Hough transform based method of estimating parameters

Claim 1

A Hough transform based method of estimating N parameters a=(a 1 , . . . .a N ) of motion of a region Y in a first image to a following image, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the method comprising the steps of: determining the total support H(Y,a) as a sum of the values of an error function for the intensities at pixels in the region Y, determining the motion parameters a that give the total support a minimum value using a Hough transform, the determining being made in steps of an iterative process moving along a series of parameter estimates a 1 , a 2 , . . by calculating partial derivatives dH i =MH(Y, a n )/Ma n,i of the total support for a parameter estimate a n with respect to each of the parameters a 1 and evaluating the calculated partial derivatives for taking a new a n+1 , wherein, in the evaluating of the partial derivatives, the partial derivatives dH i are first sealed by multiplying by scaling factors dependent on the spatial extension of the region to produce sealed partial derivatives dHN i .

Claim 52

A device for estimating values of N parameters a=(a 1 , . .. , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixel in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p and for calculating partial derivatives dH i =MH(Y,a p )/Ma p,i of the total support for parameter values a p with respect to each of the parameters a i , a decision module connected to the Hough transform module, the decision module determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a i , a 2 , . . . ., providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports and the calculated partial derivatives for the parameter estimate a n and evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step, wherein the decision module in the evaluating is arranged to use scaled partial derivatives obtained from the received calculated partial derivatives dH i by multiplying them by scaling factors independent on the spatial extension of the region Y.

Claim 53

A device for estimating values of N parameters a=(a 1 , . .. , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixel in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p and for calculating partial derivatives dH i =MH(Y,a p )/Ma p,i of the total support for parameter values a p with respect to each of the parameters a i , a decision module connected to the Hough transform module, the decision module determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a i , a 2 , . . . ., providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports and the calculated partial derivatives for the parameter estimate a n and evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step, wherein the decision module in the evaluating is arranged to use scaled partial derivatives obtained from the received calculated partial derivatives dH i by multiplying them by scaling factors independent on the spatial extension of the region Y.

Claim 74

A device for estimating values of N parameters a=(a 1 , . . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameters values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and setting, after making each of the steps of the iterative process, the resolutions r i to values smaller than the values used in the step until the resolutions, in the setting thereof, have achieved predetermined values, and in the following steps setting the resolutions r i to smaller values only after every k:th step, where k is an integer >1.

Claim 75

A device for estimating values of N parameters a=(a 1 , . . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameters values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and setting, after making each of the steps of the iterative process, the resolutions r i to values smaller than the values used in the step until the resolutions, in the setting thereof, have achieved predetermined values, and in the following steps setting the resolutions r i to smaller values only after every k:th step, where k is an integer >1.

Claim 76

A device for estimating values of N parameter a=(a 1 , . . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a control module connected to the decision module, the decision module arranged to signal each step of the iterative process to the control module, and the control module arranged to compare the number of steps in the iterative process to an iteration threshold and to stop the iterative process when the number of steps exceeds the iteration threshold.

Claim 77

A device for estimating values of N parameter a=(a 1 , . . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a control module connected to the decision module, the decision module arranged to signal each step of the iterative process to the control module, and the control module arranged to compare the number of steps in the iterative process to an iteration threshold and to stop the iterative process when the number of steps exceeds the iteration threshold.

Claim 79

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module, selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a scale estimation module connected to the Hough transform module and to the decision module, the scale estimation module arranged to calculate, at requests from the decision module, for steps of the iterative process, a value of a median estimate of scale and to communicate the calculated value of the median estimate of scale to the Hough transform module for use in determining the total supports, the decision module arranged to compare the calculated value of the median estimate o scale to a scale threshold and to stop the iterative process stopped when the value of the median estimate of scale is smaller than the scale threshold.

Claim 80

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module, selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a scale estimation module connected to the Hough transform module and to the decision module, the scale estimation module arranged to calculate, at requests from the decision module, for steps of the iterative process, a value of a median estimate of scale and to communicate the calculated value of the median estimate of scale to the Hough transform module for use in determining the total supports, the decision module arranged to compare the calculated value of the median estimate o scale to a scale threshold and to stop the iterative process stopped when the value of the median estimate of scale is smaller than the scale threshold.

Claim 83

A device for estimating values of N parameters a=(a 1 , . . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i i the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and wherein the decision module is arranged to store, for each step of iterative process, the parameter estimates and to top the iterative process when in the iterative process the same steps as before are taken.

Claim 84

A device for estimating values of N parameters a=(a 1 , . . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i i the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and wherein the decision module is arranged to store, for each step of iterative process, the parameter estimates and to top the iterative process when in the iterative process the same steps as before are taken.

Claim 85

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a scale estimate module connected to the Hough transform module and to the decision module, the scale estimation module arranged to calculate, at requests from the decision module, for steps of the iterative process, a value of a median estimate of scale and to communicate the calculated value of the median estimate of scale to the Hough transform module for use in determining the total supports, the median estimate of scale being calculated as a median absolute deviation (MAD) robust estimate of scale to which a positive constant is added.

