GRACol and uncoated

[meddington]

So when longlimb said

Uncal Coated 50% screen reads 72 - so I cut back the 50% area on my curve by 4 so I would hit my 68 (18% gain)
Uncal Uncoated 50% screen reads 77 - so I would cut back the 50% area on my curve by 9 so I would hit 68 (18% gain)


was he not correct?

Not necessarily. Deriving a cutback curve via TVI by taking the difference between the measured dot percent values between 'target' and 'sample' TVI can yield an incorrect value for the cutback curve. A more accurate method would be to derive curve from the dot percent on the 'sample' that results in the target dot percent. This is illustrated at Gordo's blog:Quality In Print: Printing at DMaxx - part 4 of 5 Even in doing so, the target and sample curves should ideally be using the same printing process/screening.

If dot gain/TVI is defined as the difference between the printed dot size and the requested dot size as it is defined in the digital file, and the printed dot comes out the same size on coated or uncoated (or even newsprint) paper, then the dot gain in both cases would be the same.

Ex.
50% dot size in digital file
68% dot size desired on coated paper = 18% TVI
68% dot size desired on uncoated paper = 18% TVI
68% dot size desired on newsprint = 18% TVI
68% dot size desired on toilet paper = 18% TVI

Now I realize that that's not actually how it works. My point is that by definition, the above is true, yes?

Not necessarily. The measured TVI may be identical, but visual tonality/density of each patch may differ. TVI (derived from the Murray-Davies equation) isn't a measure of dot size at all, but rather is a function of Density, and compares a tint patch to a solid patch. So although the 18% reported value between all your examples may be "correct", the actual perceived tonality between them could vary, as the achievable solid ink density and substrate density varies between them.

This is why a standard TVI value of say 68% for all substrates is not an ideal indicator of tonality if your goal is to create a more common visual appearance across substrates/processes. Defining tonality by density, independent of sold ink density, screening or print process, is less ambiguous.

 

[cmcfarling]

Not necessarily. Deriving a cutback curve via TVI by taking the difference between the measured dot percent values between 'target' and 'sample' TVI can yield an incorrect value for the cutback curve. A more accurate method would be to derive curve from the dot percent on the 'sample' that results in the target dot percent.

Ok I get what you're saying. There is not necessarily a linear correlation between a transfer curve for coated paper vs. one for uncoated paper for example. You wouldn't want to just use simple subtraction to determine the uncoated paper curve, you'd want to run sample patches and take actual measurements, as per gordos blog entry.

Not necessarily. The measured TVI may be identical, but visual tonality/density of each patch may differ.

Note that thsose weren't necessarily measured TVI values, but rather hypothetical TVI values strictly based on the definition of TVI. I would assume that if I took actual measuremnts of samples from each of those scenarios, the densitometer would tell me that the measured TVI would increase as you went down the list.

The gist is that it seems to me that when discussing TVI, it can mean different things in different cases. On top of that different people don't always understand how TVI is derived. Say you open a file in photoshop and find a tint that measures 50%. Now you go get a densitometer and measure the same patch on the printed press sheet which is uncoated paper. You set the densitometer to measure dot area and you take a reading of the same patch. The densitometer reports 68%. You conclude that you have 18% TVI. Now, you're probably wrong, and if you measured TVI correctly by measuring paper, solid and then your tint patch you would find that actual TVI is higher than 18% in this case.

TVI is something that seems very basic on the surface but once you really dig into it there are a lot of intricacies and caveats to be aware of. A lot of people in the industry probably don't realize this. In fact I think that it's probably not uncommon to come across people who have been in the industry for years who think that 18% dot gain means a 20% dot prints at 38% and a 70% dot prints at 88%.

Even the experts give conflicting definitions sometimes. Not calling gordo out here, just making the point that this can be a confusing topic. In a previous post in this thread gordo stated:

Dot gain specifications came from a film workflow, where the standard was a 50% dot in the film when a 50% dot was requested in the file. Nobody measured the plate. They measured the press sheet.
So, the reference dot used as the comparison for the printed dot is the tone value requested in the file - not whatever is on the plate.

And on his blog under Dot Gain/TVI (Tone Value Increase) - part 1 of 4 he states:

"Dot gain" is the term that is used to describe the difference between the requested tone value (in the film in a FtP workflow or the plate in a CtP workflow) and the resulting apparent final tone value on the substrate as measured with a densitometer.

It seems to me that the latter is a more accurate definition. The 50% dot in photoshop is not always going to start out as a 50% dot on the printing plate. And what happens to the dot from the printing plate to the substrate is what really matters from a process control standpoint.

