Atacama has asked for the wisdom of the Perl Monks concerning the following question:

Hi.

I'm trying to get SDL and Cairo working together using the code below.

I expect Cairo to draw a red square inside the SDL window.

The program segfaults instead.

This page cairographics.org/SDL says that libraries should work fine together if they "agree on the pixel format"

I tried to change pixel format to 24/rgb24 and other permutations of available formats without success.

Am I missing something?

Thank you for your time.

use strict;
use warnings;

use SDL v2.3;
use SDL::Video;
use SDL::Event;
use SDL::Events;
use SDL::Surface;

use Cairo;

my $WIDTH  = 400;
my $HEIGHT = 300;

# relevant part starts:
my $screen = SDL::Video::set_video_mode($WIDTH, $HEIGHT, 32, SDL_SWSUR
+FACE);

SDL::Video::lock_surface($screen);

my $surface = Cairo::ImageSurface->create_for_data
(
  $screen->get_pixels_ptr,
  'argb32',
  $screen->w,
  $screen->h,
  $screen->pitch
);

my $cairo = Cairo::Context->create($surface);

$cairo->rectangle(0, 0, 100, 100);
$cairo->set_source_rgb(255, 0, 0);
$cairo->fill;

SDL::Video::unlock_surface($screen);
SDL::Video::update_rect($screen, 0, 0, $WIDTH, $HEIGHT);
# relevant part ends

LOOP: while (1)
{
  my $event = SDL::Event->new();
  
  while (SDL::Events::poll_event($event))
  {
    last LOOP if $event->type == SDL_KEYDOWN
              || $event->type == SDL_QUIT;
  }

  SDL::Events::pump_events();
}

SDL::quit;

[download]

Replies are listed 'Best First'.
Re: SDL and Cairo segfault
by Khen1950fx (Canon) on Jan 08, 2011 at 07:17 UTC
    Why use Cairo? SDL::Video can produce a beautiful red rectangle all by itself.

    The segmentation fault was happening because the video mode wasn't being set right, and you were missing SDL::Rect. Here's a script that works for me: 

    #!/usr/bin/perl

use strict;
no warnings;
use SDL;
use SDL::Video;
use SDL::Surface;
use SDL::Rect;

use constant WIDTH  => 400;
use constant HEIGHT => 300;

SDL::init( SDL_INIT_VIDEO );

my $screen = SDL::Video::set_video_mode( WIDTH, 
             HEIGHT, 32, SDL_SWSURFACE );

my $mcolor = SDL::Video::map_RGB($screen->format(),
                                 255, 0, 0);
SDL::Video::fill_rect($screen,
    SDL::Rect->new( 
        WIDTH / 4, HEIGHT / 4,
        WIDTH / 2, HEIGHT / 2), $mcolor);
        
SDL::Video::update_rect( $screen, 0, 0, WIDTH, HEIGHT );
sleep 10;
SDL::quit();

    [download]
    If that works, then you can add whatever you need to add. If you run into problems, post another question. Good luck.
[reply]
[d/l]

      Thank you for reply.

      The main reasons for cairo were to draw more interesting stuff than a red rectangle (which was used to make example code short) and to do it fast enough for a simple game.

      Your code worked perfectly, but would you like to explain what is the reason for "no warnings", please? Everybody seems to be advocating for "use warnings".

      I suddenly discovered SDL::GFX::Primitives namespace which looks suitable

      Just in case, here are two pieces of code: Perl version segfaults and C one works. Either they are not equivalent or perl-cairo and perl-sdl are not compatible

      Read more... (2 kB)
[reply]
[d/l]
[select]
        I used "no warnings" after I finished the script. Always "use warnings" is always the right thing, but, with SDL, it likes to keep sending out little messsages that aren't useful. That's why I used "no warnings".

        I adjusted your script a little. It produces a red rectangle png. Give it a test drive. 

        #!/usr/bin/perl

use strict;
use Cairo;
use SDL;
use SDL::Video;
use SDL::Surface;
use SDL::Rect;

use constant WIDTH  => 400;
use constant HEIGHT => 300;

SDL::init(SDL_INIT_VIDEO);

my $screen = SDL::Video::set_video_mode(WIDTH, 
                     HEIGHT, 32, SDL_SWSURFACE);

my $surface = Cairo::ImageSurface->create(
              'rgb24', WIDTH, HEIGHT);
my $cr = Cairo::Context->create($surface);

$0 =~ /(.*)\.pl/;
my $out = "$1.png";

$cr->rectangle (0, 0, WIDTH, HEIGHT);
$cr->set_source_rgb (255, 0, 0);
$cr->fill;
$cr->show_page;

$surface->write_to_png ($out);

SDL::quit();

        [download]
[reply]
[d/l]

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