[Libexif-devel] Re: [Fwd: Re: file size decreases after exif --set-value? (relly)]

[Al E. <al...@tb...>]

At 1:02 AM +0200 8/1/03, Lutz M=FCller wrote:
>Hi!
>
>The problem has already been discovered by Al Evans. There is no fix yet
>with regard to saving the modified exif data without loosing the
>information accessible through the MakerNote.
>
>One solution could be to move (on saving) all data associated with the
>MakerNote into the MakerNote entry itself. I haven't verified if this is
>possible. But this would mean to drop the strict separation of libexif
>and libmnote.
>
>I am not sure yet.
>--
>Lutz M=FCller <lu...@us...>
>
>>----------------------------------
>>Duane H. Hesser <dh...@mo...>
>>----------------------------------
>>BTW, the size change that you observed has nothing to do with the
>>"problem" mentioned above.  Most of the size change is due to a
>>"hole" in the original image after IFD1, before the primary JPEG
>>data starts.
>>
>>--

According to my results, this is not really a "hole". The MakerNote, 
in all cases that I know of, has a similar structure to an EXIF IFD. 
That is to say, when the data doesn't fit into the last four bytes of 
an entry, those bytes contain a pointer. For the Olympus E20 
MakerNote (at least), the pointers point into this "hole". Here's 
part of an email I sent Lutz this afternoon:

-----
>Do you tell me that your camera writes EXIF data with pointers in the
>tag MakerNote to regions outside the MakerNote tag? So, if the MakerNote
>tag has a size of 600 bytes, it has in reality a size of more than that?

Precisely. If you think about it, there is no other option. If the 
data is > 4 bytes, it CAN'T put it within the MakerNote tag, since 
that tag consists of 12-byte fields. The Olympus E10 and E20 are the 
only current cameras I know that record such data in the MakerNotes, 
but any camera that did so, would have to use bytes outside the 
MakerNote tag. For example, though I have no idea what it means, 
Olympus E20 tag 0x1033 is an array of 720 shorts, with an offset of 
0x0f34 (the MakerNote is under 900 bytes):

00002d0: 0003 0000 0001 ffff 0000 1033 0004 0000  ...........3....
                                   ----
00002e0: 02d0 0000 0f34 1034 0005 0000 0001 0000  .....4.4........
          ----      ----
-----

					--Al Evans--