`Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
`Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- Subject: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- From: Moritz Heckscher <email@hidden>
- Date: Tue, 18 Jul 2006 03:54:36 -0400
Hello folks,
I am new to this list and have a general question regarding colour
profiles. Sorry if some of these seem to be rather basic to you or if
I am asking silly things, I am just getting to know this field.
Situation: I had set up my digital camera (Minolta Dimage 7Hi) to
embed the color profile. I had done this a long time ago when I got
the camera without really knowing how colour management works, so I
chose sRGB and not Adobe RGB 1998 (today I would choose Adobe RGB as
I have read that this has a larger gamut). As I have just now
realised this option was disabled about a year ago (maybe when the
camera was out of battery power or someone played with the settings),
so I have numerous files without an embedded profile in it. As the
camera allows to chose between two different profiles to be embedded,
I assume that the camera does some sort of internal conversion
appropriate for the respective profile when saving the data.
I wish to associate (embed) a profile to these files afterwards. Many
of the files are .jpg files (I think my camera makes the distinction
to use .jpg for files without a profile and .jpe for those with a
profile embedded - is this common practice or is it more widepsread
use to always use .jpg?). I have tried using ColorSync for that
purpose (Mac OS X 10.4.7). Unfortunately, it seems, after I embed the
profile and save the file, ColorSync recompresses the .jpg (applies
the lossy JPEG compression algorithm again), resulting in a possible
quality decrease. (I noticed because afterwards my files were much
smaller.)
Thus my questions (they're all a bit related to each other):
- Instead of using tricks like setting the quality settings for jpeg
to maximum (then the files would actually be larger than before) or
saving to lossless formats like .png etc., I would like to just
*embed/append* the profile without altering (ideally) any of the
actual image data bits. Is this possible, or does it not work because
I don't understand the concept of colour profiles?
In other words: Do you think that the raw data in an image without a
profile and in the same (hypethetical) one with a profile would be
the same, except for the added colour profile? Or does adding
(embedding) a colour profile actually alter the RGB data of the
pixels in the image? (I think it shouldn't.)
- If true appending/embedding of a profile is possible, which tool do
I use. ColorSync doesn't seem to work. /usr/bin/sips can extract a
profile from a file, but not embed one from a profile file, so that
wouldn't work, either. How do I do it?
- If no profile is embedded, is it (more or less) safe to assume that
embedding the sRGB profile (afterwards) will be the `right' choice?
Or which profile should I chose? Is there a way to tell, or is the
information regarding which profile is the `right' one irreversibly
lost since the profile is not embedded? I've read somewhere that it
is usually a good assumption that devices such as cameras use sRGB if
not configured specially and not embedding any profile.
That assumption could then be rephrased as follows:
- If the camera is set up to embed a profile, it captures the real
data from the sensor, performs some calculations appropriate for the
chosen profile and then writes out that data with the profile.
- If the camera is set up not to embed a profile, it does the same as
above, using the calculations for sRGB, but does not embed the sRGB
profile.
These points are interesting to me mostly from a theoretical point of
view to grasp the concepts. I do understand that especially the
question about which profile to use if none is embedded can't be
answered unambiguously.
Thanks for your info!
-Moritz
P.S./update: I have just discovered (I think) that the fabulous
GraphicConverter.app seems to do what I want, i.e. embed the profile
in a .jpg without altering the rest of the data. Would be good if
someone could confirm this and affirm that my assumption that this is
theoretically possible.
Searching terms: colour profile, color profile, ICC, embed,
`lossless', avoid recompression, jpg, JPEG.
--
I almost had a psychic girlfriend, but she left me before we met.
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