EPS Format - Understanding EPS

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Pros and cons of a crucial graphic format

Tom Arah looks at the strengths and the weaknesses of the EPS graphics format and suggests how to make the most of it with today's software.

The EPS format supports vector and bitmap

In previous publishing pieces I've often talked about the importance of Adobe Postscript. This is the underlying page description language that enables the same page to be output accurately to paper from a desktop laser and to film from an imagesetter. Obviously most page layouts tend to be based on text and fonts, but they can also include images and these must be output just as reliably and to the same high standard. This is where the Encapsulated Postscript graphics format - EPS - comes in.

Good on Paper...

Just like the raw Postscript files sent to the printer, each EPS is actually a program - as you can see if you load the file into a text editor. The graphic is stored, not as a series of bitmap pixels or vector nodes, but as a series of Postscript-language commands that describe all of the individual objects that combine to produce the end image. When output on a Postscript device this program is run and the graphic is accurately reproduced.

The only real difference between the graphic format and the continuous page output is that the EPS is "encapsulated". Effectively this means that the EPS is limited to a single page and contains commands that describe its original size or "bounding box". The resulting file can be imported into any DTP program and positioned and resized just like any other graphic. When the page is then printed it is a relatively simple process for the program to work out the new origin and scaling factor and to add its code seamlessly to that of the rest of the page. The strengths of the EPS as a graphic format all flow from this seamless integration into the Postscript-based workflow.

Most importantly, the quality of the end results is unsurpassed. Everyone knows that vector-based graphics allows resizing without losing quality, but the quality still depends on the accuracy of the defining nodes. That's fine for straight lines, but if you create a small circle in a format like the windows metafile WMF, for example, and then blow it up you will soon see the many straight lines that produce the impression of a curve. Do the same with an EPS graphic and because the circle is actually defined programatically as an object, the blown-up version will still be as smooth as the original. This might not sound too important if you use artwork size-for-size, but the hugely higher resolution of imagesetting will always show up the roughness of non-EPS graphics.

This is particularly important for graphics that contain text where the crudeness of the line-based vector formats can become apparent all too easily. EPS files can avoid this in two ways: either by including letters as high quality curve descriptions or, if there is a considerable quantity of text, by embedding the font itself into the file. This is very useful as it allows a whole page layout to be treated as just another graphic, for example, to resize an advert for slightly different column widths. The EPS format is able to do this, firstly because of the natural tie in with Postscript definition fonts and secondly because of its object-based nature.

Postscript's object-based nature has further advantages. In particular it is possible to embed bitmaps into the file so that raster, vector and font information are all dealt with together in the same graphic format. Again final quality is paramount so that all of the information of the bitmap is embedded into the EPS no matter what size it was actually used in the graphic. That way, if the EPS is scaled up in the DTP program, the maximum amount of information is still there to ensure the highest quality output possible.

The object-based nature of the EPS is also essential for colour work. Because all of the objects in the graphic are described, even if they are partially of completely obscured, this allows the accurate trapping of artwork that will be colour-separated. EPS graphics can also include named colour definitions that can be picked up by supporting applications. If you import an EPS containing Pantone colours into PageMaker, for example, these will automatically be added to the colour palette. The Pantones can then be applied to other objects allowing both the EPS and the page as a whole to be correctly colour-separated.

If you are producing commercial print the EPS format's quality and accuracy, its object-based nature and - since version 2 - its superior colour handling combine to make it the only serious choice. Just as importantly, because both the overall page and the embedded graphics are based on the same underlying language, there is no scope for incompatibility. There will be none of the problems of line weights changing on import, for example, as there can be with formats like CGM. EPS is the only graphics format where you can be sure that what you get out as final output is exactly what you put in. As this is the whole point of a graphics standard, there's no question that EPS is the best format available.

...Bad on Screen

However there are problems. Everything I've said above is true, but only if the artwork is output on a device that is able to interpret the code to recreate the page. In other words you need to have a Postscript device and Adobe naturally charge a premium for the use of their technology. Generally speaking for black and white printing it's a price well worth paying - after all there wouldn't really be a computer-based publishing and graphics industry without it. For colour output, however, the price differential between Postscript and non-Postscript printers can be very steep and can't always be justified.

