The X server cannot guarantee that a window's content is preserved when the window is obscured or reconfigured. X requires client applications to be capable of restoring the contents of a previously-invisible window region whenever it is exposed. Therefore, the X server sends events describing the exposed window and its exposed region. For a simple window, a client can choose to redraw the entire content whenever any region is exposed. For a complex window, a client can redraw only the exposed region.
| :exposure | Event Type |
An :exposure event is sent when redisplay is needed for a window region whose content has been lost. Redisplay is needed when one of the following occurs:
The regions needing redisplay are decomposed into an arbitrary set of rectangles, and an :exposure event is generated for each rectangle. For a given action causing :exposure events, the set of events for a given window are guaranteed to be reported contiguously. :exposure events are never generated for :input-only windows. All :exposure events caused by a hierarchy change are generated after any hierarchy event ( :unmap-notify, :map-notify, :configure-notify, :gravity-notify, or :circulate-notify) caused by that change. All :exposure events on a given window are generated after any :visibility-notify event on that window, but it is not required that all :exposure events on all windows be generated after all visibility events on all windows. The ordering of :exposure events with respect to :focus-out , :enter-notify, and :leave-notify events is not constrained.
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| :graphics-exposure | Event Type |
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A :graphics-exposure event is generated by a call to copy-area or copy-plane when the exposures attribute of the graphics context is :on. A :graphics-exposure event reports a destination region whose content cannot be computed because the content of the corresponding source region has been lost. For example, the missing source region may be obscured or may lie outside the current source drawable size. For a given action causing :graphics-exposure events, the set of events for a given destination are guaranteed to be reported contiguously.
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| :no-exposure | Event Type |
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A :no-exposure event is generated by a call to copy-area or copy-plane when the exposures attribute of the graphics context is :on. If no :graphics-exposure events are generated, then a single :no-exposure event is sent.
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