3.2.6 Backing Planes and Backing Pixel Attributes
You can set backing planes to indicate (with bits set to 1)
which bit planes of an
window hold dynamic data that must be preserved in backing store
and during save unders.
The default value for the backing-planes attribute is all bits set to 1.
You can set backing pixel to specify what bits to use in planes not
covered by backing planes.
The default value for the backing-pixel attribute is all bits set to 0.
The X server is free to save only the specified bit planes in the backing store
or the save under and is free to regenerate the remaining planes with
the specified pixel value.
Any extraneous bits in these values (that is, those bits beyond
the specified depth of the window) may be simply ignored.
If you request backing store or save unders,
you should use these members to minimize the amount of off-screen memory
required to store your window.
Christophe Tronche, email@example.com