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I am having a bit of trouble getting Gravatars to work properly:

When I request the following:

http://gravatar.com/avatar/8a17d0d0d8bdf6a8d527bbc943a17cf8.jpg?s=64&d=identicon

Firefox proudly displays the following:
http://files.quickmediasolutions.com/gravatar_p.png

...indicating that the file is a PNG image.

This confuses me - I thought Gravatars were JPEG images. It seems like they can be either. How can I find out if a given image is PNG or JPEG preferably without downloading it first?

Note: Some people are reporting that Gravatar only returns PNG images. Please explain this:

http://files.quickmediasolutions.com/gravatar_p2.png

http://gravatar.com/avatar/03cd042b82ac85b2c5fe0757a94e0413?s=64&d=identicon
Nathan Osman
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2 Answers2

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If Gravatar icons have accurate MIME types assigned by the server you're accessing them from, just check that. It should be image/jpeg for JPEGs and image/png for PNGs.

Failing that...

http://en.wikipedia.org/wiki/Portable_Network_Graphics#File_header

A PNG file starts with an 8-byte signature. The hexadecimal byte values are 89 50 4E 47 0D 0A 1A 0A; the decimal values are 137 80 78 71 13 10 26 10.

So just check the eight bytes at the beginning of the file; if it's a PNG, it'll have the stated values in those bytes, and if not, it won't. Just download the file, possibly store it somewhere temporarily (which shouldn't be too hard considering it shouldn't be too big), and process it differently depending on what the header contains. You can always change the file extension and then use PHP's graphics library if you saved it as the wrong type at first. (Or are you not allowed to do that?)


As a side note, my favorite bit about the PNG header:

50 4E 47 In ASCII, the letters PNG, allowing a person to identify the format easily if it is viewed in a text editor.

JAB
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  • True. But don't JPEG files have `'JFIF'` in there somewhere? Anyway, thanks. – Nathan Osman Jun 25 '10 at 20:35
  • @George: Not necessarily (though generally they do have the JFIF header). http://en.wikipedia.org/wiki/JPEG#JPEG_files It's just that the PNG header popped into my head first. – JAB Jun 25 '10 at 20:52
  • In my experience, the `Content-Type` headers on `gravatar.com` are trustworthy, so you can determine it without downloading the whole image: just fire a HTTP HEAD hequest, for example: `curl --head 'http://gravatar.com/avatar/03cd042b82ac85b2c5fe0757a94e0413?s=1'` or `curl --head 'http://gravatar.com/avatar/6dd5fae7d8f0c38b515a19a6ca4ca41d?s=1'` – ecmanaut May 30 '12 at 22:15
  • @ecmanaut: I wasn't very well-informed on the workings of the HTT protocol at the time I wrote this answer, so thank you for making that note (though it's probably a bit too late to be useful for George). Funnily enough, we never actually utilized the HEAD method or did anything with mimetypes in the networking class I just took... – JAB May 31 '12 at 12:55
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The end tag is JPG, but the string is interpreted by the server and the corresponding image is sent. In this case a PNG.

Josh K
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  • Are *all* Gravatars PNG images, then? If not, how can I tell on an individual basis which format it is? – Nathan Osman Jun 25 '10 at 19:28
  • @George, yes they are all sent as PNG's. – Josh K Jun 25 '10 at 19:33
  • @George: I thought they were all sent as PNG's. I guess some are and some aren't. I'm guessing the different type of gravatar would determine the type it sends. – Josh K Jun 25 '10 at 19:46
  • @Josh: How can I figure out which one I'm getting? I'm processing them with PHP's GD library, so I have to know ahead of time. – Nathan Osman Jun 25 '10 at 19:48