Re: Maker data

By "unsafe to modify" I mean that EXIF (unlike other chunks) consist of 
many independent values. And by modifying one kind of values (e.g. 
adding water depth or removing body serial) you may break other kind of 
values (MakerNote data). Like if changing ICC Profile name would make it 
unreadable.



Also your conditions does not prohibit dependence between ancillary 
chunks if
1) Dependence does not rely on order

2) It's declared that dependant chunk should be ignored without 
dependence(or still valid but lost part of it's information).

3) It's declared that adding dependence chunk should remove all previous 
dependant chunk(if it's impossible to appropriately modify it) OR 
dependant chunk have way to check if it fits dependence.

As for actual (yet partial) example dSIG looks like depends on all 
chunks incl. ancillary, but invalidated after any modification.

26.02.2017 20:24, John Bowler пишет:
> Note that *all* ancillary chunks are "unsafe to modify".  The
> permitted operations on ancillary chunks are:
>
> 1) The chunks can be reordered (e.g. an app might sort them in some
> way; a libpng based app always does this.)
>
> 2) Ancillary chunks may be deleted; it isn't required to delete them
> all.  For example an app might choose to delete just the big ones!
>
> 3) New ancillary chunks may be added in arbitrary positions.  Of
> course these must be *known* to the application.
>
> 4) If, and ONLY if, the app changes a critical chunk (IHDR, PLTE,
> IDAT) it must delete all the "unsafe to copy" ancillary chunks that it
> does not recognize.
>
> What this means in practice is that one ancillary chunk can NEVER
> depend on another; it can only depend on the critical chunks and then
> only if it is marked "unsafe to copy".
>
> John Bowler
>
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-- 
Have a nice DOS.

Pavel Zlatovratskii [email protected]; [email protected]


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