PDF/X and Image Resolution

[dwanehollands]

Hi all,

Just a question or perhaps an observation regarding PDF/X.

After doing a few small tests it seems that a PDF/X-1a:2001 verification does not actually check the resolution of the images in the files. Or am I missing something?

Kind of dissapointing since it's a pretty major issue. I mean it's fine if you set your Export/Distiller options to downsample to 300dpi, but that doesn't guarantee that the source files are going to be above 300dpi does it? And again it's better to catch the problems before transmission, since it saves hours or even days of re-submission issues.

Thoughts, corrections?

Dwane.

 

[John Clifford]

Re: PDF/X and Image Resolution

The PDF/X standard doesn't check for image resolution. The reason is that there might be instances where you WANT a low resolution to print. An example is a recent magazine that I worked on where the art director wanted to use a low-res, pixilated image to express the concept of "digital." Another example is the wealth of computer books that use screen captures. You can't create screen captures at anything but screen resolution. Reducing them will gain them some resolution but it still will be fairly low, or at least lower than what most people want in quality printing.

Plus, what is "right" resolution? If you use the rule of thumb that resolution should be twice line screen, then a newspaper running at 100 lpi would WANT an image at 200 dpi. Likewise a web press running 130 lpi screening would only really need 160dpi.

PDF/X is a standard for ensuring that a file will print/plate. It is not a standard for ensuring that files will print high quality. That's what we in prepress are still needed for.

You can do checks through all of the standard preflighting software (including Acrobat's preflight) to test for low-res images (you determine what you feel are low-res images), but such tests are beyond the standard for PDF/X and should be.

 

[dwanehollands]

Re: PDF/X and Image Resolution

John,

I understand there are circumstances when low resolutions "Must Run". Often our customers tell us to run with 72dpi images. That's fine.
I just think it should be a notification and indication to the designer that there are low resolution images, and where they are, so that they can find and fix immediately.

There's an opportunity to solve the problem up-stream and reduce re-submits with standardised software without trying to get designers to purchase high priced plug-ins. Particularily with 'one off jobs' which is what PDF/X is meant to be ideal for.

Dwane.

 

[davidmwe]

Re: PDF/X and Image Resolution

Hi - The +GWG+ (Ghent PDF Workgroup) set of PDF settings does check for image resolution. They set up these PDF/X-Plus series of checks exactly for these reasons mentioned above. See: [www.gwg.org|http://www.gwg.org/index.php?search...olor=all&precre=all&res=all&id=9,191,0,0,1,0]
[FlightCheck Professional|http://www.markzware-europe.com/store/fligthcheck] will of course also check not only the native files for effective image resolution (important to stress that effective part) however also the resulting PDF for any effective resolutions of used images which fall out of range.

Friendly Regards,
David Dilling
Markzware

Edited by: David Dilling on Jul 29, 2008 5:43 AM

 

[John Clifford]

Re: PDF/X and Image Resolution

Thanks David for the Ghent update. So what are the resolutions that it checks for and if a file doesn't have the proper resolution is it not a PDF/X file?

Dwane: I completely understand what you're asking for. Again, however, what standard for image resolution do you want to have PDF/X check? This is the problem with standards. If you're a designer who wants to ensure that your images are the appropriate resolution before sending them downstream to the plate room, then create a preflight, which can be based on the PDF/X standard that also includes a resolution check. That way, you can ensure that YOUR files are what YOU want them to be.

PDF/X is a standard whereby a printer (or other recipient of a "blind transfer," such as a publication) can ensure that the file will print. That is: colors are in the proper color space, fonts are embedded, images are embedded, etc.

I'm sure that there are plenty of people out there who would like PDF/X to check for more than 4 colors (so what about my 6-color job, or my varnish?), that there is bleed, or some other "common problem" with user files. Again, the STANDARD is that a job prints, not that it is IDEAL.

Even within PDF/X there are currently several versions. PDF/X-1a which checks for transparency and disallows it and also disallows color management, PDF/X-3 which allows color management but also disallows transparency, and now PDF/X4 which allows transparency. If we were to check for effective resolution, then we'd need to have another whole set of PDF/X standards for 130 or 150 lpi standard screening web, 200 lpi sheetfed standard screening, a standard for stochastic screening, etc.

