ASK ANDY




FUTURE OF MAC OS,
RESOLUTION AND LINE SCREEN,
and EPS, TIFF, PICT, WMF

This month I'm looking at a couple of questions from readers, and there's news on the future Mac operating system.


NeXT, THE FUTURE MAC OS:

In December last year, Apple announced that after an agreement with NeXT, the future Mac OS will be based on the NeXT NextStep OS. If all goes well with the OS, it could be revolutionary for DTP and graphics.

NextStep uses 'Display PostScript' to display documents and layout on screen, in comparison, the Mac and Windows OS simply appropriate the display to represent the document.

When a layout is created, in say XPress or PageMaker, and then printed to a PostScript printer, the Mac or Windows OS send the PostScript code to the printer, which in turn 'runs' the code to print the document. What is displayed on screen is a representation of the document, but not based on the actual true PostScript code, this code is not 'run' until it gets to the printer. With 'Display PostScript' the same PostScript code that is used to print, is used (after conversion) to display the image on screen.

Sometimes when printing from the Mac or Windows OS, PostScript errors occur. These can be due to faulty or corrupt fonts & EPS images etc. However, these errors might only be found when the document is printed, as that's the only time the PostScript code is 'run', as the screen display is not using this code. With Display Postscript, the PostScript code is processed on the computer, before being sent to screen or printer, so any problems with fonts or images will appear straight on the screen, before printing.


POLITICS of POSTSCRIPT:

There are a few 'political' points to this though. Adobe own all the rights to PostScript as they developed and created it. Any device that uses PostScript has to be licensed from Adobe, so if Apple wanted to use Display PostScript they'd have to pay Adobe a fee for every OS or computer sold. As Display PostScript has the potential to reduce the need for a PostScript printer, as the PostScript code could be 'rendered' on the computer, then sent to the printer in a standard 'bitmap' or raster form, Adobe may want to impose a limitation on the output quality to non-PostScript (and therefore non-licensed) printers. One idea they may have, is to limit the resolution of output to non-PostScript printers to say 300dpi. This would allow most users acceptable output, and Adobe would still be able to sell PostScript printers with higher resolution for graphics and pre-press use.

If the 'Display PostScript' principals of NeXT OS are fully developed in the future Mac OS, it technically could be revolutionary, but will Apple have to impose some limitations on it to keep Adobe happy and within any licensing agreement? There again, Adobe may jump at the chance to have a further share of every Apple and Mac OS sold. It all looks very interesting, and there are bound to be developments in the next year or so.


I've had some more questions this month via email, the first relates to resolution and line screens.


DOTS PER INCH, LINE SCREEN, HALFTONES:

Q. I've read some of your articles on the Web & have a question. When I create a file in Photoshop, it wants a resolution in 'pixels-per-inch'. My little inkjet is 600 dpi, but how do I print at 600 dpi if I'm creating in PPI? Also, I don't know what line screen if any the printer has? Finally, if I start at 150 PPI and then finish & double it to 300 PPI is it really a finer resolution? Thanks in advance.

A. Generally DPI (dots per inch) can be thought of as the same as PPI (pixels per inch), so 600 dots per inch is really the same as 600 pixels per inch, the pixel and dot being the same. However, there are some occasions where the two are used to describe different areas; dpi sometimes being an 'output' or printer resolution and ppi being the actual resolution of the image itself. In your case though, you can think of the two as being the same.

There are a few options for line screens and halftones and this really depends on the printer and it's driver software - it's very printer specific. The line screen settings as in a conventional halftone screen, are generally only available if your printer is PostScript compatible, if it isn't it'll have it's own special output settings. This can give more control and options than a single line screen. As an example, before my printer (HP1200C) was PostScript, it used it's own special Hewlett Packard driver. This almost ignored conventional line screens and used it's own diffusion method. Rather than use a normal line screen of say 60 lines per inch, it uses a method to scatter and randomly place the cells, to break up any patterning that may appear.

