Re: Endless gamma in eXIf 2017-0119

On 07/02/17 12:33 AM, John Bowler wrote:
> I think people need to consider this issue carefully when voting, it
> would also be helpful to know when a voter felt that the issue was
> decisive one way or the other, just like Pavel did.
>
> Certainly there are two possible approaches.  The one used in both the
> eXIf and zXIf proposals allows an EXIF chunk aware PNG editor to store
> the *original* information even though it may no longer apply.
>
> This happens because the "security considerations" section permits it;
> if that section was not there, regardless of the safe-to-copy bit, an
> editor would have to ensure the consistency of the information either
> by updating it or discarding it.  This is mandated by the section of
> the ISO-PNG spec that I quoted.
>
> So, by default, the chunk would have to be eXIF (not safe to copy) and
> editors which supported it would have to either modify the chunk or
> completely remove it when the image was changed.  If you want that
> vote NO on both proposals.  If you want the behavior with Soni L.
> described and are satisfied that the specification (compressed or not)
> has adequate description for decoders then you might choose to vote
> YES on on or the other, or both, proposals.
>
> Notice that the current proposals are not intended to prevent the use
> of the image-related metadata from the chunk; rather it is the case
> that it may be outdated and, as in Glenn's edited examples, there may
> be no indication of this.  As Soni L. observed:
>
>> Shouldn't eXIf be considered "sensor/camera data" and thus only useful for ppl who >know what they're doing, and not automatically mangled into the image?
> Yes; that's how I see it.  It's used to allow a digital camera image
> to be expressed in PNG and the workflows which use it will be similar
> to the ones which use EXIF in JPEG or TIFF.   Exactly the same problem
> of maintaining it without updating it exists in JPEG and TIFF too, and
> of course it already exists in the current zTXt encoding.
>

I still think that a giant "NOT FOR USE AS IMAGE METADATA" string in the 
middle of the chunk would strongly discourage developers from 
accidentally using it as image metadata.

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