Tom Balazs, Graphics for Technical Communicators

Page 3: Graphics for Technical Communicators, by Tom Balazs

Understanding Vector Formats
Vector based files are significantly different than pixel based files. In pixel based formats the entire image is divided into colored squares. Vector based files don't include information about each pixel because they use mathematical formulas to define lines, polygons, and the colors that should fill them. This can be an important advantage for graphics creation software, In many cases this results in smaller file size and better quality graphics. Like GIFs this format is best suited for "flat or simple graphic images that contain a lot of solid areas of color, including but not limited to logos, illustrations, cartoons, line art, etc."

EPS (Encapsulated Postscript Format, filename.eps)
The EPS file format can handle both vector and bitmap graphics. It is a standard format used by most graphic illustration and layout programs. It is often the best choice for professional printing. You might have problems using this format if you don't have a PostScript printer installed.

SWF (Shockwave Flash, filename.swf)
This is a vector based file used by the web browser plug-in Flash Player. The SWF format is usually used for interactive websites and short animations. There once was a concern that not enough user's web browsers would not have the necessary Flash Player Plugin. This is less of a concern today. Files that use this format are created by programs like Macromedia Flash and Adobe LiveMotion.

WMF (Windows Metafile, filename.wmf)
The Windows Metafile Format, as the name implies, is only suitable for computers running a Windows operating system. The advantage of this format is that it can be used in all Windows applications. It can also be used for vector and bitmapped images. The disadvantage is larger file size. Most web browsers cannot read this type of file.

Avoiding Screenshot GIFs that have a strange pattern

The strange pattern often seen in GIFs is called dithering. Dithering is sometimes incorrectly referred to as a moire pattern.

The GIF file format is capable of displaying up to 256 colors. So if you have an image with more than 256 colors and you save it as a GIF the program you are using has two options: 1) change the extra colors to a similar color that is one of the 256, or 2) use a mixture of colored pixels to approximate the color.

The two images above come from the same GIF. The top shows the GIF as is looks on a 32-bit monitor. The bottom shows the image as it looks on an 8-bit monitor. In the bottom image you can see the dithering of non-web safe colors. (If you are using an 8-bit monitor the images may look the same.)

This can happen when a GIF is created that uses fewer colors than the image that was captured. This can also happen on the Web when a GIF contains colors that the user's computer cannot display (i.e. colors not part of the 216 web-safe colors). The GIF format has only two options when this happens: to replace areas of similar color with one color or to use dithering to "fake" it . GIF can "fake" it, for example, by using a mixture of white and red pixels to give the illusion of a light red area. Sometimes this works well, other times it is distracting.

There are ways to prevent dithering: A) Before you do a screen capture set your computer's display to display 256 colors so you can see what the image will look like in that environment, B) Use a program like Adobe ImageReady or Macromedia Fireworks that let you carefully control what changes are made as you export to GIF format (many other programs make the changes automatically and often badly) or C) save the image in a format that can display more colors (maybe JPEG or PNG-24).

To set your computer monitor to 8-bit color (256 colors) right click on your desktop and select properties. Select the Settings tab and set Colors to 8-bit or 256 Colors. Mark L. Levinson also posted a creative way to get rid of the dithering on Techshoret.

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