Operators

# proj_match_points_distortion_ransac (Operator)

## Name

proj_match_points_distortion_ransac — Compute a projective transformation matrix between two images and the radial distortion coefficient by automatically finding correspondences between points.

## Signature

proj_match_points_distortion_ransac(Image1, Image2 : : Rows1, Cols1, Rows2, Cols2, GrayMatchMethod, MaskSize, RowMove, ColMove, RowTolerance, ColTolerance, Rotation, MatchThreshold, EstimationMethod, DistanceThreshold, RandSeed : HomMat2D, Kappa, Error, Points1, Points2)

## Description

Given a set of coordinates of characteristic points (Rows1,Cols1) and (Rows2,Cols2) in both input images Image1 and Image2, which must be of identical size, proj_match_points_distortion_ransac automatically determines corresponding points, the homogeneous projective transformation matrix HomMat2D, and the radial distortion coefficient Kappa that optimally fulfill the following equation:

/ r2 \              / r1 \
| c2 | = HomMat2D * | c1 |.
\  1 /              \  1 /

Here, (r1,c1) and (r2,c2) denote image points that are obtained by undistorting the input image points with the division model (see calibrate_cameras):

r = r' / (1+Kappa*(r'^2+c'^2)
c = c' / (1+Kappa*(r'^2+c'^2)

Here, (r1',c1') = (Rows1-0.5*(h-1),Cols1-0.5*(w-1)) and (r2',c2') = (Rows2-0.5*(h-1),Cols2-0.5*(w-1)) denote the distorted image points, specified relative to the image center, and w and h denote the width and height of the input images. Thus, proj_match_points_distortion_ransac assumes that the principal point of the camera, i.e., the center of the radial distortions, lies at the center of the image.

The returned Kappa can be used to construct camera parameters that can be used to rectify images or points (see change_radial_distortion_cam_par, change_radial_distortion_image, and change_radial_distortion_points):

CamPar = [0.0,Kappa,1.0,1.0,0.5*(w-1),0.5*(h-1),w,h]

The matching process is based on characteristic points, which can be extracted with point operators like points_foerstner or points_harris. The matching itself is carried out in two steps: first, gray value correlations of mask windows around the input points in the first and the second image are determined and an initial matching between them is generated using the similarity of the windows in both images. Then, the RANSAC algorithm is applied to find the projective transformation matrix and radial distortion coefficient that maximizes the number of correspondences under the above constraint.

The size of the mask windows used for the matching is MaskSize x MaskSize. Three metrics for the correlation can be selected. If GrayMatchMethod has the value 'ssd', the sum of the squared gray value differences is used, 'sad' means the sum of absolute differences, and 'ncc' is the normalized cross correlation. For details please refer to binocular_disparity. The metric is minimized ('ssd', 'sad') or maximized ('ncc') over all possible point pairs. A thus found matching is only accepted if the value of the metric is below the value of MatchThreshold ('ssd', 'sad') or above that value ('ncc').

To increase the algorithm's performance, the search area for the match candidates can be limited to a rectangle by specifying its size and offset. Only points within a window of 2*RowTolerance x 2*ColTolerance points are considered. The offset of the center of the search window in the second image with respect to the position of the current point in the first image is given by RowMove and ColMove.

If the transformation contains a rotation, i.e., if the first image is rotated with respect to the second image, the parameter Rotation may contain an estimate for the rotation angle or an angle interval in radians. A good guess will increase the quality of the gray value matching. If the actual rotation differs too much from the specified estimate, the matching will typically fail. In this case, an angle interval should be specified and Rotation is a tuple with two elements. The larger the given interval is the slower the operator is since the RANSAC algorithm is run over all (automatically determined) angle increments within the interval.

After the initial matching has been completed, a randomized search algorithm (RANSAC) is used to determine the projective transformation matrix HomMat2D and the radial distortion coefficient Kappa. It tries to find the parameters that are consistent with a maximum number of correspondences. For a point to be accepted, the distance in pixels to its corresponding transformed point must not exceed the threshold DistanceThreshold.

The parameter EstimationMethod determines which algorithm is used to compute the projective transformation matrix. A linear algorithm is used if EstimationMethod is set to 'linear'. This algorithm is very fast and returns accurate results for small to moderate noise of the point coordinates and for most distortions (except for small distortions). For EstimationMethod = 'gold_standard', a mathematically optimal but slower optimization is used, which minimizes the geometric reprojection error. In general, it is preferable to use EstimationMethod = 'gold_standard'.

