[Scribus] Setting linescreen when printing pdf file

[Louis Desjardins]

? (At) 11h02 +0800 21/07/04, Craig Ringer ?crivait (wrote) :

Hi again,

Please refer to LL's accurate precisions on ppi, dpi and lpi. If we 
are to post an explanation on this subject, I also think there would 
be a need for a little graphic help to keep our readers reading!

>Though a high line screen on a low-res file will get you a result where
>pixellation or scaling artifacts are really obvious, yes?
>
>Also, with a low line screen my understanding is that a high-res input
>doesn't do you any good, because it's not possible to represent the
>additional information in the coarse output.

We have to acknowledge linescreen is a destructive process. Once the 
linescreen is applied, you can literally say good bye to your image 
quality. A 50% shade means that there is ink on 50% of the area 
covered by that shade (for that one color). The rest is white, 
unprinted, paper. A magnifier will show a perfect chess board. This 
is only to say how "bad" the linescreen process will alter an image. 
Basically, you get rid of a fair percentage of the image information! 
But again, your eye will be so well fooled... Ask people around (not 
in your working area!) and chances are they never even noticed there 
where dots on their newspaper or favorite magazine!

When I say "destructive" I refer to the fact that a screened image is 
not going to be usable anymore at other size, without serious loss in 
quality. I don't mean the image will not look OK when printed. If 
there are further needs for that image to be used again on other 
publication you need to go back to the original image, at high "ppi".

>
>>  Various
>>  combinations, various results. For a newspaper, printed on a
>>  high-speed web press on cheap, uncoated, newsprint paper, the need
>>  for a low linescreen is obvious. You don't want ink to "plug" your
>>  images, so you need big dots spaced by lots of space because this
>>  paper will allow ink to penetrate deeply into its fibers and
>>  "spreading".
>
>Yep. We get 30% dot gain here (!!) due to the way the paper bleeds so
>much. My understanding is that you can't use a fine screen without
>everything mashing together and producing muddy, soggy, awful images.
>
>>  So yes, absolutely, the linescreen is applied to the entire page. To
>>  what extent does it affect the elements? First, there are no "dots"
>>  to see in a 100% of any of the CMYK color since the whole area is
>>  filled with that color.
>
>Can any press actually handle 100%, though? We're limited to 98%
>pure-black, ~96% CMKY here.

Presses can handle 100% ink coverage. Text and titles are good 
example of this. Also, a color such as "Registration" means that all 
four process colors (Cyan-Magenta-Yellow-Black) are present at 100% 
and will show on each plate. A "rich black" is made of a combination 
of CMYK and not only of Black ink. For example, here is one recipe we 
use with good results with one of our printers (sheet fed press) : 
100K - 60C - 40M - 40Y. Note that total ink coverage in that case is 
240%. Be careful, the figures might be very different in other 
environments. This is a case of good communication between the 
designer, the prepress house and the print shop.

The 98% limitation for black is to compensate dot gain on the press. 
Also, tell me if I am wrong, this limitation is only applicable to 
halftoning and doesn't mean you can't print solid ink at 100% (text 
and titles will suffer)? Right?

>
>>  Linescreen is a necessity, it is not evil. ALL printed material is
>>  screened. A 150-lines job can be of very high quality. Even newspaper
>>  don't look so bad.
>
>Sometimes ;-)
>
>Depends a _lot_ on your press, and the skill of the press staff. We used
>to have some pretty awful results, but now we get very nice photos when
>we use good near-white stock - better than a cheap home inkjet
>sometimes.

Printing a halftone is one of the big challenges for any press and press staff.

>
>>  A TIFF image has a fixed resolution. It can be anything. Usually, 72,
>>  150, 300, 600 dpi. This depends on the scanner settings or the
>  > settings at which the image was saved. If the TIFF's resolution is
>>  too low, there is no way this image will produce good result on a
>>  press. Also, if the black generation came from the RGB world, as I
>>  wrote in an earlier message, that black may not be black only and
>>  could end up beeing 5% or 7% short of a full 100% black. Then, of
>>  course, it will be screened and of course if it is too small an
>>  element, low (and even high) linescreen will reveal the weekness of
>>  the whole process if that element is not "solid" ink (100%).
>
>Hmm. It could be, then, that when clients supply us with ads in TIFF
>format (or PDFs that contain only an image), Acrobat or something at the
>printer's is downsampling the image, causing the quality issues - and
>that if we sent the image at a high resolution (say, >800dpi) we might
>get better results.
>
>This bears thinking about. Unfortunately, it might be quite tricky to
>downsample _most_ of the graphics on the page, but leave any ads
>untouched. Hmm. This is something I need to look into, anyway.
>
>>  A PDF is a vector file and has no predefined resolution until it is
>>  actually ripped (fixed). Only, the pictures in the PDF can be
>>  converted, at PDF creation, to a specific resolution (this is why the
>>  graphic art industry has come up with specifications such as
>  > PDF/X-1a, to avoid unusable PDFs).
>
>Clients? Follow specs? What a lovely thought. Maybe it'll happen one day
>;-)

See my post on Scribus Friendly Print Shops for more on that!

>
>>  Thank you for reading!
>
>Thank /you/ for your very helpful post. I'm familiar with most of what
>you write here, but not in as much detail or as solidly as I'd like, so
>it's very helpful.
>
>--
>Craig Ringer

;-)

Louis