Stocastic vrs. AM screening

[Tashi D]

Hi and thank you for reading this. I was a Heidelberg man in the late nineties, thought I would retire from printing and live in the desert. That lasted about six years, then, broke, I decided to return to the trade I "knew so well". And since '08 I've been again running a speedmaster. Sometime while I was sitting on a rock in the desert, this new screening method became popular. When I returned to the industry, I was introduced to stocastic screening, tiny dots arranged in random patterns whose density corresponds to color values. This method actually produces very smooth color transitions, but not so good when screen building solid colors. So about two years ago, we went back to traditional screenings, with good result. My questions are; how do you measure dot gain if you are running stocastic screening, and is there any pre-press magic that can produce stocastic images, and traditional solid screen builds on the same form?

 

[gordo]

[SNIP]This method actually produces very smooth color transitions, but not so good when screen building solid colors. So about two years ago, we went back to traditional screenings, with good result. My questions are; how do you measure dot gain if you are running stocastic screening, and is there any pre-press magic that can produce stocastic images, and traditional solid screen builds on the same form?

1 - Not all FM screens are the same when it comes to doing flat screen tint patches - some are smooth some are grainy. Sometimes graininess is simply a symptom of ink transfer problems or issues with plate imaging.

2 - You measure FM dot gain the same way that measure AM dot gain - with a densitometer or spectrodensitometer. Neither instrument knows about dots - just density differences.

3- Some workflows allow you to set different screening for different areas of the press form (e.g. Dots shop in Prinergy) others may require you to double burn the plate (one pass AM, one pass FM). But running AM and FM screens on the same press form is not a good idea since the AM and FM have different ink/water balance requirements - and therefore one will be compromised on press. Also, the screens react differently to solid ink density moves - AM reacts more than FM to the same SID move. That can cause color balance problems as well as confuse and press-side auto color control system.

best, gordo

 

[Alois Senefelder]

Revelation !

Revelation !

Hello Tashi D


I suggest you read Gordos - "Quality in Print " blog when ALL will be revealed.



Regards, Alois

 

[tomcatinc]

Cross-modulated screening

Cross-modulated screening

There are families of cross-modulated screening: HXM, Specta, etc. They produce FM in low and high color value range, while producing classic AM screening in the mid-tones. Maybe this would be compromise soultion for you.

 

[gordo]

There are families of cross-modulated screening: HXM, Specta, etc. They produce FM in low and high color value range, while producing classic AM screening in the mid-tones. Maybe this would be compromise soultion for you.

This is not quite correct.

Spekta is a "conventional" second order FM screen. It is not cross-modulated screening. It does not have AM anywhere in the tone scale. It is likely that the OP is currently using an FM screen that is similar to Spekta.

HXM is a so-called XM screen that is similar in function to Agfa :Sublima and Kodak Maxtone. This type of screen is a conventional "classic" AM screen. The only difference is that the smallest highlight and shadow dots are constrained to the smallest dot size that the imaging system can reliably reproduce. To achieve lighter tones, instead of getting smaller, dots at the smallest reproducible size are removed from the area. Fewer dots means a lighter tone. The dots that are left remain on the AM grid.

It is always better to use a conventional AM dot rather than an XM type dot if the system can image those small dots. BTW, wasn't the promise of CtP the ability to hold the highlight and shadow dots that film workflows lost? %-p

best, gordo

 

[tomcatinc]

I'm sorry that I made some mistakes (calling Spekta as cross-modulated).
Using FM techniques in AM grid would be cross modulation: FM uses same dot size for all tones same as you stated for XM. FM puts/removes dots of same size to achieve ink coverage on the plate.

Quote: "The only difference is that the smallest highlight and shadow dots are constrained to the smallest dot size that the imaging system can reliably reproduce. To achieve lighter tones, instead of getting smaller, dots at the smallest reproducible size are removed from the area. Fewer dots means a lighter tone." Bold text is FM by definition, right?

My statement is a simplified answer to what it really is. And no hard feelings please, I mentioned something to a guy that says he was out of printing industry for some time. In order to introduce third possible type of screening (pure AM, pure FM, combination AM/FM) I skipped some details. We would eventually come to that if it interests him, right?

