Editing 2D computer graphics (section)

===Rotation===

In [[linear algebra]], a ''[[rotation matrix]]'' is a [[matrix (mathematics)|matrix]] that is used to perform a [[rotation (mathematics)|rotation]] in [[Euclidean space]].

:<math>R = 
\begin{bmatrix}
\cos \theta & -\sin \theta \\
\sin \theta & \cos \theta \\
\end{bmatrix}
</math>

rotates points in the ''xy''-[[Cartesian coordinate system|Cartesian plane]] counterclockwise through an angle ''θ'' about the origin of the [[Cartesian coordinate system]]. To perform the rotation using a rotation matrix ''R'', the position of each point must be represented by a [[column vector]] '''v''', containing the coordinates of the point. A rotated vector is obtained by using the [[matrix multiplication]] ''R'''''v'''. Since matrix multiplication has no effect on the zero vector (i.e., on the coordinates of the origin), rotation matrices can only be used to describe rotations about the origin of the coordinate system.

Rotation matrices provide a simple algebraic description of such rotations, and are used extensively for computations in [[geometry]], [[physics]], and [[computer graphics]]. In 2-dimensional space, a rotation can be simply described by an [[Angle of rotation|angle ''θ'' of rotation]], but it can be also represented by the 4 entries of a rotation matrix with 2 rows and 2 columns. In 3-dimensional space, every rotation can be interpreted as a rotation by a given angle about a single fixed axis of rotation (see [[Euler's rotation theorem]]), and hence it can be simply described by [[Axis-angle representation|an angle and a vector]] with 3 entries. However, it can also be represented by the 9 entries of a rotation matrix with 3 rows and 3 columns. The notion of rotation is not commonly used in dimensions higher than&nbsp;3; there is a notion of a ''[[rotational displacement]]'', which can be represented by a matrix, but no associated single axis or angle.

Rotation matrices are [[square matrix|square matrices]], with [[real number|real]] entries. More specifically they can be characterized as [[orthogonal matrix|orthogonal matrices]] with [[determinant]]&nbsp;1:

:<math>R^{T} = R^{-1}, \det R = 1\,</math>.

The [[set (mathematics)|set]] of all such matrices of size ''n'' forms a [[group (mathematics)|group]], known as the [[special orthogonal group]] {{math|SO(''n'')}}.