Quick skeleton

function TSingleMatrix.ArgExtremaXY(Count: Int32; HiLo: Boolean = True; XYIntersection: Boolean = True): TPointArray;
var
  W, H: Integer;
  Buffer: TPointArray;
  function pass_x(): TPointArray;
  var
    c,X,Y: Integer;
  begin
    for Y:=0 to H-1 do
    begin
      X := 1;
      while (X < W) do
      begin
        while (X < W) and (Self[Y,X] >= Self[Y,X-1]) do Inc(X);
        Buffer[c] := [X-1,Y];
        Inc(c);
        while (X < W) and (Self[Y,X] <= Self[Y,X-1]) do Inc(X);
      end;
    end;

    Result := Copy(Buffer, 0, c);
  end;

  function pass_y(): TPointArray;
  var
    c,X,Y: Integer;
  begin
    for X:=0 to W-1 do
    begin
      Y := 1;
      while (Y < H) do
      begin
        while (Y < H) and (Self[Y,X] >= Self[Y-1,X]) do Inc(Y);
        Buffer[c] := [X,Y-1];
        Inc(c);
        while (Y < H) and (Self[Y,X] <= Self[Y-1,X]) do Inc(Y);
      end;
    end;

    Result := Copy(Buffer, 0, c);
  end;

var
  I: Integer;
  Weights: TSingleArray;
begin
  W := Self.Width();
  H := Self.Height();
  SetLength(buffer, W*H);

  if XYIntersection then
  begin
    Result := pass_x().Intersection(pass_y());
  end else begin
    Result := pass_x();
    Result.Extend(pass_y());
  end;

  // just use sort, since there arn't that many peaks
  SetLength(Weights, Length(Result));
  for I := 0 to High(Result) do
    Weights[I] := Self[Result[I].Y, Result[I].X];

  Result.Sort(Weights, not HiLo);
  if (Length(Result) > Count) then
    SetLength(Result, Count);
end;

function DistanceTransform(pts: TPointArray): TSingleMatrix;
var
  inverse: TPointArray;
  itree: TSlackTree;
  b: TBox;
  i: Int32;
begin
  inverse := pts.Invert();
  b := pts.Bounds.Combine(inverse.Bounds);
  Result.SetSize(b.x2+1, b.y2+1);
  itree.Init(inverse);
  for i:=0 to High(pts) do
    Result[pts[i].y, pts[i].x] := itree.Nearest(pts[i]).DistanceTo(pts[i]);
end;

function QuickSkeleton(pts: TPointArray): TPointArray;
var
  m: TSingleMatrix;
begin
  m   := DistanceTransform(pts);
  Result := m.ArgExtremaXY($FFFFFF, True, False);
end;