SetBkMode(hdc, TRANSPARENT) doesn't work

Question

When I use SetBkMode(hdc, TRANSPARENT); in the code below, I got the following effect when I resize the main window (and hence when the child receives the WM_PAINT message):

enter image description here

The problem is : When I resize the main window, The old area of "Find:" shoule be erased, I guess. But it just remains there.

If I don't use SetBkMode(hdc, TRANSPARENT);, I don't have this problem. It looks like:

enter image description here

, i.e it has white background. Furthermore, if I use SetBkMode(hdc, TRANSPARENT);, it looks like the same as above, before I resize the main window. So I don't think SetBkMode(hdc, TRANSPARENT); works here.

the hwnd is a static child with style SS_BITMAP.

Do you know why this issue occurs?

switch (message) {
    case WM_PAINT:
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hwnd, &ps);

        SelectObject(hdc, gDefaultGuiFont);
        SetBkMode(hdc, TRANSPARENT);

        RECT rc;
        GetClientRect(hwnd, &rc);
        DrawText(hdc, _TR("Find:"), -1, &rc, DT_SINGLELINE | DT_CENTER | DT_VCENTER);

        EndPaint(hwnd, &ps);

        return 0;

     .............
}

Looks like this is toolbar portion. Thought, it does not need to be resizable? — Maximus, Aug 05 '12 at 08:26
@Maximus, I don't understand what you mean. When I resize the main window, it call a function to resize the toolbar. I put a static child control with SS_BITMAP style and I want to display `Find:` on this static control. — user565739, Aug 05 '12 at 08:29
If I resize the main window from the right or bottom border, no such issue. But if I resize the main window from the left or upper border, the issue occurs. — user565739, Aug 05 '12 at 08:37
In any case, the height of the toolbar is not changed. But its width may change. — user565739, Aug 05 '12 at 08:38

score 0 · Answer 1 · answered Aug 05 '12 at 09:24

Try to use "fixed" rectangles. For example

    RECT rc;
    GetClientRect(hwnd, &rc);
    rc.left += ...; rc.top += ...; // shift up-left point
    DrawText(hdc, _TR("Find:"), -1, &rc, DT_SINGLELINE | DT_LEFT | DT_TOP);

The idea is you draw text in wrong position (once) and in right position (twice) while backgound updated only once. Can't say more on part of code.

score 0 · Answer 2 · answered Aug 06 '12 at 16:29

The problem is that windows is not updating the control (in time) that's behind your static control, you are now responsible for it's contents. So you want to use the background provided by the parent. Well just ask the parent to draw it for you in the child window:

RECT rc;
GetClientRectRelative(m_hWnd, GetParent(m_hWnd), &rc);
SetWindowOrgEx(m_mdc, rc.left, rc.top, NULL);
SendMessage(GetParent(m_hWnd), WM_PAINT, (WPARAM)(HDC)m_mdc);
SetWindowOrgEx(m_mdc, 0, 0, NULL);

In which

bool GetClientRectRelative(HWND hWnd, HWND hWndRelativeTo, RECT *pRect)
{
    RECT rcWnd, rcRelativeTo;
    if (!GetClientRect(hWnd, &rcWnd) ||
            !ClientToScreen(hWnd, (POINT*)&rcWnd) ||
            !ClientToScreen(hWnd, (POINT*)&rcWnd + 1) ||
            !GetClientRect(hWndRelativeTo, &rcRelativeTo) || 
            !ClientToScreen(hWndRelativeTo, (POINT*)&rcRelativeTo) ||
            !ClientToScreen(hWndRelativeTo, (POINT*)&rcRelativeTo + 1))
        return false;

    pRect->top = rcWnd.top - rcRelativeTo.top;
    pRect->left = rcWnd.left - rcRelativeTo.left;
    pRect->right = rcWnd.right - rcRelativeTo.left;
    pRect->bottom = rcWnd.bottom - rcRelativeTo.top;

    return true;
}

Now draw anything you like, I suggest you'd use the TRANSPARENT background mode.

Please create all your child windows with the styles WS_CLIPCHILDREN and WS_CLIPSIBLINGS, then these problems will become apparent immediately and you avoid flicker.

SetBkMode(hdc, TRANSPARENT) doesn't work

2 Answers2

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