Unfortunately if you use an EPS graphic on a non-Postscript printer the results are unpredictable ranging from a bounding box with a cross on it to a poor quality bitmap representation. If you cannot justify the expense of a colour Postscript printer this can easily lead to the situation where your proofs are actually worse than your client could knock up on their Windows-driven budget inkjet! The only alternative is to produce EPS versions of graphics to be sent out for final imagesetting and WMF copies to be used for local proofing - and of course to remember to swap between them.

This problem with non-Postscript printers is hardly ideal, but at least it's avoidable. The main limitation with EPS graphics, however is inescapable. The problem, of course, is that the screen you view your graphics on is not a Postscript device (unless you are using a NeXT system and, admit it, you're not). As far as your monitor is concerned the incredibly high quality and accurate Postscript code on which the graphics are built is absolutely meaningless. That's why when you import many EPS files they simply appear onscreen as boxes with a cross through them.

The final output from such files is still superb, but it's not really surprising that most designers feel uncomfortable working with blank boxes - it's not exactly wysiwyg. Of course no standard could survive if it completely ignored the screen and so the other feature that sets encapsulated Postscript apart from raw Postscript is its ability to contain a preview image. It is this preview image that the user sees and manipulates on screen although it is completely ignored when the image is output on a Postscript device.

The Preview Image

Interestingly Adobe did not define the format of the preview and instead left it to individual developers. After all, if a program didn't support the preview it could simply ignore it as it is the underlying code that really carries the information. The most obvious and near universal preview format that emerged was the TIF as this had the huge advantage that it is supported on both the PC and the Mac. With a TIF preview in place it immediately becomes possible to see and handle the EPS and to proof the graphic on non-Postscript devices, so are the problems solved?

Hardly. Representing a primarily vector-based format with a bitmap is not exactly ideal with quality inevitably poorer, particularly when the image is blown up. Moreover the bitmap is nowhere near as accurate as the underlying Postscript code. This can often lead to cases where the preview is not even the same size as the EPS bounding box. This causes trouble when adding in keylines, for example, and again results in poor quality non-Postscript proofs. Normally the problem can be solved by increasing the dpi resolution of the preview. However this is only possible if you are the one originating the EPS and even then the rapidly expanding file sizes can quickly get out of hand.

This can be balanced to some extent by choosing different formats or "flavours" of preview TIF. The actual choice you are offered depends on the program you are exporting your EPS from but covers the full range from 1-bit black and white to 24-bit true colour. Such flexibility is useful but it brings its own problems. The DTP program you are importing your EPS into might well only support some of these flavours of TIF and, if it doesn't, you are back to square one with the dreaded bounding box. The only way around this is to find a format that both your drawing and DTP program support and stick to it. And hope that you are lucky with any EPS files that clients bring in!

Suddenly EPS graphics are not looking anything like the ideal standard they first appeared. As soon as the screen is brought into the equation, they lose all of the strengths that they have on paper. Quality, accuracy, ease of use and even cross-application compatibility are all thrown out of the window. From consistent and unparalleled output quality we are reduced to the inconsistencies of shoddy bitmap previews - and that's if we are lucky! Rather than seamlessly fitting into the publishing workflow the EPS has become a potential spanner in the works. Rather than the pure and inviolable standard it's looking like a bit of a kludge.

The fact remains, however, that for final output the EPS remains unmatched and since final output is what the whole industry is built on there is no alternative. As there's no chance that another entirely different format will come along to replace the EPS, we have to make the most of what we've got. Normally this just means crossing fingers and sticking with a system as soon as it seems to work. This is obviously not ideal and so I decided to do some more investigation to come up with a guide to best practice.

Corel Options

To do this I looked at how the two market-leading PC drawing and DTP packages deal with EPS format graphics. Or, to be more accurate, how they deal with creating and importing the crucial EPS previews. This involved creating a test image in Corel Draw 7 that showed off all of the features of EPS with a CMYK-coloured rectangle, a Pantone spot coloured rectangle, a gradient-filled rectangle, an embedded bitmap and some text. This I then exported in all the available format options ready for importing into PageMaker 6.5.

