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Author Topic: the deal with calib/linearz on HP Zs?  (Read 6568 times)
Greg_E
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« Reply #20 on: January 13, 2007, 09:44:28 AM »
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Linearization corrects for out of linear response from the heads, ink, and paper. It may take a 30% signal to produce a 40% patch*. It is just another look up table, but a LUT that is normally built into the driver (but not always**). The end result is that a linear output (on paper) is obtained. The complete process would be like this:

send a 40% image, drive at 30%, receive a 40% print

Setting the ink limits by density only is not always the best method, but it is also highly dependent on the ink and paper. Sometimes you need to limit based on chroma, not density. Also since it is working both ink limit and linearization into a single step, papers that reach their limit very early will have a more course linearization correction curve. This is because there will be fewer steps making up that curve. If you have 50 steps in your linearity target (per channel), and you clip the ink limit at step 15, there are far fewer steps (15) than if you make the ink limit first, and then print that same 50 steps to get linearity (all 50 would be used).


* In reality, most of the Epson printers run with an inital ink limit (reduction) of around 60% maximum drive for inkjet coated materials. So what it really means is that an input of 40% is a drive of about 15% and a print of 40%. Uncoated materials and textiles may run right up to 100% drive, which produces ink running down your printer on almost all inkjet coated materials. Don't believe me? Get an Epson large format printer and a textile RIP and try it, the Evolution RIP springs to mind.

** In the upper level profiling packages, you can do a linearization step before creating the targets for the profile. When this is done the linearization measurements are applied to either the target, or the mathimatics involved in calculating the profile. You can also specify an ink limit (at least for CMYK and multi-channel profiles). I've done this with CMYK profiles when the RIP did not allow the system to be linearized (an old Fiery hardware RIP). Results can be night and day depending on the factors involved. Same can be said for proper ink limiting and linearization in a RIP that allows those variables to be adjusted. How the in profile linearization effects the profile differently from just a regular profile I don't really know, I just know that it does.
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opgr
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« Reply #21 on: January 13, 2007, 04:21:25 PM »
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I see where you're coming from, but this is not a situation most of us will encounter. I would venture that most here are printing on inkjet-receptor-coated media
Well, yeah, like I said, the HP offering will be really helpful in a lot of situations. I reckon it will cover 80% of the printing needs out there, say in the "good enough" color range. Although I do believe that the following scenario may not be uncommon: printshops will try to jam some of that heavy weight colorcopier paper used for greeting cards through the HP and I'd be surprised if something useful comes out the other end.

On the upside, maybe one of the manufacturers will recognize this market gap of a heavy weight double sided paper type that does allow inkjet greeting cards.

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40%M input  = 40%M on paper? Sorry, I'm not a linearization expert, but this has been my assumption in the past. As the HP engineer said, "linear response in colorimetric density as a function of input digital counts."

Conventional densitometry doesn't apply to inkjet printing at all. That is the entire core of the problem. Conventional densitometry requires 2 important ingredients:

1. Definition of a small range of most absorbing frequencies that will be taken as the reference (in reality a filter function over the entire frequency range is used),

2. Definition of the maximum absorption. (maxD).


Now, initially you would think that the maximum absorption is simply found at 100% ink deposit. You would be surprised in how many cases this is not true, even on matte papers. Even if this is the case, how does the printer know that the ink is behaving correctly? The most absorption may well occur while the ink is running...
Maybe HP has an additional test pattern like line-pairs that allows them to test this.

But the definition of the filter function also isn't as easy as it may seem. The maximum primaries of inkjets usually have a very different contorted behavior relative to the intermediated steps than for example press prints. Compare a decent Euroscale profile to any inkjet profile. This contortion is 3D through the colorcube, so testing for any combination of chroma and lightness will not mean anything.

So when you have some kind of definition of 1 and 2, you can end up with a definition of "linear", but the chances are pretty good that there is no relation to any visual linearity, and, more importantly, having "linearised" the primaries is absolutely no guarantee that the combination colors will suddenly behave properly, or predictably. This contrary to press printing, which is why they have gotten away with dot-gain control as the primary "color management" parameter for so long in that arena...
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Oscar Rysdyk
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« Reply #22 on: January 13, 2007, 04:34:37 PM »
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"Calibration garantees consistency, while Profiling guarantees accuracy"

Yes, I think this sums it up quite nicely.