I could chose which items would be printed using which method, so pictures could use the diffused method, and text use the patterned or solid method and so on.

Once I'd added the PostScript upgrade, all these options were lost if using the PostScript mode, and replaced with a standard variable halftone line screen.

Have a look in the printers manuals or help file and see if there are any recommendations, as it can be very specific. If the printer is using a 'diffused halftone' method, the rule for resolution of twice screen frequency is not applicable, although 150 to 200dpi should be fine.

If it does use a standard halftoning method, for most 300 dpi printers they would use a screen frequency of 60 lines per inch, so for 600 dpi you could be in the range of 60 to 120 lpi. As I guide I'd say that 150 dpi is a reasonable resolution to start with. Try printing the same image at different resolutions and see how they compare.

As for doubling up the resolution after you have created and finished the image, it won't really be an increase in quality. The software used to double up the resolution has to calculate those extra pixels (or dots), and it'll do this by averaging the value of neighbouring pixels. It cannot really create new information, just average what is already there, so the doubled image could be slightly pixelated or blurry. If the image quality is not paramount, and it's a fairly soft focus image, you may get away with it, but if you need a sharp detailed image at 300ppi, it would be best to start at that.


EPS IMAGES:

Q. How can I export CorelDraw Encapsulated PostScript images (EPS), so that they look good on screen when I import them into QuarkXPress for Windows? Compared to Mac Illustrator EPS's imported into QuarkXPress on the Mac, the Corel EPS's look very poor on-screen, even though they all print fine!

A. This is all due to the different images the Mac and PC use for previews in EPS images. When a EPS is exported from, say a drawing package, you don't actually see the PostScript itself, as this is a ASCII or binary 'listing' (looks like a computer program listing), and this creates the final printed image. However, to help you position and scale the EPS, in say XPress or PageMaker, a 'preview image' can be include in the EPS, so you can see a representation of the image on screen. The Mac uses the PICT format for these previews, and the PC generally uses the TIFF format but a few programs also allow the WMF format.

EPS's from Corel (and most PC 'drawing' programs including Freehand and Illustrator) use a TIFF as the preview, and this can be either black and white (1 bit) , or 255 colours (8 bit).

If you use the 255 colour preview, you will lose any 'transparency' or clear areas in the preview and any such areas will be white in the preview image, but not in the actual PostScript image itself. With any EPS's such as logos etc. that may overlay other images, the TIFF preview is pretty poor, as any 'see through' areas in the logo are actually displayed in white on screen and block out any under-laying images, although they would print correctly. If you need the 'transparency' on-screen to position the image correctly, the 1 bit black and white preview could be used, but that limits you to a black and white preview.

The answer for PC users, is to create EPS's with WMF (Windows MetaFile) previews. There is a freeware utility called EpsWmf (available on the WWW), that takes a EPS and a WMF, and merges the two to give a EPS with WMF preview. These import into XPress for Windows fine, and as they are 'vector' based they fully support transparency, and don't become pixelated on screen when zoomed in. The difference in quality on screen between the TIFF and WMF preview is amazing!

The only time the TIFF preview is preferable is when the EPS has many gradient fills and doesn't need transparency, or if the EPS contains a lot of small text, as the screen re-draw of the WMF in such circumstances is a little slower than that of a TIFF, or if the EPS is to be used on a Mac, as Mac's cannot use the WMF.

On the Mac, the previews are usually PICT, and these suffer from none of the problems of the PC's TIFF previews. However, using a WMF preview is much closer to using a PICT than a TIFF, so PC EPS's with WMF previews will give you a preview that matches the Illustrator previews you enjoy on the Mac.

If you have any DTP or graphic design questions or any comments, please feel free to email me at Andrew.Davidson@onyx.octacon.co.uk and until next month, happy DTP'ing.

Andy.
Contact: Andrew.Davidson@onyx.octacon.co.uk




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