As you would expect from Corel, the EPS export options are comprehensive with controls over trapping, fountain steps and OPI management. There are also no less than five varieties of TIF preview available - black and white, 4-bit grayscale, 4-bit colour, true grayscale and 8-bit colour - though I was rather surprised to find that 256 colours was the limit. For completeness sake I also decided to create a version without a preview and in doing this noticed that the TIFF preview format option was actually a drop-down list. Clicking on this revealed a single option to create a WMF preview which I quickly accepted.

Before attempting to load the files I took a look at them with Explorer. As the image - and therefore preview - size was quite small the majority of file sizes were actually quite similar with the EPS with no preview coming in at just under 2Mb and all the varieties of TIF coming in slightly over. The EPS with a WMF preview was considerably larger at 2.7Mb presumably due to the inclusion of the full bitmap in both the EPS code and in the WMF preview. For comparison purposes I also created a WMF version of the graphic and an EMF (enhanced metafile) version. As I expected the WMF was around 700k, but the EMF was only a tenth of the size at 70k.

PageMaker Importing

The next step was to see how the files fared when imported into PageMaker for outputting. I was pleased to find that PageMaker managed to bring in all of the TIF preview formats as this certainly hadn't been the case in the past. Even so there were some differences. In particular while the black and white preview's bounding box was transparent all of the other formats' boxes were a solid white that obscured any objects below. This is a major problem as it gives a false impression on both the screen and on non-Postscript printers of how the page will finally output.

Importing the EPS without a preview produced a major surprise as a "creating preview" dialog appeared on screen and, a few seconds later, placing the image did indeed show a bitmap representation of the EPS. This really is a major advance as it finally consigns to history the nightmare of working half-blind with just bounding boxes. Now you can be confident that you will at least be able to see the graphical elements you are working with. In fact in many ways it makes far more sense for the importing rather than exporting program to be responsible for creating the preview. After all this is the program where you will actually be looking at the EPS.

More than this PageMaker offers two important controls over the quality of the preview. The first allows the resolution to be changed. This is very important as, if the EPS is going to be scaled dramatically, you can ensure that the bitmap resolution is high enough to remain useful. This is far better than setting the preview size once-and-for-all when the image is created. Secondly it's possible to change the colour depth between 256 and a full 16 million colours. In other words it's possible to create a higher quality TIF preview when importing into PageMaker than Corel can produce when exporting.

Sadly, there are still a number of drawbacks. While the solid colours in the 24-bit preview were far smoother than the Corel 8-bit version, for example, they weren't very accurate with colours considerably darker than in the original. Worse, the gradient, which should really have benefited from the 16 million colours, only appeared as a small central band. Finally there is still the problem of the solid white background. Obviously more work needs to be done, but even so PageMaker's creation of previews on import is a major step forward.

Best of Both Worlds?

But not as big an advance as Corel's ability to create WMF previews. I wasn't sure if PageMaker would be able to import this preview at all, but there was no problem. The difference it makes to deal with a vector rather than bitmap preview is extraordinary. Resizing is simplicity itself and the quality remains very high throughout, certainly good enough for onscreen use and for proofing on non-Postscript devices. The solid colour accuracy was also good, the gradient smooth and, most importantly, the background of the bounding box was transparent so that the onscreen representation was completely accurate.

As soon as you've seen it in action it's clear that a vector-based preview is the only real option for EPS graphics. At a stroke it removes all of the problems associated with onscreen and non-Postscript use. I don't want to talk too soon, but I've certainly put the colour Postscript printer on the backburner. However, there are two slight problems. The first is the ballooning file size involved when the EPS and WMF file formats are combined. The second is that WMF format isn't entirely happy dealing with bitmaps, which come through too dark when compared to the original.

Initially I thought this might just be the case for the preview, but loading the WMF and EMF files showed that the native WMF had exactly the same problem. However the EMF file looked perfect onscreen which suggests an obvious solution. An EPS with an EMF preview would offer the best onscreen quality and with a much smaller file size. The EMF is still size dependent so even better would be the ability for PageMaker to create the EMF preview to the desired quality on the fly. A system like that really would offer the best of all possible worlds - Postscript, vector, bitmap, import and export.

More importantly it would offer the highest possible quality on screen as well as on paper and so solve the problems of EPS once and for all.

Tom Arah

December 1997


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