 

[davidmwe]

Re: PDF/X and Image Resolution

> {quote:title=John Clifford wrote:}{quote}
> Thanks David for the Ghent update. So what are the resolutions that it checks for and if a file doesn't have the proper resolution is it not a PDF/X file?

There are about 13 different flavors of PDF/X-Plus profiles from the Ghent PDF Workgroup. You can see them all on their web page. Depending on which one you want to use, there are varying resolution requirements. For instance, under the "CmykVeryHighRes" profile, it gives you an error if it is below 150 dpi and warns you if it is over 450 dpi. There are also several resolution checks within any profile. FlightCheck can see all of this of course...

Yes, if it does not pass this set of checks, it is not seen as a valid PDF/X file (based on the GWG settings v3)... Once again, you may want to check out their web site, for they have a lot of options.

 

[Dov Isaacs]

Re: PDF/X and Image Resolution


Two issues:

(1) Validating against the +Ghent PDF Workgroup+ profiles does not validate a file as PDF/X compliant, but rather, as compliant with one of the Ghent PDF Workgroup's PDF/X +subset+ specifications branded by the Ghent PDF Workgroup as +PDF/X Plus+ (although effectively it is really PDF/X Minus).

(2) Image resolution issues come up in these and many other forums. To be very clear, there are numerous situations in which it is perfectly valid to print from images that have a much lower effective resolution than that which might be considered "optimal" for a particular printing technology and device resolution. One example, quite near and dear to those of us in the software industry, is that of "screen shots." Depending upon the operating system, screen resolutions settings, etc. as well as the magnification of the screen shot's image in the document into which it is placed, the typical screen shot may have an effective resolution of anywhere from 60dpi to 150dpi, certainly much less than the 300 dpi which is all too often stated as an absolute requirement for commercial printing. The fact is that the best way to reproduce such images is to place them in the target document "as is" and not attempt to "up-rez" them in Photoshop. The target RIP device will do that for you. Prematurely attempting to up-rez an image to some artificial value does not add any quality to the image and may actually yield poorer results at RIP time than if you let the RIP do the up-rez itself. Contrary to popular belief, as part of the screen process, every image sent to a PostScript or PDF RIP is in fact resampled anyway. You gain no quality or real performance gain by trying to go for an optimal resolution of all raster data prior to the RIP process.

Although preflight profiles checking image resolution are exceptionally useful for weeding out FPO images and web crud that should be replaced by comparable, bona fide higher resolution imagery, decisions to reject lower resolution imagery should be made on a case-by-case basis.

- Dov

 

[davidmwe]

Re: PDF/X and Image Resolution

> {quote:title=Dov Isaacs wrote:}{quote}

>
> Although preflight profiles checking image resolution are exceptionally useful for weeding out FPO images and web crud that should be replaced by comparable, bona fide higher resolution imagery, decisions to reject lower resolution imagery should be made on a case-by-case basis.
>

Very good point- in other words, preflight, preflight and then postflight, time and time again.
For instance, building off of your example; a screen shot or screen grab placed within InDesign or QuarkXPress (of course normally at 72 dpi) at it's original size, would FAIL a preflight check based on it's resolution. However, take that same placed screen grab image within InDesign and scale it to 24% (or less) [72 x 24%= 300 dpi], results in an acceptable "effective resolution" of the image within InDesign or the resulting PDF (when settings are correct). This scaling should of course take place on the screen shot within Photoshop or similar application to avoid longer processing times in the RIP (note that you are not UpRezzing the image, but creating a higher resolution just by scaling the image down in size.).

Like Dov basically said, if all you have is a 150 dpi screen shot image and you have to use it at 100%, then place it and many (not all) RIP applications can handle it to some level or degree. (A contract proof is certainly recommended, for you can not magically make a low-res image print at high-res quality that easily.) I've seen manuals with screen shots come back acceptable and I have seem them come back looking like I needed glasses.

Oh, and yes, of course a screen grab is normally in RGB...

