[png-mng-misc] CFD: eXIf 2017-06-09b

[Phil Harvey]

I removed some of the JPEG-specific details so the recommendation
now reads, in the 2017-06-09b version,
While the PNG specification allows the chunk size to be as large as
2^31-1 bytes, application authors should be aware that, if the Exif profile
is going to be written to a JPEG datastream, the total length of the eXIf
chunk data may need to be adjusted to not exceed 2^16-9 bytes, so it can fit
into a JPEG APP1 marker.

Great. I apologize for making another suggestion, but calling it a "JPEG APP1
Exif segment" would be more accurate than a "JPEG APP1 marker" because the 6-byte
Exif00 header is specific to the APP1 Exif segment. Also, a "marker" is
technically the two bytes at the start of a segment (0xff 0xe1 in this case) that
identify the type of segment.

- Phil

[Glenn Randers-Pehrson]

Post by Phil Harvey

I removed some of the JPEG-specific details so the recommendation
now reads, in the 2017-06-09b version,
While the PNG specification allows the chunk size to be as large as
2^31-1 bytes, application authors should be aware that, if the Exif profile
is going to be written to a JPEG datastream, the total length of the eXIf
chunk data may need to be adjusted to not exceed 2^16-9 bytes, so it can fit
into a JPEG APP1 marker.

Great. I apologize for making another suggestion, but calling it a "JPEG APP1
Exif segment" would be more accurate than a "JPEG APP1 marker" because the 6-byte
Exif00 header is specific to the APP1 Exif segment. Also, a "marker" is
technically the two bytes at the start of a segment (0xff 0xe1 in this case) that
identify the type of segment.

OK. BTW I'm always confused about whether we should be saying
JPEG, JFIF, or JPEG/JFIF.

Glenn

[Pavel Zlatovratskii]

BTW I'm always confused about whether we should be saying JPEG, JFIF,
or JPEG/JFIF.

JFIF mostly used to specify APP0 format. I think we should say JPEG or
JIF or unabbreviated JPEG Interchange File.

[Glenn Randers-Pehrson]

The Exif spec calls it a "JPEG APP1 Marker Segment" (not a "JPEG Exif Segment").

BTW I'm always confused about whether we should be saying JPEG, JFIF, or
JPEG/JFIF.

JFIF mostly used to specify APP0 format. I think we should say JPEG or JIF
or unabbreviated JPEG Interchange File.
--
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[Phil Harvey]

Hi Glenn,

Post by Glenn Randers-Pehrson
The Exif spec calls it a "JPEG APP1 Marker Segment" (not a "JPEG Exif Segment").

(Note that I suggested "JPEG APP1 Exif segment", not "JPEG Exif Segment".)

But yes, you are correct that the Exif spec does often use "marker segment", but in at least one case they call it an "APP1 segment" (without "marker"). Technically, "APP1 marker segment" would be translated as "the application segment identifed by the APP1 marker", but this is more commonly just called the "APP1 segment" today.

There are of course other APP1 segments not mentioned in the Exif specification, so I think it would be useful to be specific and state it is the "Exif". This also hints to the fact that the segment must contain the Exif00 header (because this is what distinguishes it from other APP1 segments).

I'm just explaining my reasoning, and not trying to be argumentative. I'm quite OK if you leave it the way it was.

- Phil

[Glenn Randers-Pehrson]

Post by Phil Harvey
Hi Glenn,

Post by Glenn Randers-Pehrson
The Exif spec calls it a "JPEG APP1 Marker Segment" (not a "JPEG Exif Segment").

(Note that I suggested "JPEG APP1 Exif segment", not "JPEG Exif Segment".)
But yes, you are correct that the Exif spec does often use "marker segment", but in at least one case they call it an "APP1 segment" (without "marker"). Technically, "APP1 marker segment" would be translated as "the application segment identifed by the APP1 marker", but this is more commonly just called the "APP1 segment" today.
There are of course other APP1 segments not mentioned in the Exif specification, so I think it would be useful to be specific and state it is the "Exif". This also hints to the fact that the segment must contain the Exif00 header (because this is what distinguishes it from other APP1 segments).
I'm just explaining my reasoning, and not trying to be argumentative. I'm quite OK if you leave it the way it was.

In fact I ended up writing in the 06-10 CFD:

"...can fit into a JPEG APP1 marker (Exif) segment."

Glenn