Re: The Mysteries of Preview
Re: The Mysteries of Preview
- Subject: Re: The Mysteries of Preview
- From: Matt Deatherage <email@hidden>
- Date: Sun, 20 Jun 2004 17:40:49 -0500
On 6/20/04 at 12:59 PM, Andrew Rodney <email@hidden> wrote:
>
What I'm doing is saving an image untagged and opening it in Preview.
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I'm also viewing it in Photoshop. IF I tag the image in Photoshop to
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this Generic RGB profile, the preview in Photoshop and Preview DO NOT
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MATCH. If I tag the document in Photoshop using my display profile,
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the two DO match. If the untagged image in Preview were using this
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Generic RGB profile, then assigning that to the same numbers in
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Photoshop should produce a match to Preview. It's close but it's not
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a match. Assigning my display profile makes a perfect match. So I'm
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confused too!
Perhaps I'm approaching Clouseau-like levels of competence here, but what I read from this is, "Preview does not color-match untagged images."
Here's what I think is happening, so it's probably completely wrong.
Preview opens an untagged image and does nothing to it, so you see uncalibrated, unmatched raw pixel values on screen.
raw file --> no matching --> screen
You believe, however, that Preview is assigning the Generic RGB source profile to the untagged image, and using your display profile as the destination profile, and performing matching.
raw file --> Generic RGB (src) --> display profile (dest) --> screen
Therefore, to replicate this behavior, you tag the image with the Generic RGB profile (an embedded source profile) and open it in Photoshop. Photoshop uses your display profile as a destination for all images, so you see the matching described just above. It looks different because that's not what Preview is doing.
When you tag the image with your display profile, Photoshop does this:
raw file --> display profile (src) --> display profile (dest) --> screen
But we know that in Mac OS X, when both source and destination profiles are the same, ColorSync does no matching at all since it's a big NULL transformation. Whether Photoshop actually computes the results is irrelevant, because matching between identical source and destination profiles produces no change in pixel values. So you see:
raw file --> no matching --> screen
...just like in Preview.
If this is correct, and if Preview correctly color-matches all images that contain embedded (source) profiles, then embedding any profile should produce the same results in Preview and in Photoshop's preview. The only time you'd see a difference is with untagged images, since you believe Preview is color-matching them with an assumed source profile, and it's not.
If this is correct. :)
(Hope all this stuff today has been more helpful than annoying; time to get back to my real world for a while.)
--Matt
--
Matt Deatherage <email@hidden> <
http://www.macjournals.com>
I read this list in digest mode; copy me privately for faster responses
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