Microsoft KB Archive/69885

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DOC: SetBkColor Does Not Support Dithered Colors

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Q69885

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.1
 * Microsoft Win32 Application Programming Interface (API), used with:
 * Microsoft Windows NT Server versions 3.5, 3.51
 * Microsoft Windows NT Workstation versions 3.5, 3.51

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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) // Use the following line instead of the two above for // 32-bit Windows // case WM_CTLCOLOREDIT: {        // 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 query words:

Keywords : kbdocfix kbdocerr kbOSWinNT350 kbOSWinNT351 kbOSWin95 kbSDKWin16

Issue type :

Technology : kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch