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Author Topic: Magenta Color cast  (Read 9324 times)

Doug Gray

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Re: Magenta Color cast
« Reply #20 on: May 29, 2016, 03:11:25 pm »

That's why there's a color checker in that reference image.

It's definitely a good thing you provide that in one of your principal images Andrew. But for best comparison one really needs to print a ColorChecker image at the same size and in Absolute Colorimetric then trim the white edges to avoid perceptual white adaption. Makes a really good profile check.
« Last Edit: May 29, 2016, 03:14:32 pm by Doug Gray »
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Mark D Segal

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Re: Magenta Color cast
« Reply #21 on: May 29, 2016, 03:35:42 pm »

It's definitely a good thing you provide that in one of your principal images Andrew. But for best comparison one really needs to print a ColorChecker image at the same size and in Absolute Colorimetric then trim the white edges to avoid perceptual white adaption. Makes a really good profile check.

Yes - both print it and measure it - then you really have a quantitative grip on differences between reference values and printed values.

But you are also correct in your previous post that the GMCC could be complemented by other reference images; there is such a set - the Roman 16 bvdm Reference Images from Germany. Not cheap, but everything you asked for.
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Mark D Segal (formerly MarkDS)
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Doug Gray

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Re: Magenta Color cast
« Reply #22 on: May 29, 2016, 03:44:19 pm »

Yes - both print it and measure it - then you really have a quantitative grip on differences between reference values and printed values.

But you are also correct in your previous post that the GMCC could be complemented by other reference images; there is such a set - the Roman 16 bvdm Reference Images from Germany. Not cheap, but everything you asked for.
True, those are excellent reference images. Especially for checking out smoothness near gamut edges. Pricey though. But for a collection of images that exercises gamut boundaries they are hard to beat. Would be nice if someone provided both a few physical prints of images like Andrew has together with an in gamut,RC digital file to reproduce them on a color managed system. Business opportunity Andrew?
« Last Edit: May 29, 2016, 03:47:36 pm by Doug Gray »
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Tim Lookingbill

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Re: Magenta Color cast
« Reply #23 on: May 29, 2016, 05:36:43 pm »

It's definitely a good thing you provide that in one of your principal images Andrew. But for best comparison one really needs to print a ColorChecker image at the same size and in Absolute Colorimetric then trim the white edges to avoid perceptual white adaption. Makes a really good profile check.

You can be off by as much as 5 Lab numbers and not see much of a visual difference. Andrew's YouTube video on Delta E bares that out. I think Andrew's reference image for the OP's situation is a good enough indicator that something is messed up with his scanner as the scan and subsequent print of the correct looking print of the reference image shows a Delta E that's way over 5 evident in the overly blue white dog in snow image. My $50 Epson "All In One" does a better job of accurate duplication of documents than that.

Even the ColorChecker colors in Andrew's reference image of the print of the scan are hardly affected visually but more pronounced in the gray patches.
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Doug Gray

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Re: Magenta Color cast
« Reply #24 on: May 29, 2016, 06:54:53 pm »

You can be off by as much as 5 Lab numbers and not see much of a visual difference. Andrew's YouTube video on Delta E bares that out. I think Andrew's reference image for the OP's situation is a good enough indicator that something is messed up with his scanner as the scan and subsequent print of the correct looking print of the reference image shows a Delta E that's way over 5 evident in the overly blue white dog in snow image. My $50 Epson "All In One" does a better job of accurate duplication of documents than that.

Even the ColorChecker colors in Andrew's reference image of the print of the scan are hardly affected visually but more pronounced in the gray patches.
Agreed. Neutral tone deviations are more easily visually sensed than the more saturated CC colors. dE2k is the better metric for color difference to account for that.

No doubt something is wrong with the OP's process. He's probably looking at dE's of 10 or more.

It is hard to see differences of 5dE. Particularly, if the two prints are not adjacent or the differences are not in neutral tones. Also, many or most of the CC colors are in areas where the dE2k is much lower than the dE76 and dE2k is closer to a perceptual metric than dE76. The OP's work as described is much less sensitive to color shifts than most all photographers. He's got a problem with blue/yellow shifts that seems quite strong. It's interesting that it keeps popping up for him over many years and different printers.

One wonders where the problem is. It's not even close to normal color behavior.
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Mark D Segal

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Re: Magenta Color cast
« Reply #25 on: May 29, 2016, 07:54:14 pm »

It's interesting that it keeps popping up for him over many years and different printers.


That would point heavily to a problem with scanner colour management or performance. From what I read about it, their NextImage scanner software does support ICC colour management, so that being the case the question then turns to how the OP is using it, or whether the scanner is somehow defective.
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Mark D Segal (formerly MarkDS)
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Doug Gray

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Re: Magenta Color cast
« Reply #26 on: May 29, 2016, 08:05:31 pm »

That would point heavily to a problem with scanner colour management or performance. From what I read about it, their NextImage scanner software does support ICC colour management, so that being the case the question then turns to how the OP is using it, or whether the scanner is somehow defective.

I'm leaning that way too and for the same reasons. I'm a believed in measuring. He should scan a colorchecker and compare lab values to the published.
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Mark D Segal

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Re: Magenta Color cast
« Reply #27 on: May 29, 2016, 08:11:25 pm »

I'm leaning that way too and for the same reasons. I'm a believed in measuring. He should scan a colorchecker and compare lab values to the published.

Yes, and as necessary, adjust his colour management settings in the scanner software until he finds the combination of settings whereby the measurements of the scanned output more or less cohere with their reference values.
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Mark D Segal (formerly MarkDS)
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GWGill

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Re: Magenta Color cast
« Reply #28 on: May 29, 2016, 09:46:08 pm »

My problem: Take an old piece of paper / art / map. The paper typically has a yellow / orange tinge to it. Scan this into a Adobe RGB color space. Save as a TIF. Open in Photoshop - print to printer. The print has a magenta / pink tinge instead. The print looks the same if I use the correcr paper ICC or if I let the printer do the color management in Adobe RGB space.
I think you need to re-adjust your expectations with regard to what "out of the box" color can do for you.

Pre-canned profiles will typically be accurate to somewhere in the range of 2-10 delta E. Custom made profiles somewhere in the range 1-5 delta E. Typical workflows are all relative colorimetric based intents, where it is assumed that white is the white of the media (and it's wired up to make it that way), so the color inaccuracy manifests itself in areas other than white, and the above color accuracy is quite workable.

When using a proofing type workflow where you are trying to emulate one media color with another (Absolute Colorimetric intent), things get a whole lot more critical. I've seen many cases where 1 delta E is not enough - the media color error needs to be < 0.5 delta E to be a visual match. So "by the numbers" custom profiles may not quite enough, since the accuracy and repeatability of graphic arts instruments like the i1pro may not quite be good enough.

And then there are the other complications, such as the destination media simply being out of gamut of the source, FWA/OBE's in the paper resulting in color shifts because the instrument isn't "illuminating" the paper the same way you are viewing it. And then there is the scanner, which typically is not colorimetric (i.e. it doesn't see color the same way humans do), so profiling it with a test chart will not be accurate to better than 3-10 delta E, unless the test chart has exactly the same media and colorants as what you are scanning, and the CIE reference values are made with the same illuminant as you will evaluate the result in. One of the errors you may well see with a profiling chart being different media to what you are scanning, is a white point error - by default (i.e. Relative Colorimetric) a perfectly profiled scan should result in the media color being removed. Typically people work around any such error by manually adjusting the white point of a scan to make the media perfect white.

[ And I won't complicate this explanation any more by going into observer variability. ]

Now all this doesn't mean that you can't solve your problem, or that it's not just one of these things that is causing the majority of the color shift (hopefully some of the preceding suggestions from people will help), but saying that "you spent a lot of money" so that it should "just work" isn't realistic, when you are using a less common workflow. A serious, high accuracy color instrument could set you back in the vicinity of $20,000 and up, so what constitutes "a lot of money" is relative.

 
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Tim Lookingbill

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Re: Magenta Color cast
« Reply #29 on: May 30, 2016, 03:11:49 pm »

And then there is the scanner, which typically is not colorimetric (i.e. it doesn't see color the same way humans do), so profiling it with a test chart will not be accurate to better than 3-10 delta E, unless the test chart has exactly the same media and colorants as what you are scanning, and the CIE reference values are made with the same illuminant as you will evaluate the result in. One of the errors you may well see with a profiling chart being different media to what you are scanning, is a white point error - by default (i.e. Relative Colorimetric) a perfectly profiled scan should result in the media color being removed. Typically people work around any such error by manually adjusting the white point of a scan to make the media perfect white.

For example I made a custom scanner profile using an Agfa silver halide photographic paper it8 target that worked pretty well scanning my family photos printed from silver halide minilabs. But when I scanned old, slightly yellowish white fabric containing dye print patterns of forest green, blue, rose red and deep purple, the yellowish white looked pretty accurate but the deep purple almost looked black, forest green almost electric grass green, blue was spot on and deep rose red was an intense fuchsia.

In retrospect I wonder if I should've just reduced global saturation instead of editing global WB & Hue/Sat of individual colors within the scanner software. I've overlooked that simple approach many times to my frustration when attempting to get accurate color shooting Raw with my DSLR. Saturation appearance can really mess up color judgement in finding where the problem lies.
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Doug Gray

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Re: Magenta Color cast
« Reply #30 on: May 30, 2016, 05:54:19 pm »

Taking the (adjusted to look correct) photo of the side by side print and original, then correcting the white point to produce the same yellowed background the big difference is the highlighted "Village of Clyde" center square.

The document itself is just a faded and slightly yellowed/tanned  black & white civil engineering drawing. The more intense "yellow" portion was probably made on the original with a yellow highlighter.  These are highly fluorescent but will fade somewhat over time. So the differences could well be from uV. The actual scanned image does not have as significant yellow hue shift and it's darker as well. However, the yellowed background, unlike the highlighted square, is very close to the supplied pictures of the print and original.

My guess is that the scanner has very little uV compared to his room lighting or the luminance and yellow (high b*) would not be as intense as it is. It's also possible that the room lighting where the original and print were photographed might be an industrial very low CRI CLF. Check out the image he downloaded against the pictures.

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John Chardine

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Re: Magenta Color cast
« Reply #31 on: July 03, 2016, 08:42:41 pm »

schertz

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Re: Magenta Color cast
« Reply #32 on: July 03, 2016, 09:09:07 pm »

Have a look here: http://forum.luminous-landscape.com/index.php?topic=111295.0

It appears the problem reported here was occurring before May 28th (original post date), while the problematic Photoshop CC release occurred on June 20th. So unfortunately, Adobe's CM bug can't be blamed for this thread issue.
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