Ah, animated GIFs. Truly the best thing on the Internet since cat videos!

How do people make those? I've no idea how other people do it, but I make 'em with Perl.

GD can create animated GIFs piecemeal. The simple steps involved are:

• Print out the result of gifanimbegin to start the gif
• Create a new GD image. For each image print out the result of calling gifanimadd, making sure to pass in the previous image you output so GD only has to write out the changed pixels.
• Finally print out the result of gifanimend

And that's all there really is to it.

#!/usr/bin/perl

use strict;
use warnings;

use GD::Image;

# constants
my $FONT_FACE = '/System/Library/Fonts/Supplemental/Bodoni 72 Smallcaps Book.ttf';
my $FONT_SIZE = 14;   # in pt
my $BLACK     = 0;    # color in the GIF89a color map
my $WHITE     = 100;  # color in the GIF89a color map

# write this out to a file
open my $out, ">:raw", "/tmp/out.gif";

# load the background image
my $image = background_image();

# start an animated gif
#   first argument means to use the global GIF89a color map
#   second argument means to turn on looping
print $out $image->gifanimbegin(1,1);

# send the background as the first frame of the animated gif
print $out $image->gifanimadd;

# process each line at a time
my $prev_frame = $image;
while (<DATA>) {
    chomp;
    my @words = split /\s+/, $_;

    # for each of the words that we're going to highlight
    # we need to write a new frame
    for my $word_index (0..$#words) {
        # create the GD image for that frame
        my $frame = draw_frame(
            words      => \@words,
            word_index => $word_index,
            line       => $line,
        );

        # add the frame.  Because we're enabling compression and
        # passing in the previous frame only the pixels that are
        # changed per frame are actually saved - not the entire
        # image of santa each time
        print $out $frame->gifanimadd(
            0,                 # dummy
            0,                 # dummy,
            0,                 # dummy,
            int(240 / @words), # two and a half seconds to read a line
            1,                 # enabling comparing to previous frame
            $prev_frame        # previous frame to compare to
        );

        # remember this frame for the next time round
        $prev_frame = $frame;
    }
}

# finally write out the end of the animated gif
print $out $image->gifanimend;

sub background_image {
    return GD::Image->new("/tmp/santa.jpg");
}

sub draw_frame {
    my %args = @_;
    my $new_image = background_image();

    my @words = @{ $args{words} };

    # "draw" the whole string as a class method, which doesn't
    # actually draw at all, but lets us know how wide the whole
    # string is so we can center it
    my $width = (GD::Image->stringFT(
        $BLACK,$FONT_FACE,$FONT_SIZE,0,
        0,
        0,
        join(' ', @words),
    ))[4];
    my $x = 250 - $width/2;

    # work out the three strings (left of / before hightlight,
    # the middle / hightlight, and then right of / after highlight)
    my $left_of  = join '', map { "$_ " } splice @words, 0, $args{word_index};
    my $middle   = shift( @words ) . ' ';
    my $right_of = join ' ', @words;

    # draw each string.  stringFT returns the bounds as an array.
    # Index 4 of this array is the x in the top right corner, i.e.
    # what we want to assing to $x so we start drawing there again.
    $x = ($new_image->stringFT(
        $BLACK, $FONT_FACE, $FONT_SIZE, 0,
        $x,
        425,
        $left_of,
    ))[4];

    $x = ($new_image->stringFT(
        $WHITE, $FONT_FACE, $FONT_SIZE, 0,
        $x,
        425,
        $middle,
    ))[4];

    $new_image->stringFT(
        $BLACK, $FONT_FACE, $FONT_SIZE, 0,
        $x,
        425,
        $right_of,
    );

    return $new_image;
}

__DATA__
Twas the night before Christmas, when all through the house
Not a creature was stirring, not even a mouse;
The stockings were hung by the chimney with care,
In hopes that St. Nicholas soon would be there;
The children were nestled all snug in their beds,
While visions of sugar-plums danced in their heads;
And mamma in her kerchief, and I in my cap,
Had just settled our brains for a long winter's nap
When out on the lawn there rose such a clatter,
I sprang from my bed to see what was the matter,
Away to the window I flew like a flash,
Tore open the shutters and threw up the sash.
The moon, on the breast of the new-fallen snow,
Gave a lustre of mid-day to objects below;
When, what to my wondering eyes should appear,
But a miniature sleigh, and eight tiny rein-deer,
With a little old driver, so lively and quick,
I knew in a moment it must be St. Nick.
More rapid than eagles his coursers they came,
And he whistled, and shouted, and called them by name;
"Now, Dasher! now, Dancer! now, Prancer and Vixen!
On! Comet, on! Cupid, on! Dunder and Blitzen—
To the top of the porch, to the top of the wall!
Now, dash away, dash away, dash away all!"
As dry leaves that before the wild hurricane fly,
When they meet with an obstacle, mount to the sky,
So, up to the house-top the coursers they flew,
With a sleigh full of toys—and St. Nicholas too.
And then in a twinkling I heard on the roof,
The prancing and pawing of each little hoof.
As I drew in my head, and was turning around,
Down the chimney St. Nicholas came with a bound.
He was dressed all in fur from his head to his foot,
And his clothes were all tarnished with ashes and soot;
A bundle of toys he had flung on his back,
And he looked like a peddler just opening his pack;
His eyes how they twinkled! his dimples how merry!
His cheeks were like roses, his nose like a cherry;
His droll little mouth was drawn up like a bow,
And the beard on his chin was as white as the snow;
The stump of a pipe he held tight in his teeth,
And the smoke, it encircled his head like a wreath.
He had a broad face, and a little round belly
That shook when he laughed, like a bowl full of jelly.
He was chubby and plump—a right jolly old elf;
And I laughed when I saw him in spite of myself.
A wink of his eye, and a twist of his head,
Soon gave me to know I had nothing to dread.
He spoke not a word, but went straight to his work,
And filled all the stockings; then turned with a jerk,
And laying his finger aside of his nose,
And giving a nod, up the chimney he rose.
He sprang to his sleigh, to his team gave a whistle,
And away they all flew like the down of a thistle;
But I heard him exclaim, ere he drove out of sight,
"Merry Christmas to all, and to all a good night!"

Maybe for my next gif I'll include the entire works of Shakespeare...