Graphical Overlay Planes

Graphical overlay planes are made up of additional memory positioned logically on top of the frame buffer (thus the name overlay). Typically, graphics applications would use this extra hardware for creating windows that would not disturb the contents of existing windows in the frame buffer.

The primary goals for implementing this feature are:

• To reduce the need for redrawing underlying complex images while the user is navigating through the user interface.
• To allow simple images, such as annotations, to be written over underlying images without overwriting the images.

At X server startup/reset time, the following property is created on the root window of any screen that has overlay capabilities:

property_name:  SERVER_OVERLAY_VISUALS
property_type:  SERVER_OVERLAY_VISUALS
format:         32

The contents of the property is a list of each visual that resides in the overlays. Each element of the list contains information in the following format:

SERVER_OVERLAY_VISUALS {
        overlay_visual:          VISUALID
        transparent_type:        {None, TransparentPixel,
                                 TransparentMask}
        value:                   CARD32
        layer:                   CARD32
    }

Visuals in layer 0 (zero) are nonoverlay visuals, and would not appear in the list. Overlay visuals start in layer 1 (one). (See the -layer flag of the X command for information concerning how to specify the default visual.)

Drawing into a window created with an overlay visual (an overlay window) does not disturb the contents of windows in layers different from the layer containing the overlay window. Additionally, there is a set (perhaps empty) of transparent pixel values such that when written into an overlay window, pixels of windows in the next lower-numbered layer show through. If these pixel values also happen to be transparent, the process is applied until a window containing a nontransparent pixel value is found. Since visuals in layer 0 are not displayed in the list, they cannot have any transparent pixel values. The transparent_type and value fields specify the set of transparent pixel values as follows:

The same overlay_visual element may appear more than once in the list. If it does:

• The union of the pixel values described by the transparent_type and value fields should all be transparent.
• The value of the layer field is the same across all instances of a given overlay_visual element. (A given overlay_visual element exists in one and only one layer.)

  Note: There is no restriction on the parent-child relationships of windows with respect to the visual with which the windows were created. A window can have combinations of children in any layer on, below, or above its own layer.

GXT_OVERLAYS

On the GXT150 and GXT155 families of graphics adapters (except for the GXT150M), the GXT_OVERLAYS environment variable must be set to a value of TRUE to enable 4-bit overlays.

• If the GXT_OVERLAYS environment variable is set to a value of TRUE :
  • On low resolution monitors (1024x768), overlays and multiple hardware color maps are enabled.
  • On high resolution monitors, only overlays are enabled. The 4-bit overlays take over the video memory used to enable multiple hardware colormaps.
  • A command line argument of -layer 1 or -d 4 causes the X server to place the root window and default visual in the 4-bit overlay planes.
• If the GXT_OVERLAYS environment variable is not set to a value of TRUE :
  • On both low and high resolution monitors, only multiple hardware color maps are enabled. The 4-bit overlays are not available.
  • A command line argument of -layer 1 is ignored by the X server.
  • A command line argument of -d 4 is considered invalid by the X server.