[ITK Community] Bug in DefaultVectorPixelAccessor

[Jan Ehrhardt]

Hi Brad,

yes this is an alternative way, but the code inside the loop has approx. 
200 lines (is quite complicated stuff) and I'm still testing and in your 
solution, I have to maintain the code four times...

Best regards,
Jan

On 03/11/2014 01:55 PM, Bradley Lowekamp wrote:
> Hello,
>
> You are right that the GetPixel methods are slow, but that's why 
> iterators are recommended.
>
> Currently you are trying to have you "if( featureImage2 )" in side the 
> per-pixel loop. Not only does this cause you iterator initialization 
> issue it also is slower of have this if statement inside the loop.
>
> Here is an alternative way to construct your logic:
>
> https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Filtering/ImageFilterBase/include/itkBinaryFunctorImageFilter.hxx#L224-L297
>
> Hope this help,
> Brad
>
> On Mar 11, 2014, at 8:34 AM, Jan Ehrhardt <ehrhardt at imi.uni-luebeck.de 
> <mailto:ehrhardt at imi.uni-luebeck.de>> wrote:
>
>> Hi Matt,
>>
>> yes, I currently use the ITK iterators, but I have the following 
>> problem.
>>
>> Standard code looks like:
>>
>> itk::ImageRegionIterator<ImageType1> it( radius, myImage, region );
>> itk::ImageRegionIterator<ImageType2> featureIt1( radius, 
>> featureImage1, region );
>> itk::ImageRegionIterator<ImageType3> featureIt2( radius, 
>> featureImage2, region );
>>
>> it.GotoBegin();
>> featureIt1.GotoBegin();
>> featureIt2.GotoBegin();
>>
>> while(!it.IsAtEnd())
>> {
>>    // do some complicated processing stuff with or without feature1 
>> and/or feature2
>>
>>   ++it;
>>   ++featureIt1;
>>   ++featureIt2;
>> }
>>
>> I only know at runtime, whether featureImage1 and/or featureImage2 
>> are available. If
>> featureImage1 or featureImage2 is NULL, a segmentation fault occurs.
>>
>> Therefore, I have currently implemented:
>>
>> itk::ConstNeighborhoodIterator<ImageType1> it( radius, myImage, region );
>>
>> it.GotoBegin();
>>
>> while(!it.IsAtEnd())
>> {
>>    currentIndex=it.GetIndex();
>>    if( featureImage1.IsNotNull())
>>    {
>>         // use featureImage1->GetPixel(currentIndex) to process feature 1
>>     }
>>     if( featureImage2.IsNotNull())
>>    {
>>         // use featureImage2->GetPixel(currentIndex) to process feature 2
>>    }
>>
>>    // do some other stuff
>>
>>   ++it;
>> }
>>
>> The Image::GetPixel() method is quite time-consuming and therefore I 
>> look for a more efficient method
>> to implement this code (without writing four different versions).
>>
>> Any suggestions?
>>
>> Best regards,
>> Jan
>>
>> On 03/11/2014 06:12 AM, Matt McCormick wrote:
>>> Hi Jan,