Anyway, I'm grateful for pointing out my mistake and putting more detail about this for all. Thanks, Gordo!

BTW, screening transfer from film is a real issue. If your imaging equipment does not meet required level of being clean AND you don't use high quality plates, film and consumables AND you did not calibrate the whole chain (film -> plate -> press) AND you.... There are too many variables to achieve nice transfer from film of fine screens which would allow controlled reproduction of fine highlight/shadows. Imaging by laser directly on plate allows better control of these BUT... somehow most people do not care for reproduction quality (I'm talking about people I work with). CtP became just a cheaper and easier way to start printing. Probably it is the same in the world in most cases. I work with Screen 4300 capable of 4.000 DPI resolution - never used (extra imaging time would have to be paid more). XM and FM screens are installed (meaning invested by us to be able to work with these when the reproduction required) - never used. Plates that were capable of holding 250+ LPI AM screens and 10 micron FM - tested, never actually used. There is less art in printing then before and therefore probably there is less demand for enhancement. Without sufficient requests, AGFA, Kodak, Fuji, etc. would not allocate resources on improvement of plates, imaging devices, software, etc. I'm probably wrong on this one but this is just my perspective of the whole matter. Any thoughts would be most welcome.

 

[gordo]

[SNIP] FM uses same dot size for all tones same as you stated for XM. FM puts/removes dots of same size to achieve ink coverage on the plate.

[SNIP]To achieve lighter tones, instead of getting smaller, dots at the smallest reproducible size are removed from the area. Fewer dots means a lighter tone." Bold text is FM by definition, right?

For the sake of accuracy I'm going to split hairs. Kinda yes. Kinda not quite. First order FM screens use dots that are all the same size and use fewer dots to represent lighter tones. However most printers use "Second" order FM screens where the dots change size - usually in one dimension: length - to render darker tones. In both cases the FM dots are not placed on an angular grid the way that AM/XM halftone dots are.

[SNIP] BTW, screening transfer from film is a real issue. If your imaging equipment does not meet required level of being clean AND you don't use high quality plates, film and consumables AND you did not calibrate the whole chain (film -> plate -> press) AND you.... There are too many variables to achieve nice transfer from film of fine screens which would allow controlled reproduction of fine highlight/shadows. Imaging by laser directly on plate allows better control of these.

Correct. However it should be noted that not all CtP/plate combinations are better than film at reproducing high lpi screens. Hence the need for an XM screen in metal plate CtP.

[SNIP]BUT... somehow most people do not care for reproduction quality (I'm talking about people I work with). CtP became just a cheaper and easier way to start printing. Probably it is the same in the world in most cases. I work with Screen 4300 capable of 4.000 DPI resolution - never used (extra imaging time would have to be paid more). XM and FM screens are installed (meaning invested by us to be able to work with these when the reproduction required) - never used. Plates that were capable of holding 250+ LPI AM screens and 10 micron FM - tested, never actually used. There is less art in printing then before and therefore probably there is less demand for enhancement. Without sufficient requests, AGFA, Kodak, Fuji, etc. would not allocate resources on improvement of plates, imaging devices, software, etc. I'm probably wrong on this one but this is just my perspective of the whole matter. Any thoughts would be most welcome.

I think you're taking too narrow a view on the benefits of FM screening (BTW you don't need 4,000 dpi to do finer screens). I don't know where you are located, but in North America about 80% of Yellow pages telephone directory printing, also about 80% of newspaper flyer inserts, and Sunday comics are printed with FM screening. As are most of the major grocery store house brand packaging. Very few people who look up a phone number, or buy a can of soup, or read the Sunday comics realizes or cares about it. But the publishers do. Here are some of the benefits of using FM screening:
1 - No screen angle moiré
2 - No subject moiré
3 - No rosette artifacts
4 - Photographic/contone look
5 - Greater tone and color stability as SIDs naturally vary during press run
6 - Larger color gamut
7 - Faster drying/reduced set-off
8 - Reduced ink usage
9 - Tonal and color stability when misregistration occurs
10 - Halftone dot structure stability when misregistration occurs
11 - Competitive differentiator

Some of the benefits are more important for some print situations but not others.

best, gordo

 

[tomcatinc]

I am in SE Europe but working for customers throughout Europe.