 

[gordo]

To cmcfarling:

RE: "gordo, where did you get 78%? Is that a typo or am I missing 10% somewhere?" The 78% comes from being too long driving/living in a camper van for Print '09 and dicey internet connections making it difficult to edit my posts. I will apply a mental compensation curve once I get back home.

You are correct about my not quite saying the same thing about dot gain on my blog as I had posted here. I will correct the blog posting to bring it in line.

You are also correct about dot gain being more complex than it appears at first and there are problems with how the different equations deal with the mechanical/optical aspects. Also, because TVI is calculated from measured relationships and doesn't include the hue, you can get some interesting disconnects, e.g. SIDs can be increased but measured TVI goes down, or, using an FM screen, measured final tone values can be the same - but the FM screen can appear lighter than a 175 lpi AM/XM screen. There is a excellent book on the topic, actually a master's thesis, by (I think) a Swedish student. I'll post the publishing info once I'm back home (where my copy is) next week.

best, gordo

 

[Deleted member 16349]

You are also correct about dot gain being more complex than it appears at first and there are problems with how the different equations deal with the mechanical/optical aspects.

The M-D equation makes the assumption that the density of the dot is the same as the SID, which in reality is not true. Dots are peaked and therefore have variable density across their diameter. With FM the dots do not have the same density of the solid since their thickness is less than the solid.

Due to the thinner ink film of FM dots and their smaller diameter, more light is diffused up through the dots giving them their lighter appearance. The increased TVI is more due to the increase in optical dot gain due to the higher perimeter length around all those little dots.

In the end I think that looking at TVI will be mostly for process control on press. It should have no use in the future for colour managing printing. But that's just my personal view.

 

[cmcfarling]

To cmcfarling:
You are correct about my not quite saying the same thing about dot gain on my blog as I had posted here. I will correct the blog posting to bring it in line.

Well wait a sec. What I was inferring was that your blog posting made more sense. Forget about densitometers, Murray-Davies, densities, etc for the moment. When discussing dot gain, the core bits of information that you need to know are 1) how big did is the dot when it starts out and 2) how big is the dot once it's on paper. The phrase "when it starts out" I think is what is causing me to question the definition.

For the sake of illustration let's use really big dots that can be measured with a ruler. We create a file in Photoshop and fill it with 50% black. Using CtP we image that file to a plate. In this case there are no compensation curves applied during plate imaging so the 50% tint in the file is imaged with 50% dots. In this case each 50% dot on the plate happens to be 10mm in diameter. The plate is put on an offset press which is used to print the dots onto paper. Let's call this sample1. We measure the dots on the paper and see that they are now 12mm in diameter. From the printing plate to the paper the dot has grown by 20% (in this sense I mean physically 20%, not 20% as in the traditional sense when speaking of TVI). In this case we could also say that the printed dot has grown by 20% compared to the digital file since the value in the digital file is the same as what was imaged on the plate.

Now we take the same digital file and image it to the plate but this time we apply a compensation curve at imaging time. This curve causes the 50% tint in the digital file to be imaged with 40% dots. We measure the dots on the plate and see that they are 9mm in diameter now. The main point here is that they are smaller than before. Again, the plate is put on press and the dots are printed on paper. Let's call this sample2. We measure the dots on the paper and see that they are now 12mm in diameter, which is the same as before. From the printing plate to the paper the dot has grown by 33%. Compared to the digital file however there is no difference between sample1 and sample2.

In both cases we ended up with the same size dot on the printed paper. If the definition of dot gain is final dot size minus requested dot size from the digital file then both samples exhibit identical dot gains. However in the case of sample2, the printing conditions (whether due to blankets, paper, cylinder pressure, etc) caused the dot to physically grow more than with sample1. The dot started out smaller on the plate and ended up the same size. So in reality sample 2 experienced a higher dot gain.

So shouldn't dot gain be defined as final dot size minus plate dot size?

 

[gordo]

You are right of course, dot gain is a measure of the distortion of the halftone dot as it moves through the production process. So you need a dot so that it can be distorted.

Which makes your statement: "So shouldn't dot gain be defined as final dot size minus plate dot size?" reasonable.

Unfortunately, just considering dot gain as the dot on plate vs the dot on the press sheet does not necessarily provide the complete story. "When it starts out" can change depending on what you are trying to do.

For example, dot gain can useful as a process control metric in a color bar, by measuring the dot on plate (e.g. 50%) then on press (e.g. 68%) then determining the dot gain (18%) then monitoring any fluctuation in dot gain through the press run. But one can argue that it doesn't matter what was on the dot was on plate. One could simply monitor the fluctuations in the final tone (68%) compared with the target (68%) for a requested 50% tone patch.