Justin.
[a href=\"index.php?act=findpost&pid=95191\"][{POST_SNAPBACK}][/a]

I agree, but for obvious reasons I would like to rephrase that to:

Calibration aims for consistency, while Profiling aims for accuracy.
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Oscar Rysdyk
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Haraldo
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« Reply #23 on: January 13, 2007, 10:58:28 PM »
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Linearization corrects for out of linear response from the heads, ink, and paper. It may take a 30% signal to produce a 40% patch*. It is just another look up table, but a LUT that is normally built into the driver (but not always**). The end result is that a linear output (on paper) is obtained. The complete process would be like this:
send a 40% image, drive at 30%, receive a 40% print
Setting the ink limits by density only is not always the best method, but it is also highly dependent on the ink and paper. Sometimes you need to limit based on chroma, not density. [...]

Interesting points. I found another HP tech document that explains how the Zs do this:

1. Density ramps are printed for each ink (x11, not GE) on the target medium.
2. The embedded spectrophotometer scans the chart with densitometry [note that it's a spectro so I suppose it could do chroma too]
3. Ink limits and input/output density relationships are computed for each ink.
4. A linearization table for each ink on that specific paper is computed.

This is not a driver function, and it's clearly customized for each ink and each paper. May not be perfect, but it seems pretty good for right out of the box and built into the printer.

Harald
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Haraldo
aka Harald Johnson
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Greg_E
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« Reply #24 on: January 14, 2007, 04:28:32 AM »
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OK, well driver should maybe be a little more generic, it's putting the LUT inside the printer which is probably the best place to put it.

By doing the single pass limit/linearization they are betting that the papers that you use will not be too far off the norm. If you have one of these printers, and a spectro, you could try some Breathing Color matte polypro signage material http://www.breathingcolor.com/bc/catalog/index.php?cPath=505 and see what happens. I find the chroma based limits to come in very soon and the density limits to go quite a bit farther. Most apparent on full cyan with my inks, the limit is something crazy like 50% after a 60% reduction. Most other materials go at around 75-90% for the Cyan after the same 60% initial reduction.


If you do try that polypro material, could you put it under hot running water for a while? Just curious how well the HP pigment inks do with water.
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ternst
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« Reply #25 on: January 14, 2007, 07:50:54 AM »
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This is a stupid question I'm sure - but why on earth would you ever want to put a work of art under hot running water?
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Greg_E
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« Reply #26 on: January 14, 2007, 11:37:09 AM »
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To see what happens when the sprinkler system at the gallery fails and douses your work  . To see what happens in very humid environments  . Because you are a sadist and just like to do things like this.  

Yes I do use papers and inks that will survive a sprinkler malfunction. As long as the water coming out is clean, they will dry with no visible changes (just like a painting). To me this is important. If they don't dry completly flat, then you can steam press them back into shape


 


 

 

For signage on the material above, it is rated to be outdoor material, so it is all up to your inks when it rains. Most of the modern pigment inks should work on that material with minimum run off.
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francofit
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« Reply #27 on: January 14, 2007, 05:51:59 PM »
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[...]Yes I do use papers and inks that will survive a sprinkler malfunction. As long as the water coming out is clean, they will dry with no visible changes (just like a painting). To me this is important. [...]
[{POST_SNAPBACK}][/a]
Not sure if the HP specs could satisfy all your requirements...  
anyhow, for those interested,
HP Vivera pigment inks + some papers are rated as water resistant (by HP and/or [a href=\"http://www.wilhelm-research.com]WIR[/url]),
being "water resistant" defined by HP as:

" [...]HP uses ISO and internally developed tests and considers prints which pass the following tests to be water resistant : ISO standing water, water spray, water drip, and wet smudge. "

I have read the above from: http://www.hp.com/go/supplies/printpermanence
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Franco
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