Question:
I am having trouble getting the Windows API function PtInRegion()
to work correctly in Windows 3.1 and Windows 95 without
generating an application error or GPF. What can I do to determine
if a point is in a region reliably?
Answer:
The Windows 3.1 and Windows 95 GDI heap is limited in regards
to scan converting large and complex regions such as Ellipses,
Polygons, and PolyPolygons. One "Brute Force" technique is to
create an "in memory" monochrome bitmap and draw the
required shape in black. You can then test to see if a given point in
the bitmap is black or white. If the test pixel is black, then the given
point is "in" the given region.
Example:
procedure TForm1.Button1Click(Sender: TObject);
var
bm : TBitmap;
begin
bm := TBitmap.Create;
bm.Monochrome := true;
bm.Height := 100;
bm.Width := 100;
bm.Canvas.Brush.Color := clWhite;
bm.Canvas.FillRect(Rect(0, 0, 100, 100));
bm.Canvas.Brush.Color := clBlack;
bm.Canvas.Pen.Color := clBlack;
bm.Canvas.Ellipse(10, 10, 90, 90);
if bm.Canvas.Pixels[20, 20] = clBlack then
ShowMessage('Point : (20, 20) is in region') else
ShowMessage('Point : (20, 20) is not in region');
if bm.Canvas.Pixels[50, 50] = clBlack then
ShowMessage('Point : (50, 50) is in region') else
ShowMessage('Point : (50, 50) is not in region');
bm.Free;
end;