This page will give you some general info about icons and the MacOS handles them.<P>
Under classic MacOS (System 9.x and lower), icons are stored in the resource fork of files. You can view end adit a file's resource fork using a program like ResEdit (from <a href="http://www.apple.com">apple.com</a>, more info at <a href="http://www.resexcellence.com">ResExcellence</a>). Before system 8.5, icons had to be stored separately in the 'icl8', 'icl4', 'ICN#', ics8', 'ics4', 'ics#' resources (which stand for <U>Ic</U>on <U>L</U>arge <U>8</U>-bit, etc). The '#' elements store both the 1-bit icon and 1-bit mask together, and these resources are required to display an icon. If there is no '#' element then the system will refuse to display the icon.<P>
Since system 8.5, a new resource tagged 'icns' can now store all necessary icon data. The 'icns' resource stores all the old icon types and is also used to store the new 32-bit icons and 8-bit masks which made their debut in system 8.5. These new icon formats made exceptionally good looking icons possible with multiple levels of translucency and true 32-bit color. Unfortunately these icons only appear under OS 8.5-9.x as a file's <i>custom</i> icon. This means that the 'icns' resource must have the id -16455 and the file's <i>has custom icon</i> flag must be set (both of these can be accomplished in Icon Brush though the <B>Icon Info Window</B><P>
System 9 also added a new type of icon: the "mini" icon. These icons are 16 pixels wide and 12 pixels high, and are used in the small icon list view mode. The mini icons have no 32-bit component and no 8-bit mask component (you couldn't tell the difference at the small a size anyway) and must be stored in the 'icns' resource (there are no 'icm8' and 'icm4' resources).<P>
Under OSX server, yet another new icon type was born. The "huge" icons are 48x48 pixels and have a representation at all depths (32, 8, 4, and 1 bit).
Under OSX 10.0, the "thumbnail" icon made its debut. Thumbnail icons are 128x128 pixels and have no 1-bit mask only an 8-bit mask and a 32-bit picture (no 8, 4 or 1-bit pictures). Thumbnail icons are used in the dock (no matter what the magnification level) and in the finder if you are using an icon view size larger than 48x48 (the next smallest icon type). OSX also made 32-bit icons override their 8 and 4-bit counterparts. This allowed icons to appear in true color without using a custom icon (described above).
