Can I detect animated gifs using php and gd? - php

Question

I'm currently running into some issues resizing images using GD.
Everything works fine until i want to resize an animated gif, which delivers the first frame on a black background.
I've tried using getimagesize but that only gives me dimensions and nothing to distinguish between just any gif and an animated one.
Actual resizing is not required for animated gifs, just being able to skip them would be enough for our purposes.
Any clues?
PS. I don't have access to imagemagick.
Kind regards,
Kris

Answer 1

While searching for a solution to the same problem I noticed that the php.net site has a follow-up to the code Davide and Kris are referring to, but, according to the author, less memory-intensive, and possibly less disk-intensive.
I'll replicate it here, because it may be of interest.
source: http://www.php.net/manual/en/function.imagecreatefromgif.php#88005
function is_ani($filename) {
if(!($fh = #fopen($filename, 'rb')))
return false;
$count = 0;
//an animated gif contains multiple "frames", with each frame having a
//header made up of:
// * a static 4-byte sequence (\x00\x21\xF9\x04)
// * 4 variable bytes
// * a static 2-byte sequence (\x00\x2C)
// We read through the file til we reach the end of the file, or we've found
// at least 2 frame headers
while(!feof($fh) && $count < 2) {
$chunk = fread($fh, 1024 * 100); //read 100kb at a time
$count += preg_match_all('#\x00\x21\xF9\x04.{4}\x00[\x2C\x21]#s', $chunk, $matches);
}
fclose($fh);
return $count > 1;
}

Answer 2

There is a brief snippet of code in the PHP manual page of the imagecreatefromgif() function that should be what you need:
imagecreatefromgif comment #59787 by ZeBadger

Answer 3

Here's the working function:
/**
* Thanks to ZeBadger for original example, and Davide Gualano for pointing me to it
* Original at http://it.php.net/manual/en/function.imagecreatefromgif.php#59787
**/
function is_animated_gif( $filename )
{
$raw = file_get_contents( $filename );
$offset = 0;
$frames = 0;
while ($frames < 2)
{
$where1 = strpos($raw, "\x00\x21\xF9\x04", $offset);
if ( $where1 === false )
{
break;
}
else
{
$offset = $where1 + 1;
$where2 = strpos( $raw, "\x00\x2C", $offset );
if ( $where2 === false )
{
break;
}
else
{
if ( $where1 + 8 == $where2 )
{
$frames ++;
}
$offset = $where2 + 1;
}
}
}
return $frames > 1;
}

Answer 4

This is an improvement of the current top voted answer but I don't have enough reputation to comment yet. The problem with that answer is that it reads the file in 100Kb chunks and the end of frame marker might be split in between 2 chunks. A fix for that is to add the last 20b of the previous frame to the next one:
<?php
function is_ani($filename) {
if(!($fh = #fopen($filename, 'rb')))
return false;
$count = 0;
//an animated gif contains multiple "frames", with each frame having a
//header made up of:
// * a static 4-byte sequence (\x00\x21\xF9\x04)
// * 4 variable bytes
// * a static 2-byte sequence (\x00\x2C) (some variants may use \x00\x21 ?)
// We read through the file til we reach the end of the file, or we've found
// at least 2 frame headers
$chunk = false;
while(!feof($fh) && $count < 2) {
//add the last 20 characters from the previous string, to make sure the searched pattern is not split.
$chunk = ($chunk ? substr($chunk, -20) : "") . fread($fh, 1024 * 100); //read 100kb at a time
$count += preg_match_all('#\x00\x21\xF9\x04.{4}\x00(\x2C|\x21)#s', $chunk, $matches);
}
fclose($fh);
return $count > 1;
}

Answer 5

Reading whole file with file_get_contents may take too much memory if the given file is too large. I've re-factored the function previously given which reads just enough bytes to check frames and returns as soon as it finds at least 2 frames.
<?php
/**
* Detects animated GIF from given file pointer resource or filename.
*
* #param resource|string $file File pointer resource or filename
* #return bool
*/
function is_animated_gif($file)
{
$fp = null;
if (is_string($file)) {
$fp = fopen($file, "rb");
} else {
$fp = $file;
/* Make sure that we are at the beginning of the file */
fseek($fp, 0);
}
if (fread($fp, 3) !== "GIF") {
fclose($fp);
return false;
}
$frames = 0;
while (!feof($fp) && $frames < 2) {
if (fread($fp, 1) === "\x00") {
/* Some of the animated GIFs do not contain graphic control extension (starts with 21 f9) */
if (fread($fp, 1) === "\x2c" || fread($fp, 2) === "\x21\xf9") {
$frames++;
}
}
}
fclose($fp);
return $frames > 1;
}

Answer 6

Animated GIF must have the following string
"\x21\xFF\x0B\x4E\x45\x54\x53\x43\x41\x50\x45\x32\x2E\x30"
I've tested on a few animated gifs and it seem the string is at the pos 781 of a file (found with file_get_contents and strpos)