Operator Reference
vector_to_similarity (Operator)
vector_to_similarity — Approximate an similarity transformation from point correspondences.
Signature
Description
vector_to_similarity approximates a similarity
transformation, i.e., a transformation consisting of a uniform
scaling, a rotation, and a translation, from at least two point
correspondences and returns it as the homogeneous transformation
matrix HomMat2D. The matrix consists of 3 components: a
scaling matrix S with identical scaling
in the x and y directions, a rotation matrix
R, and a translation vector
t (also see
hom_mat2d_scale, hom_mat2d_rotate, and
hom_mat2d_translate):
The point correspondences are passed in the tuples (Px,
Py) and (Qx,Qy), where
corresponding points must be at the same index positions in the tuples.
If more than two point correspondences are passed, the transformation is
overdetermined. In this case, the returned transformation is the
transformation that minimizes the distances between the original points
(Px,Py) and the transformed points
(Qx,Qy), as described in the following equation
(points as homogeneous vectors):
HomMat2D can be used directly with operators that transform data
using affine transformations, e.g., affine_trans_image.
Attention
It should be noted that homogeneous transformation matrices refer to
a general right-handed mathematical coordinate system. If a
homogeneous transformation matrix is used to transform images,
regions, XLD contours, or any other data that has been extracted
from images, the row coordinates of the transformation must be
passed in the x coordinates, while the column coordinates must be
passed in the y coordinates. Consequently, the order of passing row
and column coordinates follows the usual order
(Row,Column). This convention is essential to
obtain a right-handed coordinate system for the transformation of
iconic data, and consequently to ensure in particular that rotations
are performed in the correct mathematical direction.
Furthermore, it should be noted that if a homogeneous transformation matrix is used to transform images, regions, XLD contours, or any other data that has been extracted from images, it is assumed that the origin of the coordinate system of the homogeneous transformation matrix lies in the upper left corner of a pixel. The image processing operators that return point coordinates, however, assume a coordinate system in which the origin lies in the center of a pixel. Therefore, to obtain a consistent homogeneous transformation matrix, 0.5 must be added to the point coordinates before computing the transformation.
Execution Information
- Multithreading type: reentrant (runs in parallel with non-exclusive operators).
- Multithreading scope: global (may be called from any thread).
- Processed without parallelization.
Parameters
Px (input_control) point.x-array → (real)
X coordinates of the original points.
Py (input_control) point.y-array → (real)
Y coordinates of the original points.
Qx (input_control) point.x-array → (real)
X coordinates of the transformed points.
Qy (input_control) point.y-array → (real)
Y coordinates of the transformed points.
HomMat2D (output_control) hom_mat2d → (real)
Output transformation matrix.
Possible Successors
affine_trans_image,
affine_trans_image_size,
affine_trans_region,
affine_trans_contour_xld,
affine_trans_polygon_xld,
affine_trans_point_2d
Alternatives
vector_to_hom_mat2d,
vector_to_aniso,
vector_to_rigid
See also
vector_field_to_hom_mat2d,
point_line_to_hom_mat2d
Module
Foundation