Friendly Regards,
David Dilling
[Markzware|http://www.markzware-europe.com]

 

[Dov Isaacs]

Re: PDF/X and Image Resolution

And if you don't want to scrunch down the 72dpi screen shot to 24% magnification or less in the publication you are printing from? I still contend that you are best leaving such images alone and not hacking around with them in Photoshop. Layout programs including both InDesign and QuarkXPress deal with the effective resolution issue in terms of the output PostScript and/or PDF.

Note that I know of no RIP that cannot properly uprez such images at RIP time - a RIP that cannot handle this is effectively in violation of the PostScript and/or PDF specifications. You really save no measurable RIP time by doing any manipulation of screen shots prior to the RIP process. And with any reasonable color management in use, there is no reason not to let the RIP do the color conversion to CMYK!

- Dov

 

[John Clifford]

Re: PDF/X and Image Resolution

Thanks Dov for clarifying the PDF/X issue as well as the screen shot issue that I originally brought up.

I'm very concerned about the idea of all of the Ghent subsets of PDF/X. The thing about standards is that the fewer there are, the easier they are to implement. ISO has standards for lots of things (such as PDF/X). As I explain to my students, the ISO has standards in a lot of industries for a lot of different things. The example I give is that you can purchase a standard lightbulb from any manufacturer and put it in any lamp. Why? Because there is a simple standard for the threads on the bulb and for where the contacts are, etc. If there were 12 standards for lightbulbs, we'd end up with a way too much diversity and nothing would really work with anything else.

I like to the idea of PDF/X-1a, PDF/X-3, and now PDF/X-4 with the anticipation that as we moved in time that 1a and 3 will be completely supplanted by 4. Using this we can create a file that we know will print regardless of where we send. Once we've determined that it will print, then we can determine if it will print the way that we want it. I'm afraid that Ghent may be trying to create specifications for too many different scenarios when that should be the purvue of applications like Acrobat preflight or Flightcheck to allow the quality user to ensure their quality. Let's KISS (Keep it Simple Stupid).

 

[Dov Isaacs]

Re: PDF/X and Image Resolution

Although I am personally Adobe's representative to the Ghent PDF Workgroup (GWG), I do share with you the concerns about too many rigid +standards+. I also like the +KISS+ principle. I think though that you need to look at the GWG PDF/X-Plus Standards more as workflow settings and preflight conditions as opposed to real standards. They are the result of a number of printers, print trade associations, and publications initially primarily from Europe wanting to be able to mandate +cookbook+ PDF settings onto their clients to try to avoid printing failures (and of course, avoid being blamed when such failures occur). Ultimately, the GWG will have standards based on PDF/X-4 although they will probably have a CMYK-only flavour of same as well as a fully-color managed version. Ironically, a PDF file that +passes+ pre-flight for any one or more of the GWG standards can still yield terrible results in terms of print quality. There is no way to automatically pre-flight against poor design, lousy taste, questionable layout, and incompetent use of the layout and editing tools prior to the actual generation of PDF.

In terms of my own content preparation and dealings with printers, I produce a PDF/X-4 file with live transparency and color management in which I keep content at the highest level of abstraction all the way to the RIP. I don't upsample, I am exceptionally conservative with images in terms of downsampling. I keep the images (and any other non-CMYK content) in their original color spaces (typically Adobe RGB or sRGB) until RIP time. And more importantly, I won't deal with printers who can't fully and properly deal with that!

- Dov

 

[[email protected]]

Re: PDF/X and Image Resolution

> David Dilling{quote:title= wrote:
> }{quote}I've seen manuals with screen shots come back acceptable and I have
> seem them come back looking like I needed glasses.
> Oh, and yes, of course a screen grab is normally in RGB...
>
> {quote:title=Dov Isaacs wrote:}{quote}
> You really save no measurable RIP time by doing any manipulation of
> screen shots prior to the RIP process. And with any reasonable color
> management in use, there is no reason not to let the RIP do the color
> conversion to CMYK!
I just want to add some additional points to this thread:


1) I have a lot of experience inserting screen shots into books and trade magazine articles. Most of the time, if you reduce the size of the images to match a 300 dpi effective resolution (don't matter if you do that within Photoshop or resizing in the layout software) they will become totally unreadable without a magnifying glass. Generally, it's much better sending then at 120 or 150 dpi and accept the fact that the text will be a little pixelate in the borders. Upsample the low res screen shot in Photoshop don't make things better, because they gain a kind of "out of focus" appearance.

