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Author Topic: Green/blue color cast with scanner profiles  (Read 3169 times)

nirpat89

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Re: Green/blue color cast with scanner profiles
« Reply #20 on: April 06, 2018, 10:50:11 AM »

I wouldn't jump to any conclusions on the quality of Faust targets. There are a number of variables at play here and unless you can isolate the particular influence of each you can't reliably pinpoint the culprit or culprits.

I understand.  If that was not true, it is an interesting coincidence to see the same outcome in two different independent cases.  We both could have been doing same thing (or different) erroneous in the process, I guess.   May be something amiss in the way VueScan does the number-crunching.  We can learn where the problem lies, the process or the target by having one of us send someone else the target and have them do calibration with the a known good process and see what results.

:Niranjan.
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Mark D Segal

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Re: Green/blue color cast with scanner profiles
« Reply #21 on: April 06, 2018, 02:54:42 PM »

Sure, but as you most likely know, coincidences are not necessarily determinative. Nor is it necessarily the case that both of you were doing the same thing in the same wrong way - your procedures may have been satisfactory but the problem is elsewhere. Based on what we have seen so far, it's largely speculative. To add to the speculation, I wouldn't rule out the possibility of technical limitations to the accuracy of reflective scanning with these scanner models. If you look at the numbers I presented yesterday - here with an Epson V850, using SilverFast Ai 8 Studio and the SilverFast target, check what happened in the panel bounded by cols H-I-J rows 10 to 17. The performance of the L* and b* channels is not too bad, but that of the a* channel deteriorates as the grayscale patches get darker. One wonders whether this could be evidence of scanner noise having increasing impact on accuracy as the signal:noise ratio decreases with lower luminance. Nothing conclusive, but again, another possibility to consider. And I remain concerned about some of the measurements themselves - still bothered by the inconsistency of readings between SilverFast and Photoshop, in particular comparing the a* channel col L (SilverFast) versus col C (Photoshop), rows 10 to 17.
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Mark D Segal (formerly MarkDS)
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aderickson

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Re: Green/blue color cast with scanner profiles
« Reply #22 on: April 06, 2018, 05:37:07 PM »

I wouldn't jump to any conclusions on the quality of Faust targets. There are a number of variables at play here and unless you can isolate the particular influence of each you can't reliably pinpoint the culprit or culprits.

I just happened to purchase and receive my Faust target a couple days ago and used it to profile my Nikon Coolscan V ED under Vuescan.

The process was very simple and it appears dead-nuts accurate from the results I've been getting.

Allan
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BrianToth

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Re: Green/blue color cast with scanner profiles
« Reply #23 on: April 07, 2018, 08:20:29 PM »

I understand.  If that was not true, it is an interesting coincidence to see the same outcome in two different independent cases.  We both could have been doing same thing (or different) erroneous in the process, I guess.   May be something amiss in the way VueScan does the number-crunching.  We can learn where the problem lies, the process or the target by having one of us send someone else the target and have them do calibration with the a known good process and see what results.

:Niranjan.

Interesting that you had some of your own issues. I probably came across your discussion during all my research.

For what it's worth, the reason I ended up with three IT8 targets is that I initially suspected a problem with the first one. That first one was $60 from LaserSoft (i.e. SilverFast). I then bought two more from Wolf Faust ($10 ea). One on Kodak, the other on Fuji.  All three exhibit the same problem.  For all I know, Wolf Faust manufactures the LaserSoft ones. :) Either way, they all seem to perform the same, and according to my profiling results, the Delta-E values should be well within acceptable margins of error… which I would take to imply that there's no noticeable discrepancies between the target itself and its reference file.

I've also tried the following scan methods (for both generating the profile and future scans that are to use the profile):

  • Epson Scan with "no color management" (turns off all settings). Output is still gamma(?) corrected or internally assigned some standard profile, but no profile is saved with the image.
  • VueScan "RAW". Gamma 1.0, no corrections. (X-Rite doesn't like to make profiles from these, so mostly tried them with Argyll CMS.)
  • VueScan with "Color: None", default values, output to Device RGB.
  • SilverFast "RAW"

All methods produced pretty much the same results once the profiles were assigned to the image. Some had slightly better shadow details, etc. but without getting too fussy, they all had the same color shifts.

Also tried with a v800, v600, v300.  The v300 has very noisy blacks, but otherwise they all look very similar.

Using Argyll CMS from the command line gave me a ton more control but the cLut profiles give the same color shifts. Some like the Shaper Curve profiles look pretty good on the screen, but that's a whole other can of worms… too many options and I don't know which I'm supposed to be focusing on. I also don't know what type X-Rite produces to compare with. Maybe there's a way to figure it out using the ColorSync utility?

