The underlay 8-bit PseudoColor visual is sometimes referred to as the 8R visual because the pixel is stored in the red channel of the frame buffer. The overlay 8-bit PseudoColor visual is referred to as the 8X visual because it is stored in the X channel.
The window pixels in the overlay visual do not interfere with the window pixels in the underlay visuals. However, window pixels in the underlay visuals do interfere with window pixels in the overlay visual. This is true because underlay windows have color data which lives in the BGR (or just R) channels but they also have window id (WID) data which resides in the X channel. This can cause an x11 expose event (damage) to be generated.
When an overlay window is occluded by an underlay window, the WID portion of the underlay data will corrupt the color data of the overlay window. When the underlay window is moved away again, an x11 expose event will be sent out for the damaged portion of the overlay window. This is different from the ZX accelerator, which has mutually non-interfering underlays and overlays. Like the ZX accelerator, the pixels of Creator 8-bit underlay windows interfere with the pixels of 24-bit underlay windows.
The Creator accelerator follows an X-channel architecture. In this architecture, some pixel values in the 8X plane group display opaque colors and some codes are used as window IDs that control the display of pixels in the underlay visuals.
The Creator 3D Series 3 has an extended overlay mode that has non-interfering overlays and underlays like the ZX accelerator. When this mode is enabled, the window id planegroup no longer shares the X or overlay channel so that an underlay window will not cause an x11 expose (damage) event.
The maxwids configuration option to ffbconfig(1m) specifies how many of the overlay pixel values are to be used as hardware window IDs. See "Hardware Window IDs"" for details. The minimum legal value for maxwids is 1. The default value is 32. Thus, the overlay visual is a partial visual because it has less than the usual 256 colormap entries. For colormaps of this visual, if the client renders with a pixel value greater than or equal to the specified number of colormap entries, no error is generated and the colors displayed are undefined.