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I'm trying to load the jpg out of an mp3 (I'm doing this in Flash, but that's neither here nor there), and I've pulled open a few MP3s in a hex editor and while the rest of the tag is pretty clear, the image part is a mystery. There is a label "image/jpeg" but then I expect something resembling a byte length for the image or a string key that demarcates the end of the image, but I see nothing of the kind and the documentation on the tag is entirely unhelpful.

Would love some guidance on what I am looking for here. Surprised at how unintuitive this is turning out!

TIA

Yevgeny Simkin
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2 Answers2

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While I'd normally advocate msw's solution, that site doesn't seem to have a lib for Flash. However, it DOES have the ID3v2 specification. If you look at section 4., ID3v2 frame overview, you'll see the frame header includes the frame's size, not including the header. I believe it's safe to assume the image size can be calculated from there.

PD: I'm assuming you know how ID3v2 tags work. In case you don't, I recommend you give at least a cursory glance at the spec. Basically, there's a header, an extended header, then one or more frames containing the tag's data (one frame per datum) and finally padding.

EDIT: For testing purposes I opened one of my MP3s with known tag data in my hex editor. Found a frame named APIC, copied the specified number of bytes into a new file and saved it as test.jpg. Didn't quite work.

Digging further, I found the frame spec. Specifically, Section 4.14, attached picture, which describes a "subheader" of sorts for the frame (1-byte text encoding, zero-delimited mimetype, 1-byte picture type and a zero-delimited description). So I trimmed that information out of the data and copied the remainder onto a new jpg, which Windows was able to show.

Therefore, the technique is:

  • Acquire frame size from frame header.
  • Parse out the beginning of the frame for the frame subheader and remove it. (Anywhere between 4 and infinite bytes)
  • Calculate image size based on remaining number of bytes.
Mauricio
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  • yep... I get that, but the documentation is a bit dense. I was hoping that the fields would be a bit more straight forward. The first few fields (track name, artist name, etc) are all totally obvious, but then the image tag, followed by a fairly long sequence of characters that are the same in every mp3, so, they must be some sort of indicator, but what do they mean? and then the image data which has no obvious END... I'm surprised that this is the "standard". :) – Yevgeny Simkin Apr 16 '11 at 20:00
  • Well, turns out it IS standard, just documented in another place. Check out the edit. – Mauricio Apr 17 '11 at 00:59
  • @Dr.Dredel: Do you get notified if you don't add the @part? I've always wondered... – Mauricio Apr 17 '11 at 16:09
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Don't reinvent that wheel unless you like pain. This is but one source for libraries that do the work for you.

msw
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  • ha ha. No, I'm not looking for misery, but I do want to build this in flash and can't find a library that does this PLUS I refuse to believe that this is very complicated. – Yevgeny Simkin Apr 16 '11 at 15:43
  • the more I look the more I am convinced that this is more complicated than I was initially convinced it wasn't :) – Yevgeny Simkin Apr 16 '11 at 20:01