[Insight-users] The Hough transform and world/pixel coordinates

[Markus Neuner]

Hi Alberto,

Nice that this solved your problem. I agree with you that every filter
that produces points as output should use coordinates in physical space.
I dont know why the itkHoughTransform2DLinesImageFilter outputs
(index)points.

Best regards,
Markus

Gomez, Alberto wrote:
> Thanks Markus,
>
> Using ContinuousIndex worked. Nevertheless, I still think that any itk
> Filter should always work in physical coordinates... Is there any
> reason why this filter does not, or is just for implementation
> easiness or efficiency?
>
> Markus Neuner wrote:
>> Hi Alberto Gomez,
>>
>> When you take a look at the source code of
>> "Insight/Code/BasicFilters/itkHoughTransform2DLinesImageFilter.txx"
>> starting at line 298 you can see how the points are calculated:
>>
>> ...
>>         LineType::PointListType list; // insert two points per line
>> ...
>>         double radius = it_input.GetIndex()[0];
>>         double teta   =
>> ((it_input.GetIndex()[1])*2*nPI/this->GetAngleResolution())-nPI;
>>         double Vx = radius*vcl_cos(teta );
>>         double Vy = radius*vcl_sin(teta );
>>         double norm = vcl_sqrt(Vx*Vx+Vy*Vy);
>>         double VxNorm = Vx/norm;
>>         double VyNorm = Vy/norm;
>> ...
>>           LinePointType p;
>>           p.SetPosition(Vx,Vy);
>>           list.push_back(p);
>>           p.SetPosition(Vx-VyNorm*5,Vy+VxNorm*5);
>>           list.push_back(p);
>> ...
>>
>> Furthermore the points are of type "itk::LineSpatialObjectPoint<
>> TPointDimension >" whith "typedef Point
>> <http://www.itk.org/Doxygen/html/classitk_1_1Point.html>< double,
>> TPointDimension > PointType".
>> <http://www.itk.org/Doxygen/html/classitk_1_1LineSpatialObjectPoint.html#993f2f73923d9e4fc69f18ca6ac95ea4>
>>
>>
>> As you can see the coordinates are of type double.
>>
>> The easiest solution is to construct a
>>  itk::ContinuousIndex
>> <http://www.itk.org/Doxygen/html/classitk_1_1ContinuousIndex.html>
>> (should be defined in the ImageType::ContiunuousIndexType)
>> and pass it to
>> TransformContinuousIndexToPhysicalPoint (const ContinuousIndex<
>> TCoordRep, VImageDimension > &        index, Point< TCoordRep,
>> VImageDimension > &  point ).
>>
>> I hope this helps.
>>
>> Best regards,
>> Markus
>>
>>
>>
>> Gomez, Alberto wrote:
>>  
>>> Hi all,
>>>
>>> I am using the itk::HoughTransform2DLinesImageFilter  to extract the
>>> lines from an image for a certain application. The point is that I
>>> don't want to draw the lines but to get the lines parameters
>>> (direction and a point ) for further computations. I realised that,
>>> following the example from the software guide, the vector and the line
>>> are in pixel coordinates and not in real world coordinates. Can anyone
>>> confirm this?
>>>
>>> So, what I did is to use the TransformIndexToPhysicalPoint() method,
>>> but it does not work properly (mi lines are slightly deviated). My
>>> assumption is that the extracted points are in pixel coordinates but
>>> aren't integers. Because of that, the TransformIndexToPhysicalPoint()
>>> method rounds them and then computes the physical coordinates, which
>>> yields in the inaccuracy I realized.
>>>
>>> The solution I found was to get two points, in pixel coordinates, very
>>> far from each other, so that the inaccuracy of the conversion is
>>> smaller. Nevertheless, this is not clean. Does anyone know a way to
>>> get the line parameters accurately in physical coordinates? I guess
>>> that the Hough map (output of the filter) is (r, theta) in real
>>> physical coordinates, but I don't know how to extract the maximums in
>>> the same way that getLines() does (using the disc, etc).
>>>
>>> Thank you,
>>> Alberto
>>> ||
>>>
>>> -- 
>>>
>>> Alberto Gómez
>>>
>>> /Division of Imaging Sciences
>>> The Rayne Institute
>>> 4th Floor, Lambeth Wing
>>> St Thomas' Hospital
>>> London SE1 7EH /
>>>
>>> phone: +44 (0) 20 718 88364
>>> email: alberto.gomez at kcl.ac.uk <mailto:alberto.gomez at kcl.ac.uk>
>>>
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>>
>>   
>
>