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Author Topic: Profile/curve for ABW - fundamental question  (Read 15796 times)

unesco

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Re: Profile/curve for ABW - fundamental question
« Reply #60 on: August 06, 2017, 05:07:11 am »

What working color space have you decided to use from here on out?  I ask because the author of the aforementioned Cameratico.com article recommends using QTR-Lab working color space (I'm not talking printer profile, but rather working space PS>Edit>Color Settings).  If your answer is QTR-Lab, then I must ask you this.   Does the QTR printing GUI (module, pipeline, driver, thingamajig, whatever you call it) bypass the Epson ABW printer driver (like PrintImage and QImage RIPs do)  and print with a 1.0 gamma response curve?  If no, then what does the QTR printing GUI actually do that the printer ABW driver does not do? My experience with QTR was limited to building an ICC profile and nothing more--I've never printed directly from QTR itself.

According to my previous post it doesn't matter what working color space the patch set was generated in PS when printed via PS, Epson Driver in ABW mode and "Printer manages color" (assuming it has been natively generated in PS). So, now I started my tests with untagged 51 random patch set I have generated with Argyll.

QTR printing is another story (I am not talking about using QTR "*.exe" scripts to generate icc). Since, QTR assumes linear gamma (=1.0), I convert my grayscale photo to QTR Gray lab .icc (linear L*) in the PS, save as LZW tif and open and print via QTR Gui.
Important thing is, as Frep has written, QTR has nothing in common with Epson driver and ABW. Prior to make your print, you have to prepare in QTR appropriate curve(s) for you print mode (resolution etc) and paper used. Spectro is needed, but you gain control on each color channel in the printer and proportion of how ink is mixed to make given shade of gray. That is not an easy task, and QTR has no clear documentation, but the result is really worth trying.

Many years ago I needed to create specialized charts with superimposed sine functions of differing frequencies for studying system MTF. Linearity was important. [...] I used standard color mode w color management disabled but it should apply to Adv. B&W just as well. Anyone tried something like that?

In retrospect I think that approach could be improved by grouping 4 pixels (at 720PPI on Epson) and adjusting them to improve the smoothness of each RGB step.

Doug, I have no problem with using Matlab, I used to be an expert in that excellent tool in the past - but would you be so kind to explain what do you mean in reference to simple patch generation? Especially, the issue of smoothness?

@unesco:  #1 surprises me a little, because depending on how you printed these various test charts, I'd have expected some differences, but perhaps you printed them in a way to prevent this.  #2 is what I would have expected.  #3 is an interesting observation that I hadn't noticed before.

#1 new image created in the PS in given color space, patches generated in my PS by posterising gradients, printed in ABW and "Printer manages color" mode
#3 visually the difference between 2880 and 1440 dpi is slight, however in 1440 mode a* vs b* curve for L* going from 100 to 0 is more straight from the white point of the paper to the black point. In 2880 it is more curvy in midgray-to-shade area. The difference is at a level of dE=~1
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Doug Gray

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Re: Profile/curve for ABW - fundamental question
« Reply #61 on: August 06, 2017, 01:52:57 pm »

Doug, I have no problem with using Matlab, I used to be an expert in that excellent tool in the past - but would you be so kind to explain what do you mean in reference to simple patch generation? Especially, the issue of smoothness?

Let's say we have the following measured results of patches created in steps of 1RGB deltas.

50,50,50  L=33.2
51,51,51  L=33.3
52,52,52  L=34.6

and the actual, smoothed over 5 RGB deltas at 51,51,51 is L=33.9

One could, perhaps, achieve better results by taking adjacent squares of 4 pixels and making two of them 51,51,51 and the other two 52,52,52.

My prior work would have only changed 50,50,50 to 51,51,51 as being the closest, printable RGB value to the smoothed target of L=33.9

My speculation is that this could be a refinement providing more smooth tones..
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unesco

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Re: Profile/curve for ABW - fundamental question
« Reply #62 on: August 06, 2017, 02:56:14 pm »

Let's say we have the following measured results of patches created in steps of 1RGB deltas.

50,50,50  L=33.2
51,51,51  L=33.3
52,52,52  L=34.6

and the actual, smoothed over 5 RGB deltas at 51,51,51 is L=33.9

One could, perhaps, achieve better results by taking adjacent squares of 4 pixels and making two of them 51,51,51 and the other two 52,52,52.

My prior work would have only changed 50,50,50 to 51,51,51 as being the closest, printable RGB value to the smoothed target of L=33.9

My speculation is that this could be a refinement providing more smooth tones..

O, I see - yes, I was considering that problem of not integer mapping of L* 0-100 range to RGB 8-bit 0-255 range. There are several options:
1) Dithering - PS is doing that, as far as I remember in case of 16-->8 bit conversion and/or posterisation.
2) Print and measure the full 256 patches set.
3) Design own dithering method as you suggest.
4) Use another software for printing which dithers at printer output generation level e.g. QImage.
5) Prepare and print patch set in 16 bit but this is virtually impossible in Windows.

