Re: DRAFT: eXIf 2017-03-03

On Fri, Mar 3, 2017 at 7:32 AM, Pavel Zlatovratskii <[email protected]> wrote:
> It will be treated as IFD[0].
>
> There is no specific pointer for IFD[0] or IFD[1]. IFD is singly linked
> list structure. TIFF header point to first(0) IFD, then first IFD point
> to second IFD (IFD[1]) in field "next IFD"...

That's not what Glenn wrote.  He defined the "0th IFD" to be the one
which contains either the EXIF or GPS offset.  It might be the first
IFD or it might be the second, which he referred to as the "1st".
Here it is again:

>> It contains a TIFF header plus a 0th IFD (Image File Directory)
>> and/or a 1st IFD as defined in the Exif specification; the 0th IFD
>> contains pointers to an Exif sub-IFD and/or a GPS sub-IFD. The
>> optional 1st IFD contains a thumbnail image.

Note that in his definition the Exif 'sub-IFD' pointer is not mandatory.

The translated CIPA spec is unambiguous, even though it mis-translates
the Japanese for "first" as 0th, because, as you observe, in figure 6
page 18 it gives the file structure.  I assume the normative Japanese
version is more clear.

What is missing, of course, is the "PNG" column for tables 17, 18 and
19.  However I believe we should assume that users of an EXIF chunk
will be more familiar with EXIF than most people on this list and will
be *better* able to get the missing columns right than I am.

I do, however, believe that pointing people in the direction of the
JPEG section of the spec is good because that is closest to the PNG
section.  I'm also unwilling to see the PNG version of the columns
written down because quite a lot of the stuff that is "M" for JPEG, is
"P" for PNG:

>>> P : It is not allowed to record (included in PNG chunk)

E.g. the Table 17 "Image resolution" fields, which are in pHYs

I'm absolutely certain software engineers will just copy stuff from
either TIFF or JPEG without checking it and I don't want to get into
the business of checking the format in a PNG decoder (as I said
before; the ICC Profile mess in libpng.)

John Bowler

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