Custom icon data can be supplied for icons of different sizes and color depths, and it can be stored in two basic formats. The software creating the custom icon determines which data it will store when creating icons.
Sizes and color depths |
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Prior to System 7, Macintosh icons were limited to black and white (1-bit) icons in small (16x16 pixels) and large (32x32 pixels) sizes. |
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System 7 introduced support for icons with sixteen (4-bit) and 256 (8-bit) colors, and custom icons. Custom icon data was stored in resources, one resource for each size and color depth. |
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Mac OS 8.6 added support for icons with millions of colors (32bit) and transparency, and introduced the icon family resource. |
How custom icon data is stored |
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Prior to Mac OS 8.6, all custom icon data was stored as individual resources in the file's resource fork, one resource for each size and color depth. |
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Mac OS 8.6 introduced a new resourcethe icon family resourcewhich stores the icon data for multiple sizes and color depths in a single resource. |
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Mac OS X introduced full color thumbnail (128x128 pixels) icons. Thumbnail icon data is always stored in an icon family resource. |
Required custom icon data |
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The rules:
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System 7 Mac OS 8.5 require custom icon data be stored as individual resources. |
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System 7 and later can use custom icon data stored as individual resources. |
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Mac OS 8.6 and later can use custom icon data stored in an icon family resource. |
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Mac OS X can only use thumbnail icon data stored in an icon family resource. |
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Which icon data do I need? |
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If a file's custom icon will only be viewed in Mac OS 8.6 and above, the individual resources used by System 7 are not needed. |
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If you are not using Mac OS X, thumbnail icon data is not used. |
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If you don't view icons in the Mac OS X Finder at the larger sizes, thumbnail icon data is not needed. |
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4-bit icons are essentially obsolete. They were originally intended for use years ago when users might be using a display with sixteen colors. |
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In general, if you're using Mac OS 9, files only need small and large icon data to display properly in the Finder. If you're using Mac OS X, you may find small and large icon data to be sufficient for your Finder display needs, or you may want the option of viewing the larger thumbnail icons. |
Why trim a custom icon? |
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Disk space. Custom icon data increases file size. Larger icons and more colors in an icon increase the amount of icon data stored in the file. An icon family with 8-bit data for small and large icons, and data for full color small, large, and thumbnail icons can contain more than 70K of icon data. Removing the thumbnail and 8-bit data can drop that to as little as 4K. |
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Depending on the application which created the icon, custom icon data may contain formats and/or sizes you never see. If this is the case, you can reduce the disk space required to store the icon by removing unnecessary icon data. |
Tip:
Picture files created with Internet Explorer® 5 in Mac OS 9 contain thumbnail data consisting of a 128x128-pixel white square with a 32-pixel icon in the center. This is useless in all versions of the Mac OS and can be removed without negative effects to save approximately 20K of disk space. |
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These files also contain icon data appropriate for displaying a custom icon in the Finder prior to Mac OS 8.6. Most File Buddy 7 users can safely removed this data as well. |
Tip:
Aliases created by the Finder in Mac OS X are often given a custom icon, and in many cases that can add as much as 70K to the size of an alias file which would otherwise require only a few kilobytes of disk space. These aliases can be good candidates for icon trimming. |
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These files also contain icon data appropriate for displaying a custom icon in the Finder prior to Mac OS 8.6. Most File Buddy 7 users can safely removed this data as well. |