Why isn't my transparent static control transparent?

Raymond Chen

A customer reported that their application uses transparent static controls positioned over a bitmap image control, but even though they set the Transparent property on the static control, the static control isn’t transparent. The customer was kind enough to provide clear steps to illustrate the problem:

• Open Visual Studio 2005 or 2008.
• From the menu, select File, New File, Visual C++, Resource Template File (RCT).

• Right-click on the RCT file, select Add Resource, and add a bitmap named IDB_BITMAP1.

• Open the dialog box (IDD_DIALOG1) and add a “Picture Control”, specifying IDC_BITMAP_1 as its ID.

• Change the IDC_BITMAP_1 type to Bitmap and change the value of the Image property to IDB_BITMAP1.

• Add a “Static Text” control IDC_TEST_STATIC and set its caption to “This is a test”.

• Reposition the static control so it overlaps the IDC_BITMAP_1 control.

• On the IDC_TEST_STATIC control, set the Transparent property to True.

• Type Ctrl+T to test the dialog and observe that the static control is not transparent.

The Transparent property in Visual Studio corresponds to the WS_EX_TRANSPARENT window style, and the documentation explains that

WS_EX_TRANSPARENT: The window should not be painted until siblings beneath the window (that were created by the same thread) have been painted. The window appears transparent because the bits of underlying sibling windows have already been painted.

The observed behavior, therefore, matches the documentation: The control underneath (the bitmap control) paints first, and then the static control paints on top of it. And a static text control paints by filling with the background brush and drawing the text on top of it. You can customize this behavior by responding to the WM_CTL­COLOR­STATIC message:

HBRUSH CTestDlg::OnCtlColor(CDC* pDC, CWnd* pWnd, UINT nCtlColor)
{
 HBRUSH hbr = __super::OnCtlColor(pDC, pWnd, nCtlColor);
 if (pWnd->GetExStyle() & WS_EX_TRANSPARENT) {
  pDC->SetBkMode(TRANSPARENT);
  hbr = GetStockBrush(HOLLOW_BRUSH);
  // even better would be to use a pattern brush, if the background is fixed
 }
 return hbr;
}

The customer appreciated the explanation but was puzzled as to why the Transparent is available if it doesn’t work. “We understand that we can use the WS_EX_TRANSPARENT window style to create a transparent window and it will be painted after its underlying siblings, but the window style by itself doesn’t make the static control transparent. But if we have to write the code above, why is the Transparent property available in the Properties box?” They included a screen shot from Visual Studio where the built-in help text for the Transparent property reads “Specifies that the control will have a transparent background.”

The WS_EX_TRANSPARENT style doesn’t mean “transparent”; it means “paint over siblings.” The style is called “transparent” not because it makes the window transparent but because it makes transparency possible. It is one of the steps (but not the only one) for making child controls render transparently. Another important step is ensuring that the control does not erase its background in its WM_ERASE­BKGND, and that’s the step that the On­Ctl­Color override performs.

Why is the Transparent property offered for static controls when it doesn’t actually work? Shouldn’t it be disabled for static controls?

The reason why it is offered is that it is a general window style that can be set on any control. Visual Studio doesn’t know which controls can render transparently and which ones don’t, or what extra steps are necessary to get the ones who can render transparently to actually do so. It just exposes the WS_EX_TRANSPARENT style and hopes that you know what you’re doing.

In retrospect, it was a poor chose of name for the style. And the incorrect online help doesn’t make things any better.

Bonus chatter: Note that the WS_EX_TRANSPARENT extended style is overloaded. In addition to affecting painting, it also affects hit-testing.

Raymond Chen

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