", 'jpeg', quality => 100);
print "done jpg\n";
return TRUE;
}

Answer 1

* unhandled exception in callback:
* `need' is not a valid cairo_status_t value; valid values are: success, no-memory, [...] at goopng2.pl line 90.
*** ignoring
at /usr/share/perl5/Gtk3.pm line 546.
By running the debugger on your code I could see that $loader->write($data) raised an exception:
need an array ref to convert to GArray
and write_to_png_stream() was not expecting this type of exception and truncated the message to the first word "need" as you can see from Glib error message at the top: `need' is not a valid cairo_status_t value ...
By some trial and error I found that I could pass the $buffer argument as an array of characters and not as a perl string:
sub write_png_clicked {
my ($but, $canvas) = #_;
print "Write PNG...\n";
my $surface = Cairo::ImageSurface->create ('rgb24', 1000, 1000);
my $cr = Cairo::Context->create($surface);
$cr->rectangle( 0, 0, 1000, 1000 );
$cr->set_source_rgb( 1, 1, 1 );
$cr->fill;
$canvas->render($cr, undef, 1);
my $loader = Gtk3::Gdk::PixbufLoader->new;
$surface->write_to_png_stream (
sub {
my ($loader, $buffer) = #_;
$loader->write([map ord, split //, $buffer]);
return TRUE;
}, $loader
);
$loader->close;
my $pixbuf = $loader->get_pixbuf;
print $pixbuf->get_bits_per_sample(),"\n";
print $pixbuf->get_colorspace(),"\n";
$pixbuf->save ("test.png", 'png');
print "done png\n";
$pixbuf->save ("test.jpg", 'jpeg', quality => 100);
print "done jpg\n";
return TRUE;
}
Edit:
To save only a part of the canvas you can pass a GooCanvasBounds parameter to the render() method:
my $bounds = GooCanvas2::CanvasBounds->new();
$bounds->x1(50);
$bounds->x2(250);
$bounds->y1(50);
$bounds->y2(250);
$canvas->render($cr, $bounds, 1);
Edit 2:
To capture a region at a specific position and a specific width and height:
my $img_width = 200;
my $img_height = 200;
my $img_x0 = 100;
my $img_y0 = 100;
my $surface = Cairo::ImageSurface->create ('rgb24', $img_width, $img_height);
$cr->translate(-$img_x0,-$img_y0);
$canvas->render($cr, undef, 1);