I attached a PDF of a small sampling of the Lab values comparing the four profile results.
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BrianToth

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Re: Green/blue color cast with scanner profiles
« Reply #24 on: April 07, 2018, 09:16:13 PM »

I just happened to purchase and receive my Faust target a couple days ago and used it to profile my Nikon Coolscan V ED under Vuescan.

The process was very simple and it appears dead-nuts accurate from the results I've been getting.

Allan


I've tried using the VueScan profiling. For reflective scanning at least, I didn't find it any more useful than using the built-in device profile. It doesn't help at all with gamma correction, white balance, etc. which still leaves me a lot of guesswork to get acceptable results. Since you're using a Nikon Coolscan V ED, I'm assuming you had to use a transparent IT8 target to do your profiling?  Is there any way that you could compare a scan of your IT8 with the profile applied to the IT8 reference Lab values? I'm not doing any slide scanning yet, but I do have some in my collection that I might need to tackle some day.
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GWGill

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Re: Green/blue color cast with scanner profiles
« Reply #25 on: April 07, 2018, 09:49:11 PM »

Either way, they all seem to perform the same, and according to my profiling results, the Delta-E values should be well within acceptable margins of error… which I would take to imply that there's no noticeable discrepancies between the target itself and its reference file.
If you are getting the same results with a range of different charts and profiling packages, then that points to either a workflow issue outside the profiling process, or a spectral mismatch between the media you are scanning and the charts. Lacking a spectral characterization of your scanner, the only way of guaranteeing optimal results using test charts and tri-stimulus profiling is to have test charts based on identical paper and colorants to the media you wish to scan.
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Doug Gray

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Re: Green/blue color cast with scanner profiles
« Reply #26 on: April 07, 2018, 09:57:30 PM »

If you are getting the same results with a range of different charts and profiling packages, then that points to either a workflow issue outside the profiling process, or a spectral mismatch between the media you are scanning and the charts. Lacking a spectral characterization of your scanner, the only way of guaranteeing optimal results using test charts and tri-stimulus profiling is to have test charts based on identical paper and colorants to the media you wish to scan.
Exactly so.  A colorimetric (accurate) scan isn't possible unless the scanner's illuminant is something close to D50. However, without a D50 illuminant and color filters that mean L/I, one can accurately scan an image printed on the same printer that printed the scanner target. It just likely will be off scanning other media with different spectral characteristics. The same applies using a scanning target printed with differing spectral responses.

Exactly the same problem occurs when profiling cameras for scene referred repro work. You just hope to get close enough.
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Mark D Segal

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Re: Green/blue color cast with scanner profiles
« Reply #27 on: April 07, 2018, 10:51:02 PM »

If you are getting the same results with a range of different charts and profiling packages, then that points to either a workflow issue outside the profiling process, or a spectral mismatch between the media you are scanning and the charts. Lacking a spectral characterization of your scanner, the only way of guaranteeing optimal results using test charts and tri-stimulus profiling is to have test charts based on identical paper and colorants to the media you wish to scan.

Taking things one step at a time, at the very least round-tripping the scanner target should produce a scan whose read values are close to the reference values, unless there are issues. Starting with that, one can determine whether there are issues (hardware, software, settings) before taking on media that differs.
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Mark D Segal (formerly MarkDS)
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Doug Gray

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Re: Green/blue color cast with scanner profiles
« Reply #28 on: April 07, 2018, 10:53:17 PM »

Taking things one step at a time, at the very least round-tripping the scanner target should produce a scan whose read values are close to the reference values, unless there are issues. Starting with that, one can determine whether there are issues (hardware, software, settings) before taking on media that differs.
Right. Highly recommended.
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BrianToth

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Re: Green/blue color cast with scanner profiles
« Reply #29 on: April 08, 2018, 06:34:51 PM »

If you are getting the same results with a range of different charts and profiling packages, then that points to either a workflow issue outside the profiling process, or a spectral mismatch between the media you are scanning and the charts. Lacking a spectral characterization of your scanner, the only way of guaranteeing optimal results using test charts and tri-stimulus profiling is to have test charts based on identical paper and colorants to the media you wish to scan.

Exactly so.  A colorimetric (accurate) scan isn't possible unless the scanner's illuminant is something close to D50. However, without a D50 illuminant and color filters that mean L/I, one can accurately scan an image printed on the same printer that printed the scanner target. It just likely will be off scanning other media with different spectral characteristics. The same applies using a scanning target printed with differing spectral responses.

Exactly the same problem occurs when profiling cameras for scene referred repro work. You just hope to get close enough.


Thanks for the feedback. I had a hunch that might be playing a role to some extent.  I've read about the importance of having the target match the materials I'm scanning, though I had also read it was more critical for some media than others.

