"Assigning" vs. "Matching" a color profile

In article ,
Wolfgang Weisselberg wrote:

isw wrote:


--snip--

and
why two methods, both provided by Apple, produce visibly different
results.

They shouldn't produce visibly different results outside
minimal adaptions for the colour space ...

It's the same conversion for both: Microtek to Adobe98.

you might be doing something wrong.

I suppose, but I have no idea what. In both cases (one manual, one
running an Applescript), it sure seems like the same thing should happen
(I've looked inside the script, which controls the same app that I use
to do the manual conversion).

The difference between the output images seems to be no more than a
change in levels, because I can make a simple adjustment to the "paler"
one to make the two visually identical.

Maybe as easy as converting to Adobe RGB, but
not attaching that information to the image, so it gets shown as
sRGB (and looks somewhat washed out).

Preview tells me the metadata in both cases is the same, so if there's a
difference, I think it must be in the actual image data. As I said, the
difference seems to be no more than a small difference in the levels of
the outputs.

Apple defines "apply" as sticking a new profile on an image without
manipulating the image data, and "match" as recalculating all the pixel
values so the image is rendered correctly in the new color space. I can
easily see the difference between those two operations; "assign" alters
the appearance of the image fairly radically, while "match" keeps the
appearance nearly identical. Performing the "match" operation manually
vs. via Applescript is where I don't understand what's happening -- the
manual way results in a noticeably more saturated output, and I can
match the scripted conversion to it by making a slight "levels"
adjustment.

We're talking about 4800 dpi scans of some 3,000 35mm slides, so there's
a lot of pixels to be converted; that's why I want to find an automated
way to do it (now that I've finally decided that what I want to do is
convert them from the problematical Microtek profile to Adobe98).

Isaac

On Sat, 02 Jan 2010 10:39:12 -0800, isw wrote:


I suppose, but I have no idea what. In both cases (one manual, one
running an Applescript), it sure seems like the same thing should happen
(I've looked inside the script, which controls the same app that I use
to do the manual conversion).

The difference between the output images seems to be no more than a
change in levels, because I can make a simple adjustment to the "paler"
one to make the two visually identical.

What options are you given in converting? In PS CS2 you are given the
Intent options of Perceptual, Saturation, Relative and Absolute
Colormetric and all but the Absolute Colormetric all you to choose
Black Point Compensation.

In article ,
me wrote:

On Sat, 02 Jan 2010 10:39:12 -0800, isw wrote:


I suppose, but I have no idea what. In both cases (one manual, one
running an Applescript), it sure seems like the same thing should happen
(I've looked inside the script, which controls the same app that I use
to do the manual conversion).

The difference between the output images seems to be no more than a
change in levels, because I can make a simple adjustment to the "paler"
one to make the two visually identical.

What options are you given in converting? In PS CS2 you are given the
Intent options of Perceptual, Saturation, Relative and Absolute
Colormetric and all but the Absolute Colormetric all you to choose
Black Point Compensation.

Based on what I've recently learned, "perceptual" is the best choice for
what I want to accomplish. I suspect that whether I use ColorSync
Utility or Preview (or the Applescript, which calls ColorSync Utility)
to do the deed, there's only one underlying algorithm (which Apple uses
for all color profile work). If any of those offers a black point
adjustment, I don't know where it is or how to find it. It's not even
clear to me if an arbitrary (i.e. user controlled) black point move is
appropriate during a color profile change.

Isaac

On Sat, 02 Jan 2010 21:02:35 -0800, isw wrote:


What options are you given in converting? In PS CS2 you are given the
Intent options of Perceptual, Saturation, Relative and Absolute
Colormetric and all but the Absolute Colormetric all you to choose
Black Point Compensation.

Based on what I've recently learned, "perceptual" is the best choice for
what I want to accomplish. I suspect that whether I use ColorSync
Utility or Preview (or the Applescript, which calls ColorSync Utility)
to do the deed, there's only one underlying algorithm (which Apple uses
for all color profile work). If any of those offers a black point
adjustment, I don't know where it is or how to find it. It's not even
clear to me if an arbitrary (i.e. user controlled) black point move is
appropriate during a color profile change.

Finally had some time to pull out my copy of Real World Color
Management by Fraser, Murphy and Bunting. The BP option is supposedly
a "proprietary Adobe feature." By mapping black to black you can
avoid: 1. clipping shadow detail if the source has a lower black
point than the destination; and 2. having no real blacks if the source
has a higher black point than the destination.