Re: EXIF support in PNG

but how can we have a PNG image with an EXIF chunk that says that the 
format of the image is JPEG ??

again, not talking about the thumbnail

yep, coordination of this will be very complicated, but it needs to be 
resolved

Willem





On 2017-01-01 17:17, John Bowler wrote:
> On Sun, Jan 1, 2017 at 1:18 PM, Willem van Schaik <[email protected]
> <mailto:[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]
> <mailto:[email protected]>>
> +1 (541) 450-9885
> PO BOX 3151
> KERBY OR 97531-3151
> USA
>
>
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-- 

Willem van Schaik
<[email protected]>
http://www.schaik.com/


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