Solaris X Window System Developer's Guide

What are Transparent Overlay Windows?

The transparent overlay extension allows the creation and manipulation of transparent overlay windows. These windows are X windows that allow the user to see through to the underlying window on a per-pixel basis. No special hardware is needed to create and use transparent overlay windows, as this functionality has been implemented in software. Complex transparent overlay manipulation on simple hardware may be time consuming; however, the X server can make use of special overlay hardware if available and the client chooses the correct visuals. Note that, depending on your hardware and needs, you may have to adapt the client color allocations for transparent overlay windows.

Overlay windows allow applications to display temporary imagery in a display window. Users of an application that provides overlays can annotate an image with text or graphical figures, temporarily highlight certain portions of the imagery, or animate figures that appear to move against the background of the imagery. When geometry in the overlay is cleared, any underlying graphics do not need to be regenerated.

The transparent overlay extension allows the client to use standard X requests to draw primitives in opaque paint, which is a name for the standard way of drawing, or transparent paint, which makes affected pixels invisible. The paint type is associated with a standard X graphics context. Window backgrounds may also be set to transparent paint. Transparent overlay windows obey all regular window rules and operating procedures. For example, a transparent overlay window can be positioned anywhere in the window stacking order, regardless of what hardware the windows are associated with. This is implemented in software with the Solaris X server multiple plane group (MPG) functionality.

The server's multiple plane group capability allows windows from different parts of the hardware to coexist. Each window is associated with a visual, which in turn is associated with hardware. Although some hardware is physically created such that there is a definite “layering” (for example, windows created in a hardware overlay plane might be expected to always be seen above the regular windows), MPG works around this limitation in software. MPG allows the stacking order of the windows to be unaffected by the physical imitations of the hardware. As a result, stacking is simply the same as in the standard server. If overlay hardware is available and requested, MPG takes care of minimizing the work and increasing performance.

In general, an overlay is a pixel buffer (either physical or software simulated) into which graphics can be drawn. When the overlay is physical (that is, not simulated in software), erasing the overlay graphics does not damage the underlying graphics. This provides a performance advantage when the underlying graphics is complex and requires much time to repaint. When the overlay is in software, erasing the overlay graphics may generate an Expose event.