Can I programmatically determine if a PNG is animated?

Question

I have PNG (as well as JPEG) images uploaded to my site.

They should be static (i.e. one frame).

There is such thing as APNG.

Answer 1

APNG images are designed to be "camouflaged" as PNG for readers that not support them. That is, if a reader does not support them, it will just assume it is a normal PNG file and display only the first frame. That means that they have the same MIME type as PNG (image/png), they have the same magic number (89 50 4e 47 0d 0a 1a 0a) and generally they're saved with the same extension (although that is not really a good way to check for a file type).

So, how do you distinguish them? APNG have a "acTL" chunk in them. So, if you search for the string acTL (or, in hex, 61 63 54 4C (the 4 bytes before the chunk marker (i.e. 00 00 00 08) are the size of the chunk in big endian format, without counting the size, marker, or CRC32 at the end of the field)) you should be pretty good. To get it even better, check that this chunk appears before the first occurrence of the "IDAT" chunk (just look for IDAT).

This code (taken from http://foone.org/apng/identify_apng.php ) will do the trick: