ID Number: Q69885
3.00
WINDOWS
Summary:
The syntax for the SetBkColor function is documented in the Microsoft
Windows Software Development Kit (SDK) as follows:
DWORD SetBkColor(HDC hDC, COLORREF crColor);
SetBkColor sets the current background color of the specified device
context (DC) to the color that the crColor parameter references, or to
the nearest physical color if the device cannot represent the RGB
color value that the crColor parameter specifies. In other words,
SetBkColor cannot be used to set the background to a dithered color
and defaults to the physical color that is closest to the requested
crColor value.
More Information:
This behavior can cause unexpected results for an application that
changes the background color of a control to a color that cannot be
represented by a color provided by the display device.
Specifically, when an application specifies a dithered color for the
background of an edit control, and specifies the same color for the
text background, Windows paints the control in two distinct colors.
For example, using the standard VGA display driver, the following
call, in which COLOR_INACTIVEBORDER is a green/gray specified by
RGB(64, 128, 128), sets the background color to gray (RGB(128, 128,
128)) rather than the dithered green/gray that is desired:
SetBkColor(wParam, GetSysColor(COLOR_INACTIVEBORDER));
To illustrate, if the application uses the function call while
processing the WM_CTLCOLOR message to change the color of an edit
control, the window background is painted green/gray, and the text
background defaults to the nearest physical color, which is gray. This
produces a gray rectangle inside a green/gray rectangle rather than
the desired green/gray for the entire edit control.
This behavior can also occur with other controls such as option
buttons and list boxes. However, an application can avoid this problem
by using the SetBkMode function to set the background mode to
TRANSPARENT. This allows the dithered brush pattern to show through
beneath the text to achieve the desired results. That solution is not
practical with a multiline edit control because if text is inserted,
and the background mode has been set to TRANSPARENT, the text that is
pushed to the right by the inserted text leaves its image behind. The
result is text superimposed on top of other text, which quickly
becomes unreadable.
To partially work around this situation for a multiline edit control,
use the GetNearestColor function to determine the nearest physical
color to the desired color, as in the code fragment below. In this
case, the entire edit control is gray:
case WM_CREATE:
{
HDC hDC;
hDC = GetDC(hWnd);
hGrayBrush = CreateSolidBrush(GetNearestColor(hDC,
RGB(64, 128, 128)));
ReleaseDC(hWnd, hDC);
hWndEdit = CreateWindow( ... ES_MULTILINE ... );
}
break;
case WM_CTLCOLOR:
if (HIWORD(lParam) == CTLCOLOR_EDIT)
{
// The following call creates the nearest physical
// color; therefore, it will be the same as the
// hGrayBrush created above.
SetBkColor(wParam, RGB(64, 128, 128));
SetTextColor(wParam, RGB(255, 0, 0)); // red text
return (DWORD)hGrayBrush;
}
else
return DefWindowProc(hWnd, identifier, wParam, lParam);
break;
Additional reference words: 3.00 MICS3 R2.4