CGAL::Aff_transformation_2<R>

Definition

Since the general form is based on the homogeneous representation, a transformation matrix multiplication by a scalar does not change the represented transformation. Therefore, any transformation represented by a matrix with rational entries can be represented by a transformation matrix with integer entries as well by multiplying the matrix with the common denominator of the rational entries. Hence it is sufficient to have number type R::RT for the entries of an affine transformation.

CGAL offers several specialized affine transformations. Different constructors are provided to create them. They are parameterized with a symbolic name to denote the transformation type, followed by additional parameters. The symbolic name tags solve ambiguities in the function overloading and they make the code more readable, i.e. what type of transformation is created.

Since two-dimensional points have three homogeneous coordinates we have a $$3 × 3 matrix ($$m_ij). Following C-style, the indices start at zero.

If the homogeneous representations are normalized such that the homogenizing coordinate is 1, then the upper left $$2 × 2 matrix realizes linear transformations and in the matrix form of a translation, the translation vector $$(v₀, $$v₁, $$1) appears in the last column of the matrix. In the normalized case, entry $$hw is always 1. Entries $$m₂₀ and $$m₂₁ are always zero and therefore do not appear in the constructors.

Creation

Aff_transformation_2<R> t ( Identity_transformation);
	introduces an identity transformation.
Aff_transformation_2<R> t ( const Translation, Vector_2<R> v);
	introduces a translation by a vector $$v.
Aff_transformation_2<R> t ( const Rotation, Direction_2<R> d, R::RT num, R::RT den = RT(1));
	approximates the rotation over the angle indicated by direction $$d, such that the differences between the sines and cosines of the rotation given by d and the approximating rotation are at most $$num/den each. Precondition: $$num/den>0.
Aff_transformation_2<R> t ( const Rotation, R::RT sine_rho, R::RT cosine_rho, R::RT hw = RT(1));
	introduces a rotation by the angle rho. Precondition: $$sine_rho2 + cosine_rho2 == hw2.
Aff_transformation_2<R> t ( const Scaling, R::RT s, R::RT hw = RT(1));
	introduces a scaling by a scale factor $$s/hw.
Aff_transformation_2<R> t ( R::RT m00, R::RT m01, R::RT m02, R::RT m10, R::RT m11, R::RT m12, R::RT hw = RT(1));
	introduces a general affine transformation in the 3x3 matrix . The sub-matrix $$hw-1 contains the scaling and rotation information, the vector $$hw-1 contains the translational part of the transformation.
Aff_transformation_2<R> t ( R::RT m00, R::RT m01, R::RT m10, R::RT m11, R::RT hw = RT(1));
	introduces a general linear transformation , i.e. there is no translational part.

Operations

The main thing to do with transformations is to apply them on geometric objects. Each class Class_2<R> representing a geometric object has a member function:

Class_2<R> transform(Aff_transformation_2<R> t).

The transformation classes provide a member function transform() for points, vectors, directions, and lines:

Point_2<R>	t.transform ( Point_2<R> p)
Vector_2<R>	t.transform ( Vector_2<R> p)
Direction_2<R>	t.transform ( Direction_2<R> p)
Line_2<R>	t.transform ( Line_2<R> p)

CGAL provides function operators for these member functions:

Point_2<R>	t.operator() ( Point_2<R> p)
Vector_2<R>	t.operator() ( Vector_2<R> p)
Direction_2<R>	t.operator() ( Direction_2<R> p)
Line_2<R>	t.operator() ( Line_2<R> p)

Miscellaneous

Aff_transformation_2<R>
	t.operator* ( s)	composes two affine transformations.
Aff_transformation_2<R>
	t.inverse ()	gives the inverse transformation.
bool	t.is_even ()	returns true, if the transformation is not reflecting, i.e. the determinant of the involved linear transformation is non-negative.
bool	t.is_odd ()	returns true, if the transformation is reflecting.

The matrix entries of a matrix representation of a Aff_transformation_2<R> can be accessed trough the following member functions:

FT	t.cartesian ( int i, int j)
FT	t.m ( int i, int j)
		returns entry $$mij in a matrix representation in which $$m22 is 1.
RT	t.homogeneous ( int i, int j)
RT	t.hm ( int i, int j)
		returns entry $$mij in some fixed matrix representation.

For affine transformations no I/O operators are defined.

See Also

Example

  typedef Cartesian<double> RepClass;
  typedef Aff_transformation_2<RepClass> Transformation;
  typedef Point_2<RepClass> Point;
  typedef Vector_2<RepClass> Vector;
  typedef Direction_2<RepClass> Direction;

  Transformation rotate(ROTATION, sin(pi), cos(pi));
  Transformation rational_rotate(ROTATION,Direction(1,1), 1, 100);
  Transformation translate(TRANSLATION, Vector(-2, 0));
  Transformation scale(SCALING, 3);

  Point q(0, 1);
  q = rational_rotate(q); 

  Point p(1, 1);

  p = rotate(p); 

  p = translate(p); 

  p = scale(p);

The same would have been achieved with

  Transformation transform = scale * (translate * rotate);
  p = transform(Point(1.0, 1.0));

See Also