Re: big lightness shift with perceptual conversions
Re: big lightness shift with perceptual conversions
- Subject: Re: big lightness shift with perceptual conversions
- From: Marc Levine <email@hidden>
- Date: Fri, 22 Nov 2002 12:56:49 -0500
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From: email@hidden
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Reply-To: email@hidden
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Date: Thu, 21 Nov 2002 12:06:18 -0800
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To: email@hidden
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Subject: colorsync-users digest, Vol 3 #503 - 16 msgs
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Subject: big lightness shift with perceptual conversions
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From: Ryan Thrash <email@hidden>
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To: email@hidden
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I have a profile generated from a Kodak Q60 target and Monaco Profiler
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of our Eversmart Supreme scanner. Here's the workflow currently in use:
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1) Scan with fixed density endpoints (same as were used to create
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profile) to an 8-bit/channel RGB Tiff file without any sharpening
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2) Open in PS 7.0.1 preserving embedded profile. (image is dark, FWIW)
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3) Convert to a different color space (i.e., BestRGB) for
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manipulation/correction/cleanup
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4) Duplicate file & sharpen appropriately
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5) Convert to desired color space(s) (i.e. Sheetfed Coated v2) for
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output & save with different name
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Step 3 is where my question comes into play:
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Files from our scanner with the correct assigned/tagged scanner profile
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result in images noticeably lightening in the quarter tones when using
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Perceptual rendering intent. With Relative Colorimetric/Blackpoint
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Compensation, everything visually looks the same.
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Is this a normal situation, or is it indicative of a profile gone awry?
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Thanks in advance.
Hi Ryan,
First or all, thank you for buying Monaco. Second, if you are using
MonacoPROFILER 4.0, I would recommend downloading the free upgrade to 4.5
from our website - it contains enhancements to both Printer and Scanner core
technologies.
Regarding your question of why the perceptual intent looks lighter in
certain areas, I would think about it this way: the perceptual intent is
primarily about preserving tone and not color accuracy. What does this mean?
It means that, if you have an area that has a given level of detail (or
tone) - and some of the tonal values are not technically in gamut - the
perceptual intent will shift the rendering to allow the tone to be
maintained - sometimes lightening the overall range. When using the
colorimetric intent, color accuracy in the primary motivation, but can come
at the expense of losing tone/detail/shape. I suspect that this is why Adobe
has implemented the BPC option. BPC applies some intelligence/stretching to
the colorimetric intent that makes behave more like the perceptual intent in
the way it renders tone, yet still providing the color accuracy a
colorimetric intent. Here are the things you need to keep in mind...
1) Will you be making transforms outside of Photoshop? If so, you will get
different results from the colorimetric intent when used without BPC. You
can get a better sense of how a non-Adobe transform in Photoshop will look
by deselecting BPC and evaluating the results. You can then make an informed
decision on what intent/applications/workflow you intent to implement
2) MonacoPROFILER CAN modify the perceptual intent behavior to better suit
your needs. Simply take your existing profile, load it into the "update
profile" window for the Printer section and click on the "Perceptual Option"
(may be called Advance Options in earlier versions). You will have 2 slides
available called "Contrast" and "Saturation". What these 2 sliders do is
customize the compression in either the Lightness axis or in the Color(ab)
axis, respectively. If you increase the "Saturation" slider, you will reduce
the compression on your colors. In other words, you will minimize the
perceptual color movement, making the intent work more like a colorimetric
intent.
Long story short, you're not crazy. The perceptual intent often does lighten
things for the purposes of keeping tone, And yes, BPC on a RelCol transform
in Photoshop is a very popular choice (and for good reason). If you're using
Photoshop for everything - use the RelCol BPC at will. If your workflow may
go outside of Photoshop, I would consider building a profile with a little
more intelligence - You've got the tools.
Please contact me off-line if you'd like to talk more about your workflow.
Best regards,
Marc
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Marc Levine
Monaco Systems
North American Accounts Manager
Technical Sales Engineer
www.monacosys.com
email@hidden
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