Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- Subject: Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- From: Andrew Rodney <email@hidden>
- Date: Tue, 01 Aug 2006 15:06:07 -0600
- Thread-topic: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
On 7/18/06 1:54 AM, "Moritz Heckscher" wrote:
> 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.
Yes.
> 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've never heard of such a convention. You can open one of each in Photoshop
and if you have your warning check boxes on in the Color Settings, you'll
get a missing profile dialog if no profile is seen.
> 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.)
Unavoidable. Anytime you edit a JPEG (and embedding a profile is altering
the data), the JPEG undergoes another round of compression. That's one
downside to JPEG.
> 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.)
This isn't a RAW file right? That is, its a rendered image (JPEG) that is in
sRGB but no profile informs anyone of this. You can have Photoshop assume
the file is in sRGB for proper viewing (set your RGB working space to sRGB).
However, IF you edit the file, you might as well now embed sRGB into the
data since you're going to recompress again anyway. If you just want to view
the files correctly, then having sRGB be the assumption for all untagged
images will allow you to properly view the files (all other untagged files
not in sRGB will not preview properly).
Andrew Rodney
http://www.digitaldog.net/
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