I had a task: to write a program that makes text from animated ‘gif’ images of letters. For example, the following image:
Usually, the PHP extension ‘GD’ is good enough to perform the job. GD is an image processing extension, that allows a developer to create canvasses, place images of common formats in there, save your picture to an image file of your favorite format or to a string, and other operations. This time, GD generated the big image, but without the animation. But GD is still useful if you learn about the structure of a GIF file, and you can get all the relevant information in this text file.
The Structure of a GIF File Made Simpler
A GIF File consists of the following:
- File Header, begins with ‘GIF’ followed by ’87a’ or ’89a’ and 7 more bytes.
- There may be a global color map, depending on byte 10. It’s size is calculated according to the 3 rightmost bytes of byte 10.
- Extensions that begin with a byte of value 21h (“\x21”), including graphic control (“\x21\xf9”), text (“\x21\x01”), app data(“\x21\xff”), and comments (“\x21\xfe”).
- Image Separator (“\x2c”).
- A file Terminator (‘|x3b”).
The file can be processed by reading the header and using a state machine as follows:
// Read the header
$gif_string = my_fread($fd, 13);
// Get the packed field;
$value = ord(substr($gif_string, 10, 1));
$globalMapExists = $value & 0x80;
if ($globalMapExists){
$tmp=($value & 0x07) + 1;
$globalMapSize = 3 * (1 << $tmp);
} else
$globalMapSize = 0;
// Read Global Color Map
if ($globalMapExists){
$globalColorMap = my_fread($fd, $globalMapSize );
$gif_string .= $globalColorMap;
}
// Data begins here.
// Data blocks begin with:
// 0x2C - Image Descriptor
// 0x21 - Extensions: Graphic Control, Comment, Text, Application.
$curr_frame = -1;
$gc_encountered = FALSE;
while (($chr = my_fread($fd, 1)) != "\x3B"){
switch (ord($chr)){
case 0x21:
$next_chr = my_fread($fd, 1);
switch (ord($next_chr)){
case 0xf9:
$curr_frame++;
$frames[] = $gif_string . chr(0x21) . chr(0xf9) .
read_frame($fd, $curr_frame);
$gc_encountered = TRUE;
break;
case 0x01:
if (!$gc_encountered){
$frames[0] = $gif_string;
$curr_frame = 0;
}
$frames[$curr_frame] .= chr(0x21) . chr(0x01) . read_text($fd);
break;
case 0xff:
$gif_string .= chr(0x21) . chr(0xff) . read_app_data($fd);
break;
case 0xfe:
$gif_string .= chr(0x21) . chr(0xfe) . read_comment($fd);
break;
}
break;
case 0x2c:
if (!$gc_encountered){
$frames[0] = $gif_string;
$curr_frame = 0;
$gc_encountered=TRUE;
}
$frames[$curr_frame] .= chr(0x2c) . read_image($fd, $curr_frame);
break; }
}
Doing it With GD
The steps of the program are as follows:
- Create an image resource for each letter using the function $res=imagecreatefromgif($url).
- Get the width and height of each letter using imagesx($res) and imagesy($res). Use these values to find the size of the output image.
- Split each image into frames using the state machine.
- Create the output frames using ‘imagecreate($total_width, $total_height);’Note: the image created by GD is not stored in GIF format.
- If the frame is new set its transparent background using the following commands:
- $tci = imagecolortransparent($frame_res); // Get the transparent color index from the original frams.
- imagepalettecopy($out_frames[$curr_frame_no],$frame_res); // Copy the palette to the output frame.
- imagefill($out_frames[$curr_frame_no], 0, 0, $tci); // Fill the output frame with the transparent color
- imagecolortransparent($out_frames[$curr_frame_no], $tci); // Set the output frame’s transparent color.
- Place each letter’s frame in the output frame using the command:
imagecopymerge($out_frames[$curr_frame_no], // Destination image $frame_res, // Source image $pos_in_pic + $left_positions[$curr_frame_no], // Destination x, $top_positions[$curr_frame_no], // Destination y, 0, //$left_positions[$curr_frame_no], // Source x, 0, //$top_positions[$curr_frame_no], // source y, $width, $height,100);
- Now, use a state machine similar to the one described above to create the output GIF file.