Claim 86

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module; selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a scale estimate module connected to the Hough transform module and to the decision module, the scale estimation module arranged to calculate, at requests from the decision module, for steps of the iterative process, a value of a median estimate of scale and to communicate the calculated value of the median estimate of scale to the Hough transform module for use in determining the total supports, the median estimate of scale being calculated as a median absolute deviation (MAD) robust estimate of scale to which a positive constant is added.

Claim 88

The device for estimating values of N parameters a=(a 1 , . . . , a 2 ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinates system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a control module connected to the Hough transform module and the decision module, the control module setting for the region Y a centre of the coordinate system for the pixels as the centre of gravity of the region.

Claim 89

The device for estimating values of N parameters a=(a 1 , . . . , a 2 ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinates system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution r i in the parameter space for each of the parameters a i , the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports, and a control module connected to the Hough transform module and the decision module, the control module setting for the region Y a centre of the coordinate system for the pixels as the centre of gravity of the region.

Claim 90

A device for estimating values of N parameters a=(a 1 , . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution t i in the parameter space for each of the parameters a i , the resolutions selected to have a relation to an expected maximum value of displacement of pixels from the region Y in the first image to the following image and to the size of the region, the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports.

Claim 91

A device for estimating values of N parameters a=(a 1 , . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p , a decision module connected to the Hough transform module, the decision module: selecting a resolution t i in the parameter space for each of the parameters a i , the resolutions selected to have a relation to an expected maximum value of displacement of pixels from the region Y in the first image to the following image and to the size of the region, the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports.

Claim 93

A device for estimating values of N parameters a=(a 1 , . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a low-pass filtering module connected to the first memory means, the low-pass filtering module arranged to prefilter the first and following images by a low-pass filter and to store the prefiltered first and following images in the first memory means, the low pass-filtering module giving the prefiltered first and following images a dynamic range larger than the dynamic range of the first and following images, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of n error function for the intensities at pixels in the region Y using the stored prefiltered first and following images, a decision module connected to the Hough transform module, the decision module determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step.

Claim 94

A device for estimating values of N parameters a=(a 1 , . . . , a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a low-pass filtering module connected to the first memory means, the low-pass filtering module arranged to prefilter the first and following images by a low-pass filter and to store the prefiltered first and following images in the first memory means, the low pass-filtering module giving the prefiltered first and following images a dynamic range larger than the dynamic range of the first and following images, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of n error function for the intensities at pixels in the region Y using the stored prefiltered first and following images, a decision module connected to the Hough transform module, the decision module determining values of the parameters a that give the total support a minimum value, the determining being made in an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step.

Claim 96

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p and for calculating partial derivatives dH i =MH(Y,a p )/Ma p,i of the total support for parameter values a p with respect to each of the parameters a i , a decision module connected to the Hough transform module, the decision module, selecting a resolution r i in the parameter space for each of the parameters a i , the resolutions selected to have a relation to an expected maximum value of displacement of pixels from the region Y in the first image to the following image and to the size of the region, the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made n an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports and the calculated partial derivatives for the parameter estimate a n and evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step; wherein the decision module in the evaluating is arranged to use scaled partial derivatives obtained from the received calculated partial derivatives dH i by multiplying them by scaling factors having values corresponding to the resolutions r i in the parameter space, so that the scaled partial derivatives are dHN i =dH i A r i .

Claim 97

A device for estimating values of N parameters a=(a 1 , . . . ,a N ) of motion of a region Y in a first image to a following image, the region having a spatial extension, the first and following images represented, in a first spatial resolution, by intensities at pixels having coordinates in a coordinate system, the device comprising: first memory means for storing the intensities at the pixels in the first spatial resolution, a Hough transform module connected to the first memory means for determining total supports H(Y,a p ) as a sum of values of an error function for the intensities at pixels in the region Y and for parameter values a p and for calculating partial derivatives dH i =MH(Y,a p )/Ma p,i of the total support for parameter values a p with respect to each of the parameters a i , a decision module connected to the Hough transform module, the decision module, selecting a resolution r i in the parameter space for each of the parameters a i , the resolutions selected to have a relation to an expected maximum value of displacement of pixels from the region Y in the first image to the following image and to the size of the region, the selected resolution giving a grid of points in the N-dimensional space of possible parameter values, determining values of the parameters a that give the total support a minimum value, the determining being made n an iterative process, the decision module executing steps, selecting in each step a parameter estimate a n in a series of parameter estimates a 1 , a 2 , . . . , the parameter estimates selected as points in the grid, providing the selected parameter estimate to the Hough transform module and receiving therefrom the determined total supports and the calculated partial derivatives for the parameter estimate a n and evaluating the received calculated partial derivatives for selecting a new parameter estimate a n+1 to be used in the following step; wherein the decision module in the evaluating is arranged to use scaled partial derivatives obtained from the received calculated partial derivatives dH i by multiplying them by scaling factors having values corresponding to the resolutions r i in the parameter space, so that the scaled partial derivatives are dHN i =dH i A r i .