Question

I am trying to draw a polygon using GDI. This code works:

type
  TPolygon: Array[0..2] of TPoint;

var 
  ACanvas: TGPGraphics;
  MyBrush: TGPLinearGradientBrush;

...

procedure DrawPolygon;
var
  Polygon: TPolygon;
begin
  Polygon[0].X := 1;
  Polygon[0].Y := 5;
  Polygon[1].X := 10;
  Polygon[1].Y := 15;
  Polygon[2].X := 1;
  Polygon[2].Y := 5;

  ACanvas.FillPolygon(MyBrush, PGPPoint(@Polygon), length(Polygon));
end;

...

This code produces a GDI Value Overflow error:

type
  TPolygon: Array of TPoint;

var 
  ACanvas: TGPGraphics;
  MyBrush: TGPLinearGradientBrush;

...

procedure DrawPolygon;
var
  Polygon: TPolygon;
begin
  SetLength(Polygon, 3);

  Polygon[0].X := 1;
  Polygon[0].Y := 5;
  Polygon[1].X := 10;
  Polygon[1].Y := 15;
  Polygon[2].X := 1;
  Polygon[2].Y := 5;

  ACanvas.FillPolygon(MyBrush, PGPPoint(@Polygon), length(Polygon));
end;

...

The only difference is that one point array is dynamic, the other is static. Obviously the underlying memory values are different, but in what way?

Answer 1

Your code is invalid. (First, there is no FillPolygon in TCanvas, and second, a polygon need at least three vertices. Also, there are some syntax errors, like a : instead of = at the const declaration.) I’d suggest the sample

Polygon[0].X := 1;
Polygon[0].Y := 1;
Polygon[1].X := 1;
Polygon[1].Y := 100;
Polygon[2].X := 100;
Polygon[2].Y := 1;

which is a nice triangle. Anyhow, while

Windows.Polygon(Canvas.Handle, Polygon, 3)

works for a static array, you have to do

Windows.Polygon(Canvas.Handle, Polygon[0], 3)

for a dynamic array. The reason is that a static array is stored ‘in-place’ in memory, that is, directly at @Polygon, just like a number (e.g., cardinal) is stored, or a ShortString, or a record of such simple types. On the contrary, if Polygon is a dynamic array, then it is really a pointer to the actual, variable-length, data (in much the same way a normal variable-length string works). That is, at @Polygon you only have a pointer, a NativeUInt. The actual data starts at this new address, which you can get by writing @Polygon[0].