This article was originally published in the Piedmont Computer Guild News, May/June 1998

 

GRAPHICS

File Formats

by Bill Barnes

Previously we talked about the dpi resolution you need to use for your pictures; but for some types of files, dpi is irrelevant.

Consider text. A word processor doesn't tell the printer what dots to use to create a letter. It merely says "draw an 'A' in 12 pt Times Roman at this location on the page. Some types of graphics can be similarly described. "Place a 2 inch square and color it blue" is simpler than "Put a dot color 0000FF at coordinates 1.003,.5; put another dot color 0000FF at 1.006,.5 ... etc." The former type of file is called a vector drawing, which describes each object and its position while the latter is a bitmap which merely defines each pixel.

Because a bitmap stores the image as so many pixels by so many pixels, the actual size of the picture is determined by its resolution. Thus 450 pixels square would be 1-1/2" at 300 dpi but only 3/4" at 600 dpi. Vector images are resolution independent because a 2" square is a 2" square to any printer. You can also scale (enlarge or reduce) a vector drawing and still output at the full capability of the printer. Scaling a bitmap just makes the dots larger.

Bitmaps are appropriate for pictures that contain random distributions of shades of gray or color such as photographs while vectors are efficient ways to create uniformly colored basic shapes such as lines or objects. While advanced programs may handle both types of data, generally if the name of the program includes "paint" or "photo" you're working with bitmaps and "draw" or "illustrate" means you'll manipulate vectors. For example, Adobe PhotoShop(r) deals with bits and CorelDRAW!(tm) handles mainly vectors. Drawing applets such as the one bundled with your office software usually create vectors. Bitmaps are the native results of scanning an image.

While every program has its own way to save a file, there are a number of generic file formats available for each type of picture. Files with the extension TIF, GIF, JPG, and BMP, as well as any number of formats a photo finisher may furnish are always bitmaps. Vector formats include EPS, WMF (or EMF), and CGM. Be aware, though, most vector file formats can also contain a bitmap and so may have resolution-dependent data.

What format should you use?

First consideration is the original and the ultimate use. If you're starting with photographic originals or screenshots, the only option is bitmaps. If your destination is onscreen, either a web page or a projected presentation; low resolution bitmaps are satisfactory. If the final product is high quality printing, especially offset color, you will get the best results from vector graphics.

Another concern is your favorite design environment. Most conventional artists prefer to work with paint-type programs while some technical illustrators find drawing programs more intuitive. If your design includes sharp-edged objects such as charts or text, the only acceptable choice is working with vectors. Even if you're creating a web illustration, you can always export a GIF from a drawing program.

Even for printing to a 300 dpi black & white laser, a vector file will usually be more efficient. Remember that any bitmap you create should have sufficient resolution for your final output at full size. A line drawing needs the same dpi as the printer. A gray-tone or color bitmap should have a dpi about 1-3/4 times the line screen you need or 1/8 the resolution of the printer.

If you're publishing to the internet the most common formats are GIF or JPG. To import a vector image into an office document, use WMF. To drop any type of picture onto a quick presentation or one-time letter the easiest technique is just copy it with the Clipboard. You should always avoid importing a program's native file format such as Corel's CDR since this technique requires the original program be available to open the image through OLE.

Work like a pro ...

In the world of commercial printing, there are only two file formats that are universally supported: TIF and EPS. Whatever you use, be sure you include the original file separate from your document when you transmit the job to the printer. Even if the document contains an embedded image rather than just links, the printer may need to manipulate the original.

For bitmaps TIF is a dependable, robust format that most high-end photo editing programs can use as a native file. Most programs on any platform that can render a bitmap will accept a TIF. (But a word or warning: if your program offers any form of compression for the TIF, choose "no.")

Using vector drawings is not so simple. Unfortunately, the world of commercial printing is dominated by the Macintosh where EPS is the native format of PostScript printers. If you try to include it in a document that will be output on a standard PC printer, all you will get is the very low resolution screen preview. On the other hand, Macs do not recognize the PC's preferred vector format of WMF. This results in the conundrum of either having to export your drawings in two formats, as well as saving the original program's native file, or buy PostScript printers for your PCs. (I chose the latter option.)

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Bill Barnes is an independent graphics consultant in Charlotte. You can reach him at 704 332-1031 or Bill@Barnes.net..


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