Dot gain can be used to evaluate ink performance. This can be done with a tone reproduction curve for each process color printed from linear plates - i.e. 50% in the source file is 50% on plate. At that point the plate and source file tone requests are the same. What is on the plate is the same as what is in the source file. So you could say that "when it starts out" is the same for plate and file.

Dot gain can be used to create dot gain compensation curves in order to deal with things like finer screen rulings, increased SIDs, changes in substrate etc. In that case the process is normally done by starting with the file request (e.g. 50%) - not what was on the plate. Then measuring the dot area on the press sheet where the 50% patch is printed (e.g. 76%) and comparing it to the target (e.g. 68%). Those three parameters (requested dot in file, response of the dot on press (76%), target for the 50% (68%) are then used by the software to create a compensation dot on the plate (e.g. when a 50% tone is requested image instead a 46% dot) to hit the target tone on press. In this case it does not matter whether the plate is linear or not, nor does it matter what the dot on the plate is - as long as it's imaged consistently. What matters is the tone I requested vs the tone that I got on the press sheet.

So, again, you are right of course, however, dot gain (plate to press) is not so useful or important, compared to the final tone compared with the requested tone. Dot gain gets you there - but the important thing is the final tone. For years dot gain was considered as the target - and in a film to plate workflow it made some sense. However if instead of looking at it as the target look at it as the final tone representing your target - then you can exercise much more control over your process. I think that is the idea of GRACoL 7 using density through the tone scale rather than dot gain.

hope this makes some sense, gordo

 

[cmcfarling]

Yes gordo that all makes complete sense. Thanks for the useful replies. And by the way, I'm glad I found your blog because it has a ton of useful and detailed explanations on a whole host of topics.

I've been around printing companies and presses for a number of years but always on the prepress and IT side. In an effort to better educate myself on pressrom topics I've been reading up lately, hence this thread. When talking TVI with pressmen and such I'll make sure were all on the same page and take compensation curves into account.

The thing is, you could have two similar presses side by side. One is old and not well maintained while the other is brand new. The older one may really expand the dots as they make their way from plate to blanket to paper while the new one hardly expands them at all. Of course, using exposure curves when burning the plates can compensate for the differences so that they both print identically (or close to it) but that doesn't eliminate the fact that both presses posses vastly different on-press dot gain characteristics.

In this situation we could say we have two presses with different dot gain but yet the same dot gain. Although seemingly contradictory, it's true.

Anyway I'll move on and accept it for what it is, and add this to the list of many caveats related to this field.

 

[gordo]

In this situation we could say we have two presses with different dot gain but yet the same dot gain. Although seemingly contradictory, it's true.

Or more accurately, in this situation we could say we have two presses with different dot gains but yet both deliver the same final target tone response.

best, gordo

 

[cmcfarling]

Or more accurately, in this situation we could say we have two presses with different dot gains but yet both deliver the same final target tone response.

Yes we COULD say that but I think it's likely that no one WILL say that. That would be a more accurate description though since we are talking about two separate things. Maybe FTTR can become a new industry standard term thanks to this thread.

Actually, now that I think about it, there are already two terms in use to describe this phenomenon, dot gain and TVI. Everything I've ever read just lumps them together as if there is no difference in the meanings. Light bulb moment here... perhaps this is why the term TVI was ever proposed. Does anyone know the origins of the term Tone Value Increase and why someone decided that there should be a new term for something that has always been called dot gain? This would actually make perfect sense. Reserve Dot Gain for discussions involving requested dot vs. printed dot and reserve TVI for discussions involving plate imaged dot vs. printed dot. Am I on to something here?

 

[gordo]

Actually, now that I think about it, there are already two terms in use to describe this phenomenon, dot gain and TVI. Everything I've ever read just lumps them together as if there is no difference in the meanings.

There is no technical difference between dot gain and TVI. TVI just more accurately describes what is important - the tone change. Also, not all processes that have gain, or proof dot gain, actually have a dot. So TVI is more inclusive of reproduction processes.

best, gordo

 

[cmcfarling]

Actually, now that you mention that I do recall reading something that made note of the fact that TVI was a better term due to some processes not having a dot. Even with that being the case, TVI could still be reserved for what could be described as device dependent gain while Dot Gain could describe so called device independent gain.

Using the same term to describe two distinct things just doesn't seem right to me.
Oc course there could still be hope for FTTR