Re: EXIF support in PNG

On Sun, Jan 1, 2017 at 1:18 PM, Willem van Schaik <[email protected]> wrote:

>
> #1 - we have to solve this!!
> looking at the EXIF metadata in eog, I noticed that one of the markers
> is "Compression" so maybe is a need for a change in the EXIF standard,
> namely to add "PNG Compression" (I'm talking here about the main
> picture, not the thumbnail)
>

PNG is not an EXIF supported format, so all that is happening is that a PNG
file is being used as a carrier for a blob of data from, probably, a
digital camera so that it can be restored at the end of the day.

Now, if PNG was an EXIF supported format then it would be necessary for
something that wrote such an EXIF to transcode the field; from
http://www.exiv2.org/tags.html (non-HTML readers can look the following up
under tag number 259):

0x0103 259 Image Exif.Image.Compression Short The compression scheme used
for the image data. When a primary image is JPEG compressed, this
designation is not necessary and is omitted. When thumbnails use JPEG
compression, this tag value is set to 6.
So 'Compression' is the compression of the last EXIF supported file format
or, maybe, depending on what PhotoShop actually does, of the original
camera data.  (I guess if I owned PhotoShop I'd put a JPEG camera file into
it and see what came out if I saved it as TIFF...)

If I were writing software to do this I would actually drop IFD[1], the
thumbnail, from the EXIF data and just store IFD[0] verbatim.  I would
probably come up with a tag exIF and use that, exactly as in JPEG APP1.


>
> it is probably tag #103 on page 28 of the JEITA/CIPA spec - or is that
> only for the thumbnail, anyway when the mail image is stored as PNG
> instead of JPEG, that must be reflected in the EXIF data
>

If it's in IFD[0] it refers to the EXIF image and *absent* means JPEG.  If
it is IFD[1] it refers to the thumbnail and *6* means JPEG.  At least I
think that is what the intention is.


>
> #2 - of lesser importance
> we could always request the CIPA folks to add in the EXIF standard PNG
> as a fourth compression method for thumbnails; which wouldn't require
> any change to the PNG standard, because from a PNG perspective we
> consider the EXIF chunk a "black box"
>

Yes, true, but personally I would rather let them sort it out.  I see the
HDR applications of 16-bit per channel PNG as something that happens as an
alternative to TIFF with floating point data or the well-known but not
widely used exponent+RGB format.  I can't imagine that camera manufacturers
would ever output it, particularly given things like SLog3, so I don't
imagine that the EXIF guys have any motivation whatsoever to support PNG.

-- 
John Bowler <[email protected]>
+1 (541) 450-9885
PO BOX 3151
KERBY OR 97531-3151
USA

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