“Close enough” would be good. The color shift happens across all kinds of photos that I'm scanning, from very glossy to matte, color or black & white.
It looks like most of the photos I'm scanning have much whiter paper than the IT8 target white patches, which is maybe why the ColorChecker 24 handles the white point better? Yet the black patch on the ColorChecker does't get anywhere near as black as the black and white photos I have. 

Do you think my next best option would be to use a target that has a wider gamut that might encompass the media I am scanning better?

When I veered off into making DNG camera profiles as an experiment, the Adobe DNG Profile Editor suggested that the color temperature of the scanned ColorChecker was 6800k. I'm guessing it just estimated that based on the white balance correction that it applied to the profile, but that was something I tried to learn about… how that might affect the whitepoint of the ICC profiles.

I can try to do some test readings using Argyll and my i1Studio spectrophotometer if that would help with anything.
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BrianToth

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Re: Green/blue color cast with scanner profiles
« Reply #30 on: April 08, 2018, 06:35:54 PM »

Taking things one step at a time, at the very least round-tripping the scanner target should produce a scan whose read values are close to the reference values, unless there are issues. Starting with that, one can determine whether there are issues (hardware, software, settings) before taking on media that differs.


I think the targets do an ok job of profiling themselves. Especially the ColorChecker. The IT8 profiles raise the luminance value too much though, unless I use the -u or -ua flags in Argyll to make the profile. Alternatively assigning the IT8 profile and then converting to the working space using absolute colorimetric brings the values closer to the reference. Seems like a roundabout way to do it, but I’m sure I’m just not understanding the internal workings of the profiles very well, at least when it comes to input profiles.  Profiles made from the ColorChecker seem to do better in this regard.

Is there a recommended tool I could use to generate automated comparisons between the reference data and the profiles in a meaningful way? Right now I have to manually sample each patch in Photoshop, which is way too time consuming. I have the target reference files, scans of the target, and if I use Argyll I have .ti3 files from scanin.  I attached the results of Argyll’s profcheck against some of my profiles, as well as a sample of manually read Lab values from Photoshop, but it’d be great if I could script some way to mass-compare profiles somehow.  Going just off Delta-E seems to not mean much, sometimes the smaller error values actually yield greater color shifts than the simpler profiles.
« Last Edit: April 08, 2018, 06:41:17 PM by BrianToth »
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Mark D Segal

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Re: Green/blue color cast with scanner profiles
« Reply #31 on: April 08, 2018, 06:48:53 PM »


Is there a recommended tool I could use to generate automated comparisons between the reference data and the profiles in a meaningful way?

Chromix ColorThink Pro.
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Mark D Segal (formerly MarkDS)
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BrianToth

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Re: Green/blue color cast with scanner profiles
« Reply #32 on: April 14, 2018, 03:52:07 PM »

I did get a Digital ColorChecker SG. At the moment, I haven't seen any advantage to it. According to ColorThink (which is super buggy), the gamut volume of the SG profiles are less than that of the IT8, which isn't what I was expecting… considering the SG colors are more saturated. I did attach some preliminary ColorThink info. (I don't own a licensed copy and considering how much it's crashed, I think I'll just do things the old fashioned way.)

I found this interesting paragraph on the ICC's website (http://www.color.org/faqs.xalter#pa1):

Quote
Q. How do I assess the accuracy of an input profile?

A. Make sure you have a CIELAB colour space profile on your system, and select an input test chart such as the one you made the profile with (and which you have measurement data for. Then use Photoshop 'convert to profile' command to convert from your input profile to the CIELAB colour space, using the absolute colorimetric intent and no black point compensation. The CIELAB values (which you can display as a CIELAB image in Photoshop) should match the measurement data for the chart. Note that you are only evaluating the colorimetric rendering intent by this method, and not the perceptual rendering intent.

It hadn't occurred to me to do this. I was comparing the LAB values after assigning my profile to the scanned image. In hindsight it makes sense as most of the profiles use the target white patch as the whitepoint, which doesn't make sense when trying to compare to the target reference file. Also, as I had already observed, when I would convert to my working space using different intents, it would sometimes bring the luminosity values back in line with the target reference file.

None of this hasn't magically solved any of my problems. But I wonder what other detail I might be missing that's leading to my poor scan results. I suspect I've had a misunderstanding of how and when to manage the white point throughout the scanning > display > printing process.  I may have been trying to force an absolute intent to get the values to match, but what I really might want is to keep the white point relative to the media I'm viewing the image on.  I don't know.
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Mark D Segal

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Re: Green/blue color cast with scanner profiles
« Reply #33 on: April 14, 2018, 04:03:08 PM »

ColorThink Pro is not buggy at all.

