Re: EXIF Color Space: sRGB
Re: EXIF Color Space: sRGB
- Subject: Re: EXIF Color Space: sRGB
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Fri, 09 May 2003 09:47:49 -0500
Joachim Hartmann wrote:
>
"EXIF Color Space: sRGB"
>
>
Being curious I checked the file for any included Profile and found none. So
>
what does this line tell me?
A JPEG file may have up to three "profiles" associated with it:
1) Embedded ICC profile. This could be a custom input ICC profile or more
typically, a working space profile.
2a) Embedded EXIF information. When EXIF is included in a JPEG file, its
format is actually TIFF format. (Yes, a TIFF image inside a JPEG image. But
it can be even stranger than that -- this TIFF image may have the thumbnail
encoded as JPEG, so then we have a JPEG image inside a TIFF image which is
inside a JPEG image. But I digress...) TIFF format has provisions for
defining the RGB colorimetry (the equivalence of a profile) via its
WhitePoint, PrimaryChromaticities and TransferFunction tags. The EXIF data
may contain these tags.
2b) Embedded EXIF information. Inside this EXIF/TIFF format is a private
TIFF tag called the "Exif IFD Pointer" which contains, among other things, a
single value reference to "Color Space Information." There are only two
legal values defined so far: "sRGB" and "Uncalibrated." So if the RGB
information is calibrated to a non-sRGB reference (e.g. Adobe RGB), it
cannot be handled, except by omitting the Color Space Information, which of
course would leave it ambiguous.
So a JPEG image file may contain any combination of the above three
"profiles," all of which may be different. I would think that a JPEG reader
should use the precedence I have indicated (i.e. use embedded ICC if
present, else use TIFF colorimetry tags if present, else use EXIF Color
Space Information if present). I believe the information Joachim sees is
that from (2b) above, and as has already been mentioned by others, Photoshop
has provisions for optionally ignoring this.
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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