As it is visible in the Finder, icons can be displayed at more than one size. Although it is possible to create just one size and let the operating system generate the rest, better results are obtained when each size is created by hand, such that you can tweak each one for the best possible appearance. However, if you do decide to not include certain sizes, it is better to create the large ones and leave out the smaller ones than vice-versa, since graphics look better scaled down than scaled up. Here are all the sizes that Iconographer supports:
 Mini |
Dimensions:16 x 12 Depths:8-bit, 4-bit, 1-bit, 1-bit mask Description: The only icon size that is not square. Used very rarely, it's main use is as the smallest icon size in list view under Mac OS 8.x/9.x Supported formats: Mac OS Old, Mac OS New, Mac OS Universal. |
 Small |
Dimensions:16 x 16 Depths:32-bit, 8-bit, 4-bit, 1-bit, 8-bit mask, 1-bit mask Description: A common size when viewing folders in as a list. Also visible in the menu bar and as a proxy icon. Supported formats: Mac OS Old, Mac OS New, Mac OS Universal, Windows, Mac OS X Server. |
 Large |
Dimensions:32 x 32 Depths:32-bit, 8-bit, 4-bit, 1-bit, 8-bit mask, 1-bit mask Description: The most common traditional icon size. The default under pre-Mac OS X systems. Supported formats: Mac OS Old, Mac OS New, Mac OS Universal, Windows, Mac OS X Server. |
 Huge |
Dimensions:48 x 48 Depths:32-bit, 8-bit, 4-bit, 1-bit, 8-bit mask, 1-bit mask Description: Although it is mainly used under Mac OS X Server, this size is also available under Mac OS X when choosing to scale icons down. Supported formats: Mac OS New, Windows, Mac OS X Server. |
 Thumbnail |
Dimensions:128 x 128 Depths:32-bit, 8-bit mask Description: The default icon size under Mac OS X. Often scaled down to a more manageable size. Supported formats: Mac OS New |
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Within each icon size, there can be various bit depths. This means that you can have a separate icon for black and white (1-bit), 16 colors (4-bit), 256 colors (8-bit) and millions of colors (32-bit). The Finder automatically chooses the depth based on the number of colors available on the screen. Again, although you can simply draw one depth and leave it to the OS to extrapolate the others, you will usually get better results by drawing all the depths by hand.
It may not be immediately apparent that each icon is actually composed of two parts, the icon picture itself, and a mask. This mask acts as a cutout, determining which pixels of the icon show through, and which ones are left alone. The picture below shows an icon mask in action:
As you can see, the desktop is visible though the parts of the mask which have been left out. This simple mask only has two possible values, black, where the icon shows through, and white where the background is visible. Mac OS 8.5 introduces an additional mask, an 8-bit one. This new mask allows 256 possible values, ranging from white to black. As before, white means that the background is completely visible while black means that the icon shows through. The gray values in-between are used to blend the background and the icon. For example, a dark gray would yield a blend of the background and icon which would be weighed towards the icon. The picture below shows an example of an 8-bit mask:
It is clearly visible that the edge of the circle blends in with the desktop. If the icon were to be moved onto a different background, then it would blend in with that. The 8 bit mask can be used to create visual effects such as semi-transparency/translucency, anti-aliasing and drop shadows.
Old: This is the format used by systems previous to Mac OS 8.5. They support only 1-bit through 8-bit icons, at the sizes of 32 x 32, 16 x 16 and 16 x 12 pixels. This format is still required for document icons and for maximum compatibility (Mac OS 8.5 and above can still display this format) at a minimum file size.
New: This is the format introduced by Mac OS 8.5. It supports 32-bit icons with 8-bit masks, as well as the additional 48 x 48 size (which, although it is not used by the Finder directly, does appear in other places, like Sherlock 2's search button). These icons use a completely different method of storage, and thus they are not visible on older systems. Mac OS X
Universal: This is a pseudo-format which combines both the Old and the New resources into the same file with the same ID. The result is that the icon is visible on both systems, although usually things will look better when viewed in Mac OS 8.5 and above. This results in a slight increase in file size.
There is more information describing the icon formats of other platforms.