Endless gamma in eXIf 2017-0119

> On February 6, 2017 at 5:05 AM Pavel Zlatovratskii <[email protected]> wrote:
> [...]
> At least colourspace data have special way to handle: absence of chunks 
> is 'output as-is'. So 'Where conflicting information is present in other 
> chunks in the stream that data shall be assumed to be correct unless it 
> can be determined to be incorrect.' is not enough.
> 
> My previous comment about 'endless gamma' was missed so I try to provide 
> more detailed explanation:
> 
> 1) Alice and Bob have same Jpeg/EXIF file with same gamma = 2.2
> 
> 2.1) Alice recode jpeg to PNG trying to achieve best compatibility so 
> she use both eXIf and gAMA chunks.
> 
> 2.2) Bob use Willem's pngexif tool, which write only eXIf (that's not 
> Willem's fault! proposal have no word about this!)
> 
> 3) Both PNG goes to Charles which publish them to web. Charles knows 
> nothing about eXIf, but he knows that some browsers and image viewers 
> doesn't process gAMA. So he runs 'truepng /g1'(apply and remove gamma) 
> on all his files.

Would making the chunk copy-unsafe ("exIF") prevent this error?
Charles changes the image data and therefore knows that he must not
copy exIF.

> 
> 4) Now if we give Alice's file to Bob and ask him to recode it back to 
> Jpeg/EXIF he ignore absence of gAMA chunk(he doesn't write it) and write 
> gamma-applied Jpeg with original meta.(gAMA is absent so there is no 
> conflict mentioned in proposal)
> 
> 5) Repeat this scenario again and again and gamma will be applied again 
> and again.
> 
> In fact after step 3 we already have different images with same 
> metadata. So we need some 'meta-metadata' to find out if eXIf could be 
> used as source of colourspace data.

I'm interested in testing to find out if this scenario happens without
using PNG or exIf.  What happens if I use ImageMagick to read a JPEG,
change gamma, and write a new JPEG?  I don't think the Exif profile
gets modified, but I could be wrong about that, and will do some
experimenting later (at the moment I don't have access to ImageMagick
or GraphicsMagick).

And I expect that the scenario you described can happen with existing
ImageMagick/GraphicsMagick, which preserve Exif in a zTXt chunk and
read it back, when converting a JPEG to PNG and back again.

Glenn
 
> 
> Pavel Zlatovratskii scondo-JGs/[email protected]; [email protected]
> 
> 
> 
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