A ColorChecker SG is not meant for profiling scanners. Don't use it. You need proper scanner profiling targets. ColorThink Pro Color Worksheets will give you a reliable reading of the performance of properly made and correctly scanned targets.
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Mark D Segal (formerly MarkDS)
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GWGill

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Re: Green/blue color cast with scanner profiles
« Reply #34 on: April 14, 2018, 09:47:13 PM »

A ColorChecker SG is not meant for profiling scanners.
Sure you can use a ColorChecker SG for profiling scanners, if it better (spectrally) represents the type of media you want to scan. For artwork people have gone to the trouble of making custom test charts using the exact canvases and paints of the type of work they want to scan, and if that is the type of thing you are using your scanner for, then a ColorChecker is going to be closer to the mark than a photographic IT8 print.
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Doug Gray

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Re: Green/blue color cast with scanner profiles
« Reply #35 on: April 14, 2018, 10:05:24 PM »

Sure you can use a ColorChecker SG for profiling scanners, if it better (spectrally) represents the type of media you want to scan. For artwork people have gone to the trouble of making custom test charts using the exact canvases and paints of the type of work they want to scan, and if that is the type of thing you are using your scanner for, then a ColorChecker is going to be closer to the mark than a photographic IT8 print.

As I see it the main issue with both ColorChecker and Colorchecker CG is whether the scanner is impacted by the air gap between the color patches and glass since they are slightly recessed. My sense is that this depends on the scanner.

I got better scene referred profiles from printed colorcheckers than an actual colorchecker. I believe this is because the printed colorchecker spectral content is spectrally less diverse than an actual colorchecker. Thus a matrix profile fits quite well.

So I have used profiles from a printed colorchecker for repro work of printed material that is CYMK and LUT profiles from a regular ColorChecker and/or SG for items that are more diverse spectrally.

But I don't have any hard evidence one way or the other. Just very limited data sets and the scanner I used was retired some years ago.
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Mark D Segal

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Re: Green/blue color cast with scanner profiles
« Reply #36 on: April 14, 2018, 10:39:56 PM »

Sure you can use a ColorChecker SG for profiling scanners, if it better (spectrally) represents the type of media you want to scan. For artwork people have gone to the trouble of making custom test charts using the exact canvases and paints of the type of work they want to scan, and if that is the type of thing you are using your scanner for, then a ColorChecker is going to be closer to the mark than a photographic IT8 print.

If you have evidence of a scanner profile made from a CCSG being superior (for the vast majority of photographs people scan) to say the Hutchcolor scanner profiling targets, or the Faust targets, or the LSI targets, that would be interesting to see. As for targets bespoke to certain kinds of artwork, judging from all I've been exposed to, to get results those folks consider satisfactory the selection of the palettes for such profiles seem a lot more painstaking and customized than just using a CC.
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Mark D Segal (formerly MarkDS)
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WayneLarmon

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Re: Green/blue color cast with scanner profiles
« Reply #37 on: May 30, 2018, 09:31:46 AM »

I maybe be naïve, but how can scanners be expected to be profiled into yielding accurate color?  For example, here is the light in my Epson V600 scanner:



If this can be profiled into giving accurate color, then why are we wasting time searching for lights that provide accurate color rendering?  Why don't we just grab whatever LED bulbs are in our local hardware store and profile our cameras just like scanners are profiled?

Sorry if this is an elementary question but I've had cognitive dissonance ever since I measured my V600's LED.
« Last Edit: May 30, 2018, 09:54:22 AM by WayneLarmon »
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Doug Gray

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Re: Green/blue color cast with scanner profiles
« Reply #38 on: May 30, 2018, 03:57:24 PM »

I maybe be naïve, but how can scanners be expected to be profiled into yielding accurate color?  For example, here is the light in my Epson V600 scanner:



If this can be profiled into giving accurate color, then why are we wasting time searching for lights that provide accurate color rendering?  Why don't we just grab whatever LED bulbs are in our local hardware store and profile our cameras just like scanners are profiled?

Sorry if this is an elementary question but I've had cognitive dissonance ever since I measured my V600's LED.

It's a problem, but not as big as one might think.

Take a look at the spectrum of a white patch on your monitor. Pretty peaky. But can it be profiled reasonably accurately? Yes. So it's not the issue of an illuminant alone but the interaction of the illuminant with the spectral diversity of objects being illuminated.

This is why a matching print of a colorchecker can be profiled with a lower dE than an actual colorchecker. This is also why using a scanner to  scan photos can yield good results. Both photos (color dyes) and prints (from CYMK) printers have less diversity than things like colorcheckers which are made from spot colors from a variety of pigments. However, both photos and prints need separate profiles made from patch sets using the two processes for optimal scanning color accuracy.

For other objects scans will introduce more color distortion.
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smilem2

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Re: Green/blue color cast with scanner profiles
« Reply #39 on: May 30, 2018, 04:22:12 PM »

I always wondered same questions
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