Do not confuse the term Window ID used in this context with the X protocol term window ID. An X protocol window ID is an XID which uniquely identifies a window. A hardware window ID is a value rendered into the frame buffer that controls window appearance.
Some overlay pixel codes are treated as opaque pixels, which display visible colors. Other overlay pixel codes are used to control the display attributes of underlay windows. These codes are referred to as hardware window IDs (WIDs). Specifically, a certain number of codes at the high end of the colormap (toward 255) are used as WIDs. The actual number of WIDs is configurable through the ffbconfig -maxwids option. For each overlay code used as a WID, the number of overlay colormap entries is reduced by one. The default value of maxwids is 32, so there are 224 opaque pixel values in the overlay.
One hardware WID is always reserved for windows of the default visual. The remaining WIDs are assigned on a priority basis to windows that have the following characteristics:
Assigned a unique WID for the purposes of hardware WID clipping. (This clipping technique is used by the Sun 3D rendering libraries).
Non-default visual windows can share a WID with other windows of the same visual. However, like unique WID windows, double buffered windows always require a unique dedicated WID.
maxwids must be a power-of-two. Thus, legal values for maxwids are: 1, 2, 4, 8, 16, and 32. The default value of maxwids is 32.