Inside Macintosh: Macintosh Human Interface Guidelines / Part 2 - The Interface Elements: Chapter 8 - Icons

Small Icons

If you do not provide a 16-by-16 pixel icon, the Finder reduces the 32-by-32 pixel icon based on an algorithmic formula. The algorithm simply shrinks the icon and typically creates black areas, creating less pleasing visual results. If you provide a 16-by-16 pixel icon, however, you can optimize its design by removing pixels when necessary.

When you design a small version of your 32-by-32 pixel icon, preserve as many graphical elements of the icon as possible. In essence you want to provide the same icon in a smaller size. Typically you have to remove some pixels to reduce the visual clutter. However, don't eliminate significant elements, or the smaller version of the icon may look different from the larger version. Figure 8-28 shows icons that a designer carefully scaled and tuned
to preserve key elements of the icons' designs.

Figure 8-28 Consistently designed small icons

After you've created the small icon, verify the accuracy and clarity of the small icon by trying to design a large icon based on its design. If the large icon you end up with based on your small icon is different from the original large icon, something is not working about your small icon. In this case, you should consider redesigning the small icon, making sure to incorporate the key graphical elements of the original large icon.

In Figure 8-29 the small icons don't match their corresponding 32-by-32
pixel versions. If you have difficulty distinguishing the consistencies or inconsistencies, it's a good idea to consult with someone who specializes in graphic design to design or review your icons.

Figure 8-29 Inconsistently designed small icons