The value Error indicates the overall quality of the estimation procedure and is the mean symmetric euclidian distance in pixels between the points and their corresponding transformed points.

Point pairs consistent with the above constraints are considered to be corresponding points. Points1 contains the indices of the matched input points from the first image and Points2 contains the indices of the corresponding points in the second image.

The parameter RandSeed can be used to control the randomized nature of the RANSAC algorithm, and hence to obtain reproducible results. If RandSeed is set to a positive number, the operator returns the same result on every call with the same parameters because the internally used random number generator is initialized with RandSeed. If RandSeed = 0, the random number generator is initialized with the current time. In this case the results may not be reproducible.

## Parallelization

• Multithreading type: reentrant (runs in parallel with non-exclusive operators).
• Processed without parallelization.

## Parameters

Image1 (input_object)  singlechannelimage object (byte / uint2)

Input image 1.

Image2 (input_object)  singlechannelimage object (byte / uint2)

Input image 2.

Rows1 (input_control)  point.y-array (real / integer)

Input points in image 1 (row coordinate).

Restriction: length(Rows1) >= 5

Cols1 (input_control)  point.x-array (real / integer)

Input points in image 1 (column coordinate).

Restriction: length(Cols1) == length(Rows1)

Rows2 (input_control)  point.y-array (real / integer)

Input points in image 2 (row coordinate).

Restriction: length(Rows2) >= 5

Cols2 (input_control)  point.x-array (real / integer)

Input points in image 2 (column coordinate).

Restriction: length(Cols2) == length(Rows2)

GrayMatchMethod (input_control)  string (string)

Gray value match metric.

Default value: 'ncc'

List of values: 'ncc', 'sad', 'ssd'

Default value: 10

Typical range of values: 3 ≤ MaskSize ≤ 15

RowMove (input_control)  integer (integer)

Average row coordinate offset of corresponding points.

Default value: 0

ColMove (input_control)  integer (integer)

Average column coordinate offset of corresponding points.

Default value: 0

RowTolerance (input_control)  integer (integer)

Half height of matching search window.

Default value: 200

Restriction: RowTolerance >= 1

ColTolerance (input_control)  integer (integer)

Half width of matching search window.

Default value: 200

Restriction: ColTolerance >= 1

Rotation (input_control)  angle.rad(-array) (real / integer)

Estimate of the relative rotation of the second image with respect to the first image.

Default value: 0.0

Suggested values: 0.0, 0.1, -0.1, 0.7854, 1.571, 3.142

MatchThreshold (input_control)  number (integer / real)

Threshold for gray value matching.

Default value: 0.7

Suggested values: 0.9, 0.7, 0.5, 10, 20, 50, 100

EstimationMethod (input_control)  string (string)

Algorithm for the computation of the projective transformation matrix.

Default value: 'gold_standard'

List of values: 'gold_standard', 'linear'

DistanceThreshold (input_control)  number (real / integer)

Threshold for the transformation consistency check.

Default value: 1

Restriction: DistanceThreshold > 0

RandSeed (input_control)  integer (integer)

Seed for the random number generator.

Default value: 0

HomMat2D (output_control)  hom_mat2d (real)

Computed homogeneous projective transformation matrix.

Kappa (output_control)  real (real)

Error (output_control)  real (real)

Root-Mean-Square transformation error.

Points1 (output_control)  integer-array (integer)

Indices of matched input points in image 1.

Points2 (output_control)  integer-array (integer)

Indices of matched input points in image 2.

## Example (HDevelop)

points_foerstner (Image1, 1, 2, 3, 50, 0.1, 'gauss', 'true', \
Rows1, Cols1, _, _, _, _, _, _, _, _)
points_foerstner (Image2, 1, 2, 3, 50, 0.1, 'gauss', 'true', \
Rows2, Cols2, _, _, _, _, _, _, _, _)
get_image_size (Image1, Width, Height)
proj_match_points_distortion_ransac (Image1, Image2, Rows1, Cols1, \
Rows2, Cols2, 'ncc', 10, 0, 0, \
Height, Width, 0, 0.5, \
'gold_standard', 1, 42, \
HomMat2D, Kappa, Error, \
Points1, Points2)
CamParDist := [0.0,Kappa,1.0,1.0,0.5*(Width-1),0.5*(Height-1), \
Width,Height]
CamParDist, CamPar)
CamParDist, CamPar)
concat_obj (Image1Rect, Image2Rect, ImagesRect)
gen_projective_mosaic (ImagesRect, MosaicImage, 1, 1, 2, HomMat2D, \
'default', 'false', MosaicMatrices2D)