Ben Bullock


Image::PNG::Cairo - extract PNG data from a Cairo::ImageSurface


    use Image::PNG::Cairo 'cairo_to_png';
    use Cairo;
    my $surface = Cairo::ImageSurface->new ('argb32', 100, 100);
    # Draw something on surface.
    my $png = cairo_to_png ($surface);
    # Now can use the methods of Image::PNG::Libpng on the PNG,
    # e.g. write to file.

(This example is included as in the distribution.)


This documents Image::PNG::Cairo version 0.09 corresponding to git commit 615fa499479dd235024385a5f4d09f558e3d1ec8 released on Thu Oct 5 13:15:48 2017 +0900.


This is a bridge between Cairo and Image::PNG::Libpng which allows the user to extract the image data from a Cairo::ImageSurface into a structure which can then be manipulated to add other kinds of data.



    my $png = cairo_to_png ($surface);

The input, $surface, is a Cairo::ImageSurface object. Only Cairo surfaces of type 'argb32' are supported. The return value is an Image::PNG::Libpng object, with its image dimensions, bit depth, colour type, and image data taken from $surface. $png corresponds to the return value of the Image::PNG::Libpng method create_write_struct with the methods set_IHDR and set_rows and set_transforms applied to it using data extracted from $surface.

$png is set up using the Image::PNG::Libpng method set_transforms such that the PNG image data is transformed on writing from the Cairo image data format to the PNG format.



This example CGI program makes a captcha from random letters. It also includes the captcha text and the date and time of creation of the image in the PNG.

    use lib '/home/protected/lib';
    use CGI::Carp 'fatalsToBrowser';
    use Cairo;
    use Image::PNG::Cairo 'cairo_to_png';
    use Image::PNG::Libpng ':all';
    use Image::PNG::Const ':all';
    use constant { M_PI => 3.14159265 };
    my $xsize = 200;
    my $ysize = 50;
    my $surface = Cairo::ImageSurface->create ('argb32', $xsize, $ysize);
    my $cr = Cairo::Context->create ($surface);
    # Make a background
    $cr->set_source_rgb (0, 0, 0);
    $cr->rectangle (0, 0, $xsize, $ysize);
    $cr->fill ();
    # Get six randomly-chosen letters
    my $captcha = random_letters (6);
    # Draw the captcha text in white
    my $gap = 10;
    $cr->set_source_rgb (1, 1, 1);
    $cr->set_font_size ($ysize - $gap);
    $cr->move_to ($gap, $ysize - $gap);
    $cr->show_text ($captcha);
    $cr->fill ();
    # Obscure the text with translucent circles in random colours.
    for (0..50) {
        $cr->set_source_rgba (random_colours (), 0.4);
        $cr->arc (rand ($xsize), rand ($ysize), rand (25), 0, 2 * M_PI);
        $cr->fill ();
    # Get the PNG data out of it.
    my $png = cairo_to_png ($surface);
    # Put the captcha into the PNG itself, and set a modification time.
    $png->set_text ([{compression => PNG_TEXT_COMPRESSION_NONE,
                      key => 'captcha', text => $captcha}]);
    $png->set_tIME ();
    # Get the PNG data from $png and print it out.
    my $data = write_to_scalar ($png);
    binmode STDOUT;
    print "Content-Type: image/png\r\n\r\n$data";
    sub random_colours
        my @r;
        for (1..3) {
            push @r, rand (1);
        return @r;
    sub random_letters
        my ($length) = @_;
        my @letters = ('0' .. '9', 'a' .. 'z', 'A' .. 'Z');
        my $r = '';
        for (1..$length) {
            $r .= $letters[rand (@letters)];
        return $r;

(This example is included as in the distribution.)

Please do not use this example program in the stead of a real captcha.



Cairo is a general-purpose drawing program.


Image::PNG::Libpng is a Perl wrapper to the C PNG library libpng.


Ben Bullock, <>


This package and associated files are copyright (C) 2014-2017 Ben Bullock.

You can use, copy, modify and redistribute this package and associated files under the Perl Artistic Licence or the GNU General Public Licence.