[Bug 783625] typefinding: add support for jpc or j2c

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=783625

--- Comment #4 from Aaron Boxer <boxerab at gmail.com> ---
Right. Just to summarize the different formats:

JP2 boxes are in the format 

L T BBBBBBBBBBBBBBBBBBBBBBBBB

where :

L is a 4 byte number storing the length of the box, including L,
T is a 4 byte type, 
BBBBBBBBBBBBBBBBBBBBBBBBBb are the bytes in the box.



0) JPC is the raw jpeg 2000 code stream, starting with the SOC code stream
marker : magic bytes 0xFF4FFF51


1) JP2 has the JP2 signature at the beginning of the file

  /* jp2 signature */
  if (memcmp (data, "\000\000\000\014jP  \015\012\207\012", 12) != 0)
    return;


JP2 also contains a contiguous codestream box of type T = "jp2c", and length L
= 8;

It also contains a JPC file inside (raw code stream)



2) J2C format is identical to JPC except that it has a contiguous codestream
box at the beginning, but unlike in JP2, the length L equals the length of the
entire frame.

So, to find J2C, we can search for XXXX MAKE_FOURCC("jp2c") 0xFF4FFF51
where XXXX match any bytes.


And to find JPC, we need to just search for 0xFF4FFF51. Not sure if this is
enough to be unique.

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