 The magazine of the Melbourne PC User Group DTP and PostScript files - tips and tricks Stephen Davey |
Today, with the aid of relatively inexpensive software and hardware you can produce much of your own publishing needs with greater flexibility and substantially less cost than previously possible. Software packages such as CorelDRAW!, Adobe Illustrator, PageMaker, Quark Express and Corel Ventura enable you to easily design and layout your own brochures, leaflets and magazines.
The crunch comes when you have finished and you need to output some "masters" that the printing company can use to produce the final printed item.
While most people involved in desktop publishing have 300-800 dpi laser printers, these are not of a high enough quality to produce artwork or film negatives suitable for good print production. However there is a new breed of service companies called either "Imagesetting Bureaus" or "Service Bureaus" that have high resolution (2600-3000 DPI) image-setting or output devices that can output your files onto "bromide" paper or film.
For full-colour brochures you can do your own scans (it may take a bit of trial and error!) or use these bureaus to provide you with high-quality colour scans that you can place into your own files. Even if you do not have a very powerful system it is still possible to set up a full-colour production by using black and white "positional" images in your layout, which are then replaced with high resolution colour images as the final film is produced on the service bureau's imagesetter.
Software choices
It does not matter what software you use, these tips and techniques apply to most.
PostScript--the standard
Virtually all graphic arts programs use a standard page description language called PostScript to describe (in computer terms) how the page looks. So long as your program supports PostScript, then image-setting bureaus can output your files to bromide or film on a high resolution imagesetter.
Most computer graphics programs are capable of printing the jobs you have created in two ways
- Printing direct to a printer connected to the same computer
- Printing to a file that can later be processed by a printer in a different location (e.g. your imagesetting bureau). These PostScript print files simply capture the stream of information that would normally be sent to your printer. Not all imagesetting bureaus can process PostScript files, check with your particular bureau.
PostScript print files can be processed faster and more efficiently than application files such as CorelDRAW!, PageMaker or Quark files.
To print your application files to its imagesetter the bureau must
- Firstly start the appropriate program (Corel, PageMaker etc),
- Open your file, checking that any linked graphics are present and the fonts you have used match those installed on its system.
- If there are any font mismatches the correct fonts have to be loaded.
- Finally either a PostScript print file is produced, which is then processed by the imagesetter, or the job may be sent directly to the imagesetter.
Advantages of PostScript print files
- Properly created PostScript print files contain all the font and graphic information required to print the job in just one file.
- The bureau doesn't need any separate linked graphic files, used by programs such as PageMaker, Quark and Ventura, as they are automatically included in the PostScript print file.
- The bureau doesn't need copies of your special fonts. The PostScript print file includes information about the fonts in your system, so what you thought you would get is what you get.
- PostScript print files can generally be processed by the imagesetter much faster than application files and as a result bromides and film produced from PostScript print files are less expensive.
- A PostScript print file can be substantially larger than the corresponding application file.
- PostScript print files take a little extra time to create. However time you spend is time the bureaus don't, and therefore they should charge you less.
To enable imagesetting bureaus to produce bromides and or film from your graphic file they need either
- PostScript print files of your job or
- Your application files, special fonts and any linked graphics.
Fonts!@#**!
Computer graphics programs use font files (and sometimes separate font management programs) to create, display and print text in different typefaces. Unless you specifically arrange otherwise, your graphics file only contains the name of the font file so that the text can be created each time the file is opened or printed. This system works fine while you are working with you own system.
However when you move files from one system to another, if the same font files are not found then the text may be displayed with a different typeface or in some cases where there are conflicts between font types and names the text may completely disappear!
To make matters worse there are two main types of fonts (PostScript and TrueType, on both Mac and PC) supplied by several different vendors, who may use the same name for a font file describing similar but not identical typefaces.
If, for instance, you use a TrueType face called France and the bureau has an Adobe PostScript face with the same name, then your job, on their system, will use the PostScript font instead, which may result in different character spacing.
Supplying them with the correct fonts from your system helps, but problems can still arise where they install both the PostScript and TrueType versions of the same font. Conflicts occur and part of the job may not print.
(It may be illegal to copy your font files and give them to someone else. Fonts are just like any other software and may have a specific licence that allows you to use them only on one machine. Some fonts may be shareware or freeware, and other font manufacturers may allow you to install your font on another system just to output your job provided the font is then removed.)
To solve these problems I recommend that you use PostScript print files, or at least, use Adobe PostScript fonts. These fonts are more accurate and cause less problems with imagesetters than TrueType fonts.
Creating PostScript print files
Before creating PostScript print files for the first time, you must set up your system so that the correct printer drivers will be available. You will need to ask your bureau to give you a particular printer driver file, or to tell you which of the standard Windows drivers is best suited to their imagesetter.
For instance they may use the "Linotronic 330" printer driver that comes as part of Microsoft Windows. All you have to do in pretend you have that printer and install it on your system.
Once installed the new printer will now appear as an available printer in the print dialogue boxes of any of your Windows applications.
When you have carefully checked and printed your job out on your own printer you simply reprint the job using the bureau's printer driver and selecting the "Print to file" option, if available.
If your particular application does not have a print-to-file option then set up the printer driver in Windows, not connected to LPT1 or LPT2 but connected to FILE.
Make sure that you download the fonts with the job.
