Windows SDK GDI: How do I calculate the area of a window exposed?

[Pravin Kumar]

Q: How do I calculate the area of a window exposed?

A: How to find the area exposed (not covered by other windows) on your desktop? There is no default function for that. Here is an easy method to find out.

The method followed is to create a region corresponding to an area covered by the original window and keep on subtracting the overlapping region covered by other windows on the desktop above the specified window in Z-order. Once the exposed region is calculated, its area can be calculated by counting the points in that region.

I have written two functions 'Visible()' to calculate the percentage area exposed by the window and 'Area()' to calculate the area of a region. Both the functions are self-explanatory but commented.

long Area(HRGN hrgn)
{
  RECT Rect;
  GetRgnBox(hrgn, &Rect);
  for (int N = 0, X = Rect.left; X < Rect.right; X++)
    for (int Y = Rect.top; Y < Rect.bottom; Y++)
      if (PtInRegion(hrgn, X, Y))
        N++;
  return N;
}

float Visible(HWND hwnd)
{
  // Calculates full area of total window
  RECT Rect;
  GetWindowRect(hwnd, &Rect);
  int FullArea = (Rect.right - Rect.left) * (Rect.bottom - Rect.top);

  // Creates a region corresponding to the window
  HRGN hRgn = CreateRectRgn(Rect.left, Rect.top, Rect.right, Rect.bottom);

  // Gets the topmost window 
  HWND hWnd = GetWindow(hwnd, GW_HWNDFIRST);
  while (hWnd != NULL && hWnd != hwnd)  // If the window is above in Z-order
  {
    if (IsWindowVisible(hWnd))      // If the window is visible
    {
      // Gets window dimension
      GetWindowRect(hWnd, &Rect);

      // Creates a region corresponding to the window
      HRGN hrgnWnd = CreateRectRgn(Rect.left, Rect.top, Rect.right, Rect.bottom);

      // Creates a region corresponding to region not overlapped
      CombineRgn(hRgn, hRgn, hrgnWnd, RGN_DIFF);
    }

    // Loops through all windows till the specified window
    hWnd = GetWindow(hWnd, GW_HWNDNEXT);
  }

  // Returnes exposed area in percentage
  return (float) (100.0 * Area(hRgn) / FullArea);
}

I did the testing by creating a timer in my project and repeatedly calculating the area exposed. The area currently exposed is made as the application window title. By manipulating other windows, I could see that the area exposed is displayed as the title of the application window.