Re: EXIF support in PNG

Ok, my opinion on current situation with eXIf chunk:


As  EXIF designed as full-featured header, not only photo-specific 
proposal (and appropriate part of specification) should contain 
recommendations how to treat generic features.

My opinion:
1) If EXIF contain image size and size mismatch size of picture from 
IHDR chunk - EXIF chunk should be completely ignored. (disputable, may 
be was resize or crop which does not affect metadata).
2) Resolution of EXIF should be ignored if pHYs chunk is present. It may 
ignored even if pHYs chunk is absent, so encoder should write pHYs chunk 
if it treat resolution as valid.
3) Colorspace data should be ignored anyway. Other behaviour could 
result to double-apply colorspace: if colorspace data was duplicated to 
appropriate chunks (cHRM, gAMA, iCCP) and then some re-compressor 
unaware of eXIf apply and remove them.

Anyway, I think most of EXIF data should obtain other way to be stored 
within PNG (chunks or keywords) and later eXIf chunk will be used as 
gIFx: "If a GIF-to-PNG converter recognizes the Application Identifier 
and is aware of a corresponding PNG chunk, it may choose to convert the 
Application Extension into that PNG chunk type rather than using gIFx".

According to compression - I support way with compressed chunk. Not to 
keep space, but to keep some uniformity with iCCP.

*Sorry if my English is bad.
Pavel Zlatovratskii.

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