[Insight-users] HoughTransform2DCirclesImageFilter (itk)

[Luis Ibanez]

Hi Ab,

You got it right,

If you take a group of (let's say) 4x4 pixels that
include the pixels with a  local maximum in the
cumulator image, then you will be able to compute
a center with subpixel precision.

Note that there are already classes for BSpline
interpolation in the toolkit. You may want to
take advante of them.


     Regards,


        Luis


-------------------
Ab Sayeed wrote:

> Many Thanks 
> 
> Method 2 sound a bit more useful.
> 
> I'm going through the code and it seems very difficult
> to follow. Are there any papers describing the Hough
> Transform implantation other than the Manual (pdf on
> the website)?
> 
> 
> My understanding so far. 
> 
> itkHoughTransform2DCirclesImageFilter.txx
> Line 242: center[0] = it_input.GetIndex()[0];
> Line 243: center[1] = it_input.GetIndex()[1];
> 
> Retrieves the index  of the highest pixel count in the
> Hough Transformed image. The Hough transform image is
> located in a the image variable 
> 
> postProcessImage
> 
> 
> In theory if I get the pixel counts from the maximum
> and it's neighbors, using 
> 
> postProcessImage->GetPixel(index);
> 
> i can interpolate with a BSpline to find where the
> true max would be. 
> 
> 
> Am I talking gibberish. 
> 
> 
> Kind Regards
> 
> Ab
> 
> 
> --- Luis Ibanez <luis.ibanez at kitware.com> wrote:
> 
>>
>>Hi Ab,
>>
>>
>>There are two things you could do in this filter
>>for making it compute the center of the circle
>>with sub-pixel precision.
>>
>>Unfortunately both of them involve modifying the
>>code of the filter.
>>
>>
>>They are:
>>
>>
>>1) Modify the size and spacing of the output
>>    image in this filter.  The output image
>>    is used as the accumulator for the exploration
>>    of the circle parametric space.  Using
>>    an image with higher resolution than your
>>    input image will in principle give you
>>    access to sub-pixel resolution. Note however
>>    that the centers will still be limited by
>>    the new resolution of the output.
>>
>>
>>OR
>>
>>
>>2) Introduce a BSpline interpolator in the
>>    GenerateData() method of this filter.
>>    With the BSpline you could compute a
>>    maximum with subpixel precision in the
>>    accumulator image, instead of just
>>    selecting the index of the highest
>>    pixel as it is done right now.
>>    (see lines 242 in Insight/Code/BasicFilters/
>>     itkHoughTransform2DCirclesImageFilter.txx)
>>
>>
>>
>>Option (2) is probably the best approach.
>>
>>If you feel motivated to introduce this
>>modifications, we will be happy to assist
>>you in the process, and of course will
>>welcome the modified class back into the
>>toolkit.
>>
>>
>>
>>   Regards,
>>
>>
>>
>>      Luis
>>
>>
>>-----------------------
>>Ab Sayeed wrote:
>>
>>
>>>I want to get the
>>
>>HoughTransform2DCirclesImageFilter 
>>
>>>to give me the centre of a circle in sub pixel 
>>>precision. Any idea's on how I could do this or 
>>>is there another function I can use to do this. 
>>>
>>>Thanks
>>>
>>>Ab 
>>>
>>>
>>
>>
> 
> 
> 
> 	
> 		
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