2) RGB screen shots print better if you convert then to CMYK using a "maximum GCR" setting in Photoshop (Edit / Convert to Profile / Custom CMYK) instead of using a standart ICC profile setting. Doing this way, the black text goes to K100 in the black channel, instead of spreading to some C80 M70 Y60 K90 like composition – a registration nightmare, especially in web offset presses (despite it is not such a problem in digital printers).

3) Another common use of low res images is when you want to place a gradient background. Bitmap gradients generated in Photoshop (with fine color adjustments and some level of noise added) often do a better job than layout programs gradients, especially in avoiding banding problems. And you don't need more than 100 or 120 dpi in a gradient background image.

4) Automatic preflight verification of images resolution is a very usefull tool to avoid mistakes, like "position only" rough files missed in pages. But it's important to know that this kind of check is not a real guarantee against "in fact" low res images. Any "smart operator" can easily usample a mobile phone capture to reach 300 dpi in full page size, deceiving the preflight software. In this cases, there is no substitute to a carefull visual proof check.

Best regards;

André Borges Lopes
Bytes & Types
São Paulo, SP - Brazil

Edited by: André Borges Lopes on Jul 31, 2008 8:42 AM

 

[John Clifford]

Re: PDF/X and Image Resolution

André,

You'll actually get better screen captures if you use UCR instead of GCR. Using GCR you get rich black text which, because it's small and needs to be tightly registered, is not ideal. Using UCR makes the truly black text black only which increases the readability.

The main problem I had was when UCR was used on screen captures, the operators might forget to change back to GCR and then regular images would be messed up. But if you're working in a good environment, it's just a matter of training.

 

[[email protected]]

Re: PDF/X and Image Resolution

> {quote:title=John Clifford wrote:}{quote}
>
> You'll actually get better screen captures if you use UCR instead of GCR. Using GCR you get rich black text which, because it's small and needs to be tightly registered, is not ideal. Using UCR makes the truly black text black only which increases the readability.
>
> The main problem I had was when UCR was used on screen captures, the operators might forget to change back to GCR and then regular images would be messed up. But if you're working in a good environment, it's just a matter of training.
Hi John.

I'm afraid you're doing a mistake in this specific case.

The terms UCR and GRC are used with a lot of different meanings in image manipulation software. But, as long as I know, when using Photoshop "Custom CMYK" feature (aka "CMYK Setup") the only way to convert RGB black (R0 G0 B0) to pure 100% K in CMYK mode is using GCR, with Black Generation adjusted to Maximum, Black ink limit at 100% and UCA Amount 0%. Using UCR, as you suggest, (or CGR with any other level of black generation) will give us some "rich black" formula, mixing the four CMYK inks.

Alternatively, we can build some customized ICC profile with a similar black generation setting. But to do that, we will need some specialized (and expensive) software – like Profile Maker, Monaco Profiler, etc. Despite not being a real ICC Profile generator, CMYK Setup tables are a practical and cheap way to customize black generation in Photoshop.

And is exactly to avoid the problem you've mentioned (forget to change back the settings) the reason that I've recomended to use the feature "Edit - Convert to Profile" with Custom CMYK option, instead of changing the usual Color Settings configuration.

Best regards;

André Borges Lopes

 

[davidmwe]

Re: PDF/X and Image Resolution

Good exchange- thank you everyone, I have learned a lot the last days here. Generally speaking, when I print something with screen shots used, I communicate with our printer first to understand the best way to go about it for that job, for their workflow. (Communication and Preflighting can go a long way!)

Friendly Regards,
David Dilling

 

[Lukas Engqvist]

Re: PDF/X and Image Resolution

sorry to barge in on a "closed subject" Just one thing I notice with modern screendumps. A traditional CRT scrren will give better text screendumps, the CoolType antialising is good for CRT monitors but does not make good type for colourseparations, If you can do screendumps to a CRT or with CoolType (or similar font optimising for LCD) dissabled you will get better results.