Magical Bundles of Numbers

The other day, a colleague on a mailing list asked for some background info on EPS graphic files. It seems her school wanted something professionally printed and the printer asked that the artwork be provided in EPS format. Thanks to the ubiquity of the web, we are mostly only exposed to the more common web graphic formats like JPEG and GIF files these days, so some of the more exotic types like EPS are not as well understood as they could be. For what it’s worth, here is my reply…

Ah, EPS files! A thing of beauty and a joy forever!

For anyone who works with graphics Encapsulated Post Script, or EPS files, are the holy grail of image formats. They are the ideal format for storing original artwork, although they are generally pretty useless for actually applying that artwork to a final format. By that I mean that most final applications for graphics, whether it be the web or printed documents, cannot use EPS files natively. But for a way of storing the original artwork, EPS files reign supreme. Here’s why…

Imagine you have a school logo image that gets used for a variety of purposes. It gets printed on school letterheads and business cards in very small sizes, reproduced on school coffeemugs and Tshirts at slightly larger sizes, and even gets blown up to very large sizes to make a banner for outside the school on open day. To get an image that looks good at all these different sizes, the artwork needs to be at different resolutions… A small jpeg file suitable for printing on the letterhead will look awful when stretched to be a large image for a banner for example. What you need is a way of creating these images that is resolution independent, so you can have the right resolution for the right situation.

That’s where EPS files come in. In an EPS, all the shapes and lines in the image are described mathematically. A very simple example would be a circle… Imagine you had a circle in an image, all you’d really need is the formula for creating a circle and the specific information about that particular circle. Mathematically, a big circle is exactly the same as a little circle but with different values for the key factors of radius, colour, edge thickness, etc… In an EPS file, this technique of using functions to describe objects is how the shapes are defined. This is very different to a JPEG or GIF, where shapes are created by a mosaic of pixels placed next to each other, a technique known as Bitmapping. Using bitmapping, a small circle and a big circle are two totally different patterns of bitmaps, which is why small graphics look so awful when they get stretched to be larger… You cannot just create new pixels from out of thin air to fill the gaps… the original pixels need to stretch, making the image look blocky and rather awful.

So in an EPS, the basic mathematical descriptions of the shapes that make up the image (called vectors) are embedded into the file using a language called PostScript. Postscript is a language that many laser printers use to describe how an image will be printed… When you print to a postscript-enabled laser printer, your printed image (whether that be text or pictures) is bundled up as little mathematical descriptions and sent to the printer where it is decoded back into the images and then printed. EPS files are that bundle of mathematical descriptions which define the image, but instead of being decoded by a printer they are just stored as a file.

Here’s the good bit… When you open an EPS file (which is vector based) in a graphics editing program like Photoshop (which is bitmap based), it asks you what size and resolution you’d like the image to be created at. Once you tell it the desired size and resolution, the EPS is then converted to a bitmap using the exact values you specify. This is the real magic of an EPS file… EPS gives you a single vector-based, size and resolution-independent file that unpacks to a bitmap-based, size and resolution-specific file at whatever specs you like. No wonder people who work in the printing industry love ‘em! They can be all things to all people. A single EPS file can be used to convert the image into whatever size and quality is required, be it the tiny artwork for the business card or the huge artwork for banner.

Tags: graphics, file formats, eps, postscript

Magical Bundles of Numbers by Chris Betcher is licensed under a Creative Commons Attribution 4.0 International License.