How to calculate an area of a Windows region (HRGN) in pixels?

Question

What is the fastest way of getting the area of any arbitrary Windows region?

I know I can enumerate all points of bounding rectangle and call the PtInRegion() function but it seems not very fast. Maybe you know some faster way?

Probably [`GetRegionData`](http://msdn.microsoft.com/en-us/library/windows/desktop/dd144920(v=vs.85).aspx) and use the points to calculate the area of a polygon — Seth Carnegie, Jul 18 '12 at 14:17

score 10 · Accepted Answer · answered Jul 18 '12 at 14:31

When you call GetRegionData, you'll get a list of non-overlapping rectangles that make up the region. Add up their areas, something like this:

function GetRegionArea(rgn: HRgn): Cardinal;
var
  x: DWord;
  Data: PRgnData;
  Header: PRgnDataHeader;
  Rects: PRect;
  Width, Height: Integer;
  i: Integer;
begin
  x := GetRegionData(rgn, 0, nil);
  Win32Check(x <> 0);
  GetMem(Data, x);
  try
    x := GetRegionData(rgn, x, Data);
    Win32Check(x <> 0);
    Header := PRgnDataHeader(Data);
    Assert(Header.iType = rdh_Rectangles);

    Assert(Header.dwSize = SizeOf(Header^));
    Rects := PRect(Cardinal(Header) + Header.dwSize);
    // equivalent: Rects := PRect(@Data.Buffer);

    Result := 0;
    for i := 0 to Pred(Header.nCount) do begin
      Width := Rects.Right - Rects.Left;
      Height := Rects.Bottom - Rects.Top;
      Inc(Result, Width * Height);
      Inc(Rects);
    end;
  finally
    FreeMem(Data);
  end;
end;

Thanks, Rob! This must be fast enough – Andrew Jul 18 '12 at 14:39 — Andrew, Jul 18 '12 at 14:39

How to calculate an area of a Windows region (HRGN) in pixels?

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