GLUT- A binding for the OpenGL Utility Toolkit

MaintainerSven Panne <>
Safe HaskellNone




When overlay hardware is available, GLUT provides a set of routines for establishing, using, and removing an overlay for GLUT windows. When an overlay is established, a separate OpenGL context is also established. A window's overlay OpenGL state is kept distinct from the normal planes' OpenGL state.


Overlay creation and destruction

hasOverlay :: StateVar BoolSource

Controls the overlay for the current window. The requested display mode for the overlay is determined by the initial display mode. overlayPossible can be used to determine if an overlay is possible for the current window with the current initial display mode. Do not attempt to establish an overlay when one is not possible; GLUT will terminate the program.

When hasOverlay is set to True when an overlay already exists, the existing overlay is first removed, and then a new overlay is established. The state of the old overlay's OpenGL context is discarded. Implicitly, the window's layer in use changes to the overlay immediately after the overlay is established.

The initial display state of an overlay is shown, however the overlay is only actually shown if the overlay's window is shown.

Setting hasOverlay to False is safe even if no overlay is currently established, nothing happens in this case. Implicitly, the window's /layer in use/ changes to the normal plane immediately once the overlay is removed.

If the program intends to re-establish the overlay later, it is typically faster and less resource intensive to use overlayVisible to simply change the display status of the overlay.

X Implementation Notes: GLUT for X uses the SERVER_OVERLAY_VISUALS convention to determine if overlay visuals are available. While the convention allows for opaque overlays (no transparency) and overlays with the transparency specified as a bitmask, GLUT overlay management only provides access to transparent pixel overlays.

Until RGBA overlays are better understood, GLUT only supports color index overlays.

overlayPossible :: GettableStateVar BoolSource

Contains True if an overlay could be established for the current window given the current initial display mode. If it contains False, setting hasOverlay will fail with a fatal error.

Showing and hiding an overlay

overlayVisible :: SettableStateVar BoolSource

Controls the visibility of the overlay of the current window.

The effect of showing or hiding an overlay takes place immediately. Note that setting overlayVisible to True will not actually display the overlay unless the window is also shown (and even a shown window may be obscured by other windows, thereby obscuring the overlay). It is typically faster and less resource intensive to use the routines below to control the display status of an overlay as opposed to removing and re-establishing the overlay.

Changing the layer in use

data Layer Source

The layer in use.



The normal plane.


The overlay.


layerInUse :: StateVar LayerSource

Controls the per-window layer in use for the current window, which can either be the normal plane or the overlay. Selecting the overlay should only be done if an overlay exists, however windows without an overlay may still set the layer in use to Normal. OpenGL commands for the window are directed to the current layer in use.


postOverlayRedisplay :: Maybe Window -> IO ()Source

Mark the overlay of the given window (or the current window, if none is supplied) as needing to be redisplayed. The next iteration through mainLoop, the window's overlay display callback (or simply the display callback if no overlay display callback is registered) will be called to redisplay the window's overlay plane. Multiple calls to postOverlayRedisplay before the next display callback opportunity (or overlay display callback opportunity if one is registered) generate only a single redisplay. postOverlayRedisplay may be called within a window's display or overlay display callback to re-mark that window for redisplay.

Logically, overlay damage notification for a window is treated as a postOverlayRedisplay on the damaged window. Unlike damage reported by the window system, postOverlayRedisplay will not set to true the overlay's damaged status (see damaged).

Also, see postRedisplay.