[win32]Draw on a background bitmap

[Member 2965471]

i've tried, when i maximize the window bitmap disappear and when i restoring the window the bitmap reappears, it seems that the coordinates are different when the window is maximize, it may be true?

[Mark Salsbery]

You're joking, right?

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Member 2965471]

no no of course, i call it from WM_MOUSEMOVE.

[Member 2965471]

i don't understand, why? i post my code in the first msg...

[Mark Salsbery]

Well it's not as bad as the recursion you'd get from calling it in response to WM_PAINT, but it certainly doesn't need to be called on every WM_MOUSEMOVE message. Maybe call it from WM_SIZE handling code instead...

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Member 2965471]

I need to call it on mouse move because i need to draw two lines converging at the mouse cursor position.

[Mark Salsbery]

Ok then I would double-buffer as mentioned by DavidCrow and use Invalidate();UpdateWindow() combination.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

Since this is fun and I haven't touched C++ in years, here's a double-buffered version... Note I moved stuff out of the WM_PAINT handler that didn't need to be done every paint. For example, the offscreen buffer is only (re)created when the window size changes, the client rect is only updated when the window size changes, and the bitmap and its associated memoryDC are only loaded/created once (in WM_CREATE handler in the sample code). Flicker free! :)

POINT pt;
BITMAP Bitmap;
HANDLE hBitmap = 0;
HDC hBitmapMemoryDC = 0;
RECT ClientRect;
HBITMAP hOffScreenBitmap = 0;
HDC hOffScreenMemoryDC = 0;

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
PAINTSTRUCT ps;
HDC hdc;

switch (message)
{
case WM\_CREATE:
    hBitmap = ::LoadImage(hInst, MAKEINTRESOURCE(IDB\_BITMAP1), IMAGE\_BITMAP, 0, 0, LR\_CREATEDIBSECTION);
    ::GetObject(hBitmap, sizeof(Bitmap), &Bitmap);
    hBitmapMemoryDC = ::CreateCompatibleDC(0);
    ::SelectObject(hBitmapMemoryDC, hBitmap);
    break;
case WM\_PAINT:
    hdc = BeginPaint(hWnd, &ps);
    // blt the bitmap to the offscreen buffer
    ::StretchBlt(hOffScreenMemoryDC, 0, 0, ClientRect.right, ClientRect.bottom, hBitmapMemoryDC, 0, 0, Bitmap.bmWidth, Bitmap.bmHeight, SRCCOPY);
    // draw crosshairs on the offscreen buffer
    ::MoveToEx(hOffScreenMemoryDC, pt.x , 0 , NULL); 
    ::LineTo(hOffScreenMemoryDC, pt.x, ClientRect.bottom); 
    ::MoveToEx(hOffScreenMemoryDC, 0, pt.y, NULL); 
    ::LineTo(hOffScreenMemoryDC, ClientRect.right, pt.y); 
    // blt the offscreen buffer to the screen
    ::BitBlt(hdc, 0, 0, ClientRect.right, ClientRect.bottom, hOffScreenMemoryDC, 0, 0, SRCCOPY);
    EndPaint(hWnd, &ps);
    break;
case WM\_MOUSEMOVE:
    pt.x = LOWORD(lParam);
    pt.y = HIWORD(lParam);
    ::InvalidateRect(hWnd,NULL,FALSE); // changed to FALSE since we draw the entire client area
    ::UpdateWindow(hWnd);
    break;	
case WM\_ERASEBKGND:
    // We draw the background in WM\_PAINT - don't do it here!
    return 1;
case WM\_SIZE:
    ClientRect.left = 0;
    ClientRect.top = 0;
    ClientRect.right = LOWORD(lParam);
    ClientRect.bottom = HIWORD(lParam);
    // (Re)Create the offscreen buffer
    if (hOffScreenBitmap != 0)
        ::DeleteObject(hOffScreenBitmap);
    if (hOffScreenMemoryDC != 0)
        ::Delete

[David Crow]

Member 2965471 wrote:

it seems that the coordinates are different when the window is maximize, it may be true?

That depends on if you are using screen or client coordinates. With the former, the coordinates will always change. With the latter, top and left will always be 0.

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[bob16972]

It almost sounds like your asking how to rubberband a line. If not, ignore my comments but if you are looking to have the bitmap drawn in your WM_PAINT handler and you want to stretch a line from some starting point to where the mouse cursor is, this is normally referred to as "rubberbanding". Here's an old post of mine describing how to rubberband a line (btw, the idea is basically the same for a rectangle). Rubberbanding Lines[^] NOTE: One you commit to the line being at some location permanently, you would need to keep track of that object and also draw it in your WM_PAINT handler but only after you no longer want to rubberband it. I reference the book by Ivor Horton which is truly a must have if your going to pursue a drawing project as he basically builds one in the second half of the book. Also, this is all using MFC but the concepts can be easily adjusted to WIN32.