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

[Luis Ibanez]

Markus, Alberto,

Would you like to log a feature request in the bug Tracker,

http://public.kitware.com/Bug/my_view_page.php

for adding to this filter the functionality of computing the
line parameters in physical coordinates ?


     Thanks


           Luis


-------------------------------
On Wed, Jan 27, 2010 at 12:10 PM, Markus Neuner <neuner.markus at gmx.net> wrote:
> 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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>>>
>>>
>>
>>
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