PageMaker and CorelDRAW! download the fonts by default, but you may need to check the user manual of other applications.
When you print to file, make sure you choose trim/registration marks, colour separation, etc as required. When you press SAVE/PRINT you will be asked to name the PostScript print file.
Name the file and choose the directory where you want it to be placed. Don't print directly to a floppy disc, as it takes considerably longer and you may not have sufficient space.
Trapping
When producing jobs where two or more colours are printed, colour "trapping" can be very important if you want your job to print correctly.
For example (and use your imagination here!) let's say we have some light blue text printing over a black background. The film for the light blue would have just the text, while the film for the black would have a hole corresponding to the text "knocked out" of the black background. If the film for this job is made with no allowance for movement on the press or stretch in the paper, it is possible that the two components will print with a slight unintended white (or paper coloured) gap between them.
To avoid these gaps when misregistration occurs, light colours that overprint dark colours must be slightly extended underneath the darker background.
Some software programs allow you to control colour trapping, others don't. The important thing is that you are aware of it and if in doubt talk to your service bureau about it.
Different printing process require different amounts of trapping. For example, screen printing of fabrics normally is set up without trapping (butt fit) because the colours will "bleed" in the fabric to create the colour trap; however screen printing of vinyl normally requires a much greater amount of trapping than for any other print process.
The best way to trap . . . avoid it!
The best way to avoid costly trapping or trapping problems is to avoid situations where misregistration will cause problems.
The gaps between misregistered objects are generally most noticable when the gap is white and the other colours fairly dark. Very light objects on very dark backgrounds don't cause too much concern.
If possible make the lighter colour have some component of the darker colour. In this way the gap between the two will not be white. For example, if the background colour is 100% cyan and 50% magenta and the other colour is 100% yellow and 20% magenta, then any gap will be no lighter than 20% magenta.
If possible let a dark object overprint rather than knock a hole in lighter objects.
Graphic file formats
Apart from the application files and PostScript print files already discussed, there are several other different types of graphic image file formats that you may have to include in your publication. It is important that you use the correct format for the particular application you are using and the result you require. Here is a brief summary.
Black and white images
In computer terms, black and white images should not be confused with what many people refer to as black and white photographs.
Black and white images are those that contain only black or white--no shades of grey.
Grey scale images
These are black and white photographs or illustrations where there may be black, shades of grey and white in the image.
If you are scanning artwork that is only black (i.e. line artwork) make sure you do not scan as greyscale. Scanning as black and white, 1 bit, or line art will both reduce the file size and increase the sharpness.
Colour images
To describe colour, the computer breaks a coloured photo down into tiny bits (called pixels) and describes the colour of each bit in one of two ways
- RGB--as a combination of different intensities of Red, Green and Blue.
- CMYK--as a combination of different intensities of Cyan, Magenta, Yellow and blacK
- PCX, PICT: Native IBM and Mac picture format. Limited resolution, RGB.
- TIFF (Tagged Image File Format): Picture format for photographs specifies colours in either RGB (most flatbed scanners produce RGB TIFF files) or CMYK.
- EPS (Encapsulated PostScript): A text file that defines a picture using PostScript commands. EPS files often contain a PICT or PCX preview or header that is used only for screen display purposes. Illustration programs such as Adobe Illustrator, Freehand and CorelDRAW! can save illustrations in this format.
- DCS (Desktop Colour Separations): An enhanced EPS file format that consists of five files for each image:
- A low resolution preview file
- The Cyan separation
- Magenta
- Yellow and
- Black.
- OPI (Open Prepress Interface): A specification that integrates high resolution pictures into desktop publishing documents. (Similar to DCS but where the four separation files are located elsewhere) It is possible for you to get high resolution scans done of an illustration and all they give you is the preview file that you use to position and size the illustration in your application. When the job is printed at the bureau the other four file parts (which are much larger) are linked into you document.
- GEM, WPG, WMF, GGM, BMP, DXF: Specialised formats not very well suited for compatibility.
The scanning resolution of colour or greyscale images has a dramatic influence on their file size.
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Image size--Postcard 100 x 150mm |
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| Image size--A4 210 x 297 mm | |||||||||||||||
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Scanning images at too high a resolution causes the file sizes to be too large -
wasting your hard disk space and your bureau's processing time.
Scanning images a too low a resolution causes the images to look jaggy or stepped.
What is the correct resolution?
The first thing to consider is the final size (cms, inches) the image will be printed on the page. Plan ahead so you know the image size before you
scan - this will save you many megabytes of space.
Next, know how and at what screen ruling the image will be printed. For instance, images for newspaper advertisements are generally printed at 75 LPI, while high quality brochures may be 133-175 LPI.
At actual image size, images should be scanned at 1.5 times the final LPI. For instance, the 75 LPI newspaper advertisement image is best scanned at 75 x 1.5 = 112.5 or say 115 DPI; a 133 LPI brochure image is best at 133 x 1.5 = 199.5 or say 200 DPI.
These are only a few of the DTP tips and tricks you'll need but normally your service bureau is happy to point you in the right direction.
About the author
Stephen Davey owns and operates an imagesetting bureau in Geelong called Pagination Design Services.
Reprinted from the September 1996 issue of PC Update, the magazine of Melbourne PC User Group, Australia
