There are two ways to store information about a graphic element (photograph or clip art): as a mathematical expression or as a description of dots of color.
In a vector format, the picture is represented by a mathematical description, typically only possible in clip art or illustrations.
For example, a vector format file decribing a piece of clip art of the moon might describe it as a circle with black line and white fill.
As another example, a vector format file describing a company logo would describe each line and curve mathematically, and indicate that the fill color is red.
Typical file types for vector formats include:
- Windows Metafile (WMF) - typical clip art format on a Windows system.
- Enhanced PostScript (EPS) - typical file format from graphic artists using Adobe Illustrator.
You may have to experiment with file types to determine what looks good for a particular project. For example, some vector graphics look fantastic when printed, but the way Windows displays them on screen make them a poor choice for an on-screen presentation. Other vector graphics, like most clip art, look perfect when printed or displayed on-screen.
In a raster or "bitmap" format, the picture is represented by dots (pixels) arranged in rows and columns.
For example, a raster format file describing the company logo would describe each dot of red and, possibly, white.
- Bitmap (BMP or TIFF) - an uncompressed and therefore very large raster image… archenemy #1 of PowerPoint presentations.
- JPEG (JPEG or JPG) - a raster image compressed with the JPEG compression scheme. JPEG files preserve most image and color detail while reducing file size significantly from bitmap formats. JPEG is the best format for high-color, detailed graphics, including photos. Digital cameras typically store their images as JPEG files.
- GIF - a raster image compressed with the GIF compression scheme. GIF files preserve all image detail and minimize file size at the expense of color detail. GIF images typically have a low number of colors (from 2 to 256). Low-color images such as logos and simple art are best saved as GIFs. GIFs have the ability to support transparency, meaning that one color can be designated astransparent. Logos often use white as the transparent color in a transparent GIF, so that the logo can be placed without a white 'box' surrounding the logo.
You should try to use raster graphics (.GIF, .JPG, .TIF, .BMP) only when the picture was produced with the correct size in the first place, so you are not having to scale the picture.
Raster versions of high-contrast art such as a logo can look jagged on a computer monitor, even in their correct/original sizes. That's why raster graphics are often antialiased to soften the edges. Antialiasing makes a high-contrast graphic look good on a monitor, but the soft edges look very poor when printed on paper.
Note the blurry edges in the company logo above, which are particularly noticeable as the object is enlarged. These edges, even in the smaller graphic, would be highly apparent if printed on a quality laser printer.
- GIF is the best file format for logos and other low-color, high-contrast, crisp images.
- JPG is the best file format for photographs.
- Anti-aliasing high-contrast art is useful for viewing on a monitor, but does not look good on paper.
- If creating a raster image, it is best to produce the image in the size it will be used, so that it does not have to be scaled (enlarged or reduced).