Re: DRAFT: eXIF 2017-05-28

On Wed, May 31, 2017 at 8:36 AM, Pavel Zlatovratskii <[email protected]> wrote:

> But I think that purely historical safe-to-copy eXIf should be explicitly
> forbidden to use any other way, while current eXIF draft allows it to use
> EXIF data as property of current image itself.
>
> This includes any kinds of PNG-to-JPEG (or PNG-to-TIFF or PNG-to-WebP)
> conversions. And for current eXIF proposal recommendations about fitting
> into APP1 looks like suggestions of direct using eXIF chunk as APP1. Actual
> eXIF may be used for direct APP1 in some cases, but historical eXIf never
> should be used this way.

About simple conversions to/from APP1, we might want to admit the
possiblility that the 6-byte Exif00 might be present (see the most recent
zXIf proposal, which has among the "Recommendations for Decoders"
section that Exif00 might be present, is harmless, and should be skipped/
ignored).

>
> So no simple PNG-to-EXIF conversion will be allowed for eXIf (I still doubt
> that such conversion could be simple enough for eXIF).

If the image data has been changed.  It is safe to do that when the image
has not changed.  I would expect all of the Exif profile to survive a simple
JPEG -> PNG -> JPEG round trip, although it would not bother me if
the JPEG-specific data were to be lost.

> And vice versa: no
> simple EXIF-to-PNG conversion with eXIf as any valuable data that could be
> applied to image must be copied to appropriate PNG chunks (incl. resolution,
> gamma, change date, software etc..).
>
> Also some values of EXIF are unsafe-to-copy and have no appropriate PNG
> chunk: orientation, subject area/location. Not sure about digital zoom and
> camera elevation.

In fact I've added a private "orNT" chunk to ImageMagick/GraphicsMagick
to store image->orientation.  I haven't tried to address subject area/location.

Glenn

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot