An image of a fern-like fractal (Barnsley's fern) that exhibits affine self-similarity. Each of the leaves of the fern is related to each other leaf by an affine transformation. For instance, the red leaf can be transformed into both the dark blue leaf and any of the light blue leaves by a combination of reflection, rotation, scaling, and translation.
More generally, an affine transformation is an automorphism of an affine space (Euclidean spaces are specific affine spaces), that is, a function which maps an affine space onto itself while preserving both the dimension of any affine subspaces (meaning that it sends points to points, lines to lines, planes to planes, and so on) and the ratios of the lengths of parallel line segments. Consequently, sets of parallel affine subspaces remain parallel after an affine transformation. An affine transformation does not necessarily preserve angles between lines or distances between points, though it does preserve ratios of distances between points lying on a straight line.
If X is the point set of an affine space, then every affine transformation on X can be represented as the composition of a linear transformation on X and a translation of X. Unlike a purely linear transformation, an affine transformation need not preserve the origin of the affine space. Thus, every linear transformation is affine, but not every affine transformation is linear.
A generalization of an affine transformation is an affine map (or affine homomorphism or affine mapping) between two (potentially different) affine spaces over the same fieldk. Let (X, V, k) and (Z, W, k) be two affine spaces with X and Z the point sets and V and W the respective associated vector spaces over the field k. A map f: X → Z is an affine map if there exists a linear mapmf : V → W such that mf (x − y) = f (x) − f (y) for all x, y in X.
Let (X, V, k) be an affine space of dimension at least two, with X the point set and V the associated vector space over the field k. A semiaffine transformationf of X is a bijection of X onto itself satisfying:
If S is a d-dimensional affine subspace of X, f (S) is also a d-dimensional affine subspace of X.
If S and T are parallel affine subspaces of X, then f (S) || f (T).
These two conditions express what is precisely meant by the expression that "f preserves parallelism".
These conditions are not independent as the second follows from the first. Furthermore, if the field k has at least three elements, the first condition can be simplified to: f is a collineation, that is, it maps lines to lines.
If the dimension of the affine space (X, V, k) is at least two, then an affine transformation is a semiaffine transformation f that satisfies the condition: If x ≠ y and p ≠ q are points of X such that the line segments xy and pq are parallel, then
If the dimension of the affine space is one, that is, the space is an affine line, then any permutation of X would automatically satisfy the conditions to be a semiaffine transform. So, an affine transformation of an affine line is defined as any permutation f of the points of X such that if x ≠ y and p ≠ q are points of X, then
By the definition of an affine space, V acts on X, so that, for every pair (x, v) in X × V there is associated a point y in X. We can denote this action by v→(x) = y. Here we use the convention that v→ = v are two interchangeable notations for an element of V. By fixing a point c in X one can define a function mc : X → V by mc(x) = cx→. For any c, this function is one-to-one, and so, has an inverse function mc−1 : V → X given by mc−1(v) = v→(c). These functions can be used to turn X into a vector space (with respect to the point c) by defining:
This vector space has origin c and formally needs to be distinguished from the affine space X, but common practice is to denote it by the same symbol and mention that it is a vector space after an origin has been specified. This identification permits points to be viewed as vectors and vice versa.
Then L(c, λ) is an affine transformation of X which leaves the point c fixed. It is a linear transformation of X, viewed as a vector space with origin c.
Let σ be any affine transformation of X. Pick a point c in X and consider the translation of X by the vector , denoted by Tw. Translations are affine transformations and the composition of affine transformations is an affine transformation. For this choice of c, there exists a unique linear transformation λ of V such that
That is, an arbitrary affine transformation of X is the composition of a linear transformation of X (viewed as a vector space) and a translation of X.
This representation of affine transformations is often taken as the definition of an affine transformation (with the choice of origin being implicit).
As shown above, an affine map is the composition of two functions: a translation and a linear map. Ordinary vector algebra uses matrix multiplication to represent linear maps, and vector addition to represent translations. Formally, in the finite-dimensional case, if the linear map is represented as a multiplication by an invertible matrix and the translation as the addition of a vector , an affine map acting on a vector can be represented as
Affine transformations on the 2D plane can be performed by linear transformations in three dimensions. Translation is done by shearing along over the z axis, and rotation is performed around the z axis.
Using an augmented matrix and an augmented vector, it is possible to represent both the translation and the linear map using a single matrix multiplication. The technique requires that all vectors be augmented with a "1" at the end, and all matrices be augmented with an extra row of zeros at the bottom, an extra column—the translation vector—to the right, and a "1" in the lower right corner. If is a matrix,
Ordinary matrix-vector multiplication always maps the origin to the origin, and could therefore never represent a translation, in which the origin must necessarily be mapped to some other point. By appending the additional coordinate "1" to every vector, one essentially considers the space to be mapped as a subset of a space with an additional dimension. In that space, the original space occupies the subset in which the additional coordinate is 1. Thus the origin of the original space can be found at . A translation within the original space by means of a linear transformation of the higher-dimensional space is then possible (specifically, a shear transformation). The coordinates in the higher-dimensional space are an example of homogeneous coordinates. If the original space is Euclidean, the higher dimensional space is a real projective space.
The advantage of using homogeneous coordinates is that one can combine any number of affine transformations into one by multiplying the respective matrices. This property is used extensively in computer graphics, computer vision and robotics.
Example augmented matrixEdit
If the vectors are a basis of the domain's projective vector space and if are the corresponding vectors in the codomain vector space then the augmented matrix that achieves this affine transformation
This formulation works irrespective of whether any of the domain, codomain and image vector spaces have the same number of dimensions.
For example, the affine transformation of a vector plane is uniquely determined from the knowledge of where the three vertices () of a non-degenerate triangle are mapped to (), regardless of the number of dimensions of the codomain and regardless of whether the triangle is non-degenerate in the codomain.
An affine transformation preserves:
collinearity between points: three or more points which lie on the same line (called collinear points) continue to be collinear after the transformation.
parallelism: two or more lines which are parallel, continue to be parallel after the transformation.
convexity of sets: a convex set continues to be convex after the transformation. Moreover, the extreme points of the original set are mapped to the extreme points of the transformed set.
ratios of lengths of parallel line segments: for distinct parallel segments defined by points and , and , the ratio of and is the same as that of and .
The invertible affine transformations (of an affine space onto itself) form the affine group, which has the general linear group of degree as subgroup and is itself a subgroup of the general linear group of degree .
Each of these groups has a subgroup of orientation-preserving or positive affine transformations: those where the determinant of is positive. In the last case this is in 3D the group of rigid transformations (proper rotations and pure translations).
If there is a fixed point, we can take that as the origin, and the affine transformation reduces to a linear transformation. This may make it easier to classify and understand the transformation. For example, describing a transformation as a rotation by a certain angle with respect to a certain axis may give a clearer idea of the overall behavior of the transformation than describing it as a combination of a translation and a rotation. However, this depends on application and context.
An affine map between two affine spaces is a map on the points that acts linearly on the vectors (that is, the vectors between points of the space). In symbols, determines a linear transformation such that, for any pair of points :
We can interpret this definition in a few other ways, as follows.
If an origin is chosen, and denotes its image , then this means that for any vector :
If an origin is also chosen, this can be decomposed as an affine transformation that sends , namely
followed by the translation by a vector .
The conclusion is that, intuitively, consists of a translation and a linear map.
Given two affine spaces and , over the same field, a function is an affine map if and only if for every family of weighted points in such that
In their applications to digital image processing, the affine transformations are analogous to printing on a sheet of rubber and stretching the sheet's edges parallel to the plane. This transform relocates pixels requiring intensity interpolation to approximate the value of moved pixels, bicubic interpolation is the standard for image transformations in image processing applications. Affine transformations scale, rotate, translate, mirror and shear images as shown in the following examples:
The affine transforms are applicable to the registration process where two or more images are aligned (registered). An example of image registration is the generation of panoramic images that are the product of multiple images stitched together.
The affine transform preserves parallel lines. However, the stretching and shearing transformations warp shapes, as the following example shows:
This is an example of image warping. However, the affine transformations do not facilitate projection onto a curved surface or radial distortions.
In the planeEdit
A central dilation. The triangles A1B1Z, A1C1Z, and B1C1Z get mapped to A2B2Z, A2C2Z, and B2C2Z, respectively.
Affine transformations in two real dimensions include:
scaling in a given direction, with respect to a line in another direction (not necessarily perpendicular), combined with translation that is not purely in the direction of scaling; taking "scaling" in a generalized sense it includes the cases that the scale factor is zero (projection) or negative; the latter includes reflection, and combined with translation it includes glide reflection,
To visualise the general affine transformation of the Euclidean plane, take labelled parallelogramsABCD and A′B′C′D′. Whatever the choices of points, there is an affine transformation T of the plane taking A to A′, and each vertex similarly. Supposing we exclude the degenerate case where ABCD has zero area, there is a unique such affine transformation T. Drawing out a whole grid of parallelograms based on ABCD, the image T(P) of any point P is determined by noting that T(A) = A′, T applied to the line segment AB is A′B′, T applied to the line segment AC is A′C′, and T respects scalar multiples of vectors based at A. [If A, E, F are collinear then the ratio length(AF)/length(AE) is equal to length(A′F′)/length(A′E′).] Geometrically T transforms the grid based on ABCD to that based in A′B′C′D′.
Affine transformations do not respect lengths or angles; they multiply area by a constant factor
area of A′B′C′D′ / area of ABCD.
A given T may either be direct (respect orientation), or indirect (reverse orientation), and this may be determined by its effect on signed areas (as defined, for example, by the cross product of vectors).
Over the real numbersEdit
The functions with and in , are precisely the affine transformations of the real line.
Over a finite fieldEdit
The following equation expresses an affine transformation of GF(28) viewed as an 8-dimensional vector space over GF(2), that is used in the crypto-algorithm Rijndael (AES):
where is the matrix below, is a fixed vector and Specifically,
For instance, the affine transformation of the element in big-endianbinary notation is calculated as follows:
In plane geometryEdit
A simple affine transformation on the real plane
Effect of applying various 2D affine transformation matrices on a unit square. Note that the reflection matrices are special cases of the scaling matrix.
In , the transformation shown at left is accomplished using the map given by:
Transforming the three corner points of the original triangle (in red) gives three new points which form the new triangle (in blue). This transformation skews and translates the original triangle.
In fact, all triangles are related to one another by affine transformations. This is also true for all parallelograms, but not for all quadrilaterals.
Anamorphosis – artistic applications of affine transformations