Accelerated-X™ Summit Series
X server?driver feature

Overlays vs Underlays            

The use of image overlays and video windows, often in the same display, is common in X graphics systems in industrial/military applications.

Image Overlays use what is logically a transparent plane "overlaying" the main image. Generally the main image plane has 24-bit color, (TrueColor), while the image overlay plane uses 8-bit (PseudoColor).

An XVideo window is logically a plane that "underlies" the main image. Its color depth can be 8, 16. or 24-bit, and it can be seen in the display by the use of a transparent "window" in the main image plane. See the figures below. P 

The image at the left depicts a 1920x1200 monitor showing a main image plane with 24-bit color depth, a PseudoColor overlay image plane with colored text and lines, and an XVideo window from a traffic monitoring camera.

Overlays are useful when the data on the overlay are mostly text and lines that do not unduly obscure the main image, and when that data are changing frequently while the main image is mostly static. An example is the Air Traffic Control application.

Video windows are used to present real-time information to augment the computer generated images. The incoming video data are synchronous to the X server operation, being previously captured by frame-grabber hardware. An example is the Auto Traffic Control application. P 

The image at the right illustrates the "planar effect" of using an overlay and an underlay.

Image overlay planes can be can be of any size, up to the full monitor resolution, or just small temporary popup windows.

Some GPUs support PseudoColor with a hardware Color Lookup Table that can hold up to 255 image-color specifications and one transparent bit color specification.

XVideo windows can be scaled to any window size. Unlike an overlay window, the XVideo Window is generally not "dragable" with the mouse.