I was talking about XM principle: (first order) FM algorithm of putting/removing dots of same size to do more or less ink coverage but only in highlight and shadows; AM angular grid as distribution rule and principle in midtones.

XM is by far better option in flexo printing: polymers used in flexo printing are kind of lower-LPI then offset printing plates and reproduction is much better when using XM/FM. And flexo is by far better in price ratio then offset when it comes to very high number of printed copies.

And I was not talking about FM, I was talking about printing industry as a whole. Publishers do not care about moire and contone look, they care for speed of production (to which faster drying and mis-registration resilience helps) with less ink spent, all equaling in more money with same machine hours and man hours for higher number of copies and less ink spent.

I do some security printing with no raster/screen in it and only rarely use 4K DPI (it is solid lines varying in thickness, density and color or solid patterns but always solid). Other then that, commercial printing is what covers the vast part of services. Very rarely we do something with artistic and permanent value that requires high screen rulings AM or FM or XM (in my opinion, phone books, labels, flayers, etc. do not have permanent value). Only some specialized color magazines (like home decoration, maritime or automotive) require good reproduction while still in (sort of) higher-midrange of screen rulings/dot sizes.

Economy kills art in printing and that is my whole point.

However, one point you mentioned is something I'm unaware of: larger color gamut when using FM. I was always thinking that gamut is limited by CMYK color space and that no matter the screening, that color space is limited by CMYK ability to optically mix itself in a limited number of steps (let's say). And I thought that this was the reason to broaden CMYK either with Orange and Green or with Light Cyan and Light Magenta. I mean no offense here. Would you be so kind to elaborate on this? Thank you in advance.

 

[gordo]

XM is by far better option in flexo printing: polymers used in flexo printing are kind of lower-LPI then offset printing plates and reproduction is much better when using XM/FM.

Absolutely correct.

And I was not talking about FM, I was talking about printing industry as a whole. Publishers do not care about moire and contone look, they care for speed of production (to which faster drying and mis-registration resilience helps) with less ink spent, all equaling in more money with same machine hours and man hours for higher number of copies and less ink spent.

That's why I noted that the benefits of FM vary with the type of work that's being done.

Very rarely we do something with artistic and permanent value that requires high screen rulings AM or FM or XM (in my opinion, phone books, labels, flayers, etc. do not have permanent value). Only some specialized color magazines (like home decoration, maritime or automotive) require good reproduction while still in (sort of) higher-midrange of screen rulings/dot sizes.

I used those examples to show that the benefits of FM screening can apply to print jobs that people do not think of as being "quality" or permanent value.

Economy kills art in printing and that is my whole point.

I can understand that feeling. However there's a saying that "Just because people are willing to eat $hit doesn't mean that I have to serve it." Printers who take pride in their work will try to produce the best printing possible and then find ways to deliver that printing at a price that allows them to remain competitive. For example, printing with 20 micron FM does not cost more than printing at 150 or 175 lpi AM, but it is an easy difference in printing quality that any buyer can appreciate. And if the customer doesn't care, well so what, the printer can benefit by lower production costs and wastage.

However, one point you mentioned is something I'm unaware of: larger color gamut when using FM. I was always thinking that gamut is limited by CMYK color space and that no matter the screening, that color space is limited by CMYK ability to optically mix itself in a limited number of steps (let's say). And I thought that this was the reason to broaden CMYK either with Orange and Green or with Light Cyan and Light Magenta. I mean no offense here. Would you be so kind to elaborate on this? Thank you in advance.

A post on my blog explains how the extra gamut is achieved. Click here: Quality In Print: AM and FM gamuts compared - part 1 of 2

It won't reach the extra gamut that you can get by using extra process colors but, for example, a 20 micron FM screen (or a 350+ lpi AM/XM screen) will deliver a larger gamut than 150-175 lpi AM/XM.

best gordo

 

[tomcatinc]

Point taken and thanks a lot for enlightenment, Gordo!