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Date: Fri, 01 Oct 2004 20:43:19 -0700
From: Chris Cox <ccox@adobe.com>
Subject: Re: [Tiff] Re: BigTIFF extension issue
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At 3:21 AM +0200 10/2/04, Joris wrote:
>  > And we can't go back and update Photoshop 4.0 (still in use!) to
>>  support BigTIFF, or to make it give a reasonable  error message --
>>  it's just going to say "this isn't a valid TIFF file".
>
>I assume that Photoshop 1.0 is saying the same about a tiled TIFF. (I may of
>course have version numbers and dates incorrect, but I mean to address the
>general issue, of course.) Why was this not a problem back then?

It has always been a problem with newer versions of some file formats.



>As far as
>Photoshop 4.0 is concerned, BigTIFF is indeed not a valid TIFF file. It is not
>uncommon for applications to not be able to foresee future 
>incompatible versions
>of file formats, users do know that, and expect Photoshop 4.0 to eg not know
>what a DNG file extension means. That is merely logical, no user expect up to
>date behaviour from an outdated version.

No, users DO expect that TIFF will be compatible across versions.
They raised quite a stink (threatening boycotts) when Photoshop added 
FLATE compression and transparency support to TIFF because their 
older applications couldn't deal with it.



>Incidently, Photoshop 7.0 is unable to read my YCbCr subsampled 
>images. There's
>plenty of other TIFFs it can't read. And that's pretty normal, anyone used to
>TIFF knows that even the best of apps usually supports the reading 
>of a limited
>subset of TIFF.

No, Engineers know that - customers don't.



>I'd like to bring some focus to the most general question: is this a new file
>format, or a new version of an existing format? It wouldn't hurt to draw the
>logical conclusion about the file extension and such, after answering that
>question - that is, I think, the way it's always been done before. 
>Right? Nobody
>even suggested changing name to 'Tiled TIFF' and extension to '.ttf', I'm
>reasonably sure.

It is a new version of a file format.
But that doesn't mean that customers will understand that distinction.
(they didn't understand PSD 2.0, so we had to give it a new name and 
extension: PSB).



>From this point of view, current applications refusing the new BigTIFF, has
>nothing to do with the current applications, but everything with the fact that
>we chose to not design BigTIFF to be backwards compatible with classic TIFF
>readers, for which we had very good reasons. One backwards 
>compatible option was
>explored (http://www.asmail.be/msg0055356585.html), but it was quite rightly
>quickly judged to be way too much hassle and way to ugly.
>
>I feel the primary issue is: Is BigTIFF a new version? Or is it a new file
>format? Let's decide, and be consistent, is what I propose. Do you 
>plan to write
>a new spec, or write a new version of the spec, that should give 
>some indication
>as to this question, and do we plan to design new codecs, or write 
>new versions
>of the existing ones? Seems that's all very related. If this is merely a new
>version of an existing file format... It is extremely uncommon at 
>the very least
>to have version dependent file extensions, is it not?

Engineering concerns can be consistent.
But end users are only consistent in one way: they just want it to work.

Chris
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