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From: "Joris" <joris.at.lebbeke@skynet.be>
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Subject: Re: [Tiff] Saving Exif with TiffLib
Date: Tue, 29 Jun 2004 15:04:11 +0200
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Alex wrote:
> Exif gets promoted to an IFD and there is nothing criminal because
> this directory shoudn't be visible in a common way. Offset to this
> directory will not be placed anywhere except the ExifTag. And
> any "good" reader will just skip unknown tag, as specified by
> Tiff standart.

Not true, I believe. For instance, a tiff concat that builds a multipage file
out of a series of exiffed single page files, will look upon the private tag as
an ordinary value tag, not a IFD pointer tag. I'm guessing the type is long and
count is 1, right? In that case, the concat app will see this as a normal 32bit
value, situated inside the IFD, and simply copy that value. Next, an EXIF aware
reader might come along, interpret the value as a pointer to an IFD, but this
pointer will now be invalid and the EXIF reader will not be able to make sense
of what is being pointed.

I believe the standard says that there should be no offsets in data (other then
the 'known offsets' like in the TileOffsets and SubIFD data and such). I believe
this is where the EXIF scheme fails to comply.

> The problem is how to write such subdirectory using
> another offset tag (not  subifd tag provided by TiffLib).

That's where you need the LibTiff Gods, Andrey and/or Frank. I couldn't say.

> But in
> any way, thanks for your letter, it is rather pleasant to get answer
> so quick.

No problem. The only TIFF testfile I've got that has EXIF data, is going about
it in another way. The first IFD contains the image. It points to the second IFD
in the usual way. This second IFD is the horrible EXIF IFD. This scheme is worse
then what you're after, in some regards, and better in others... But anyway, I'm
always on the lookout for new files in my testfile library, and I'd very much
appreciate receiving an EXIF TIFF that complies with the scheme you've talked
about, if you've got any. You can send it to joris.at.lebbeke@skynet.be, if you
wish.


Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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