Window Background
The window background is the color or pattern used to fill the client area
before a window begins drawing. The window background covers whatever was on the
screen before the window was moved there, erasing existing images and preventing
the application's new output from being mixed with unrelated information.
Windows paints the background for a window or gives the window the opportunity
to do so by sending it a
WM_ERASEBKGND message when the application calls
BeginPaint. If an application does not process the message but passes it to
DefWindowProc, Windows erases the background by filling it with the pattern in the
background brush specified by the window's class. If the brush is not valid or the
class has no background brush, Windows sets the
fErase member in the
PAINTSTRUCT structure
BeginPaint returns, but carries out no other action. The application then has a second
chance to draw the window background, if necessary.
If it processes WM_ERASEBKGND, the application should use the message's
wParam parameter to draw the background. This parameter contains the handle of the
display DC for the window. After drawing the background, the application should
return a nonzero value. This ensures that
BeginPaint does not erroneously set the
fErase member of the
PAINTSTRUCT structure to a nonzero value (indicating the background should be erased)
when the application processes the subsequent
WM_PAINT message.
An application can define a class background brush by assigning a brush handle
or a system color value to the
hbrBackground member of the
WNDCLASS structure when registering the class with the
RegisterClass function. The
GetStockObject or
CreateSolidBrush function can be used to create a brush handle. A system color value can be
one of those defined for the
SetSysColors function. (The value must be increased by one before it is assigned to the
member.)
An application can process the WM_ERASEBKGND message even though a class
background brush is defined. This is typical in applications that enable the user to
change the window background color or pattern for a given window without
affecting other windows in the class. In such cases, the application must not pass
the message to
DefWindowProc.
It is not necessary for an application to align brushes, because the system
draws the brush using the window origin as the point of reference. Given this,
the user can move the window without affecting the alignment of pattern brushes.
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