private void rect(Point2d pt1, Point2d pt2, Point2d pt3)
{
Vector2d vec = pt1.VectorTo(pt3);
Vector2d vec2 = pt2.VectorTo(pt3);
if (vec.Length < vec2.Length)
{
vec = vec2;
}
Point2d pt;
vec.Normalize();
Vector2d normal = vec.Copy();
Matrix2d mtx = invApp.TransientGeometry.CreateMatrix2d();
mtx.SetToRotation(3.1415926 / 2, invApp.TransientGeometry.CreatePoint2d(0, 0));
normal.TransformBy(mtx);
vec.ScaleBy(b / 2);
normal.ScaleBy(a / 2);
pt = mid(pt1, pt2);
pt.TranslateBy(vec);
pt3 = pt.Copy();
pt3.TranslateBy(normal);
ps.SketchLines.AddAsThreePointCenteredRectangle(mid(pt1, pt2), pt, pt3);
}