The rounding max error is at 0.4 L* for given point so it could give 0.8 L* consecutive points difference. I think, that errors due to print non uniformity is at least comparable sometimes even at a level close to 2 L* (that's why not only multiple measures but also multiple prints are required). So, nice area to research in the future :-)
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Doug Gray

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Re: Profile/curve for ABW - fundamental question
« Reply #63 on: August 06, 2017, 03:16:18 pm »

O, I see - yes, I was considering that problem of not integer mapping of L* 0-100 range to RGB 8-bit 0-255 range. There are several options:
1) Dithering - PS is doing that, as far as I remember in case of 16-->8 bit conversion and/or posterisation.
2) Print and measure the full 256 patches set.
3) Design own dithering method as you suggest.
4) Use another software for printing which dithers at printer output generation level e.g. QImage.
5) Prepare and print patch set in 16 bit but this is virtually impossible in Windows.

The rounding max error is at 0.4 L* for given point so it could give 0.8 L* consecutive points difference. I think, that errors due to print non uniformity is at least comparable sometimes even at a level close to 2 L* (that's why not only multiple measures but also multiple prints are required). So, nice area to research in the future :-)

Exactly so. Non-uniformity is one reason I used the Epson 9800 rather than the Canon 9500 II even when just printing 8.5x11 targets. The 9800 was much more uniform. Generally to the point where even the lower variations in the I1Pro required multiple passes to average out.

And there's the issue of changes, possibly due to thermal effects, that could occur based on the ink channels and activity level from the immediately preceding printing.

All in all, a complex set of issues.
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Ferp

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Re: Profile/curve for ABW - fundamental question
« Reply #64 on: August 07, 2017, 10:23:54 pm »

According to my previous post it doesn't matter what working color space the patch set was generated in PS when printed via PS, Epson Driver in ABW mode and "Printer manages color" (assuming it has been natively generated in PS). So, now I started my tests with untagged 51 random patch set I have generated with Argyll. ..... #1 new image created in the PS in given color space, patches generated in my PS by posterising gradients, printed in ABW and "Printer manages color" mode

I don't have a lot more to offer on this, since you are on your way and getting seemingly consistent results.  However one thing still puzzles me.  If I print test charts with 21 equally-spaced steps in both gray gamma 2.2 and sRGB then I get different results, and the gray gamma 2.2 one looks a little odd, which I assume is from the silent conversion to sRGB.  You don't seem to get that.  It's possible that the effect is less marked on the PK paper that you're using.  You should see it on MK papers.

Since, QTR assumes linear gamma (=1.0), I convert my grayscale photo to QTR Gray lab .icc (linear L*) in the PS, save as LZW tif and open and print via QTR Gui.

If you look in C:\Program Files (x86)\QuadToneRIP\icc you'll see a number of generic ICC profiles provided by Roy Harrington.  The matte paper ones are, unsurprisingly, intended for printing on matte paper and the photo paper ones are for printing on gloss and semi-gloss papers.  I'd need to dig around, but I seem to recall that the lab ICCs were intended as alternative generic editing spaces and not for printing with.  You may want to check this, or perhaps ask on the Yahoo QTR group, where most expertise on QTR resides.  You may not be doing what you think you're doing by converting to the lab ICC.
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Ferp

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Re: Profile/curve for ABW - fundamental question
« Reply #65 on: August 08, 2017, 08:44:32 pm »

If you look in C:\Program Files (x86)\QuadToneRIP\icc you'll see a number of generic ICC profiles provided by Roy Harrington.  The matte paper ones are, unsurprisingly, intended for printing on matte paper and the photo paper ones are for printing on gloss and semi-gloss papers.  I'd need to dig around, but I seem to recall that the lab ICCs were intended as alternative generic editing spaces and not for printing with.

p.s.  In my C:\Program Files (x86)\QuadToneRIP\icc folder there is a document gray-readme.rtf which is worth reading, as it explains Roy Harrington's intentions when creating the profiles in that folder.
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donbga

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Re: Profile/curve for ABW - fundamental question
« Reply #66 on: August 10, 2017, 02:58:30 pm »

p.s.  In my C:\Program Files (x86)\QuadToneRIP\icc folder there is a document gray-readme.rtf which is worth reading, as it explains Roy Harrington's intentions when creating the profiles in that folder.

It always helps to RTFM.

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Ferp

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Re: Profile/curve for ABW - fundamental question
« Reply #67 on: August 11, 2017, 08:09:00 pm »

It always helps to RTFM.

Yes, generally, but in this case the manual doesn't tell the whole story.  My copy is dated from early 2005, and so the manual doesn't explain that the photo and matte ICCs are generic alternatives to ones that nowadays many people create themselves with an i1.  Nor does it discuss the pros and cons of printing without color management, which some people in this thread want to try.  But it does explain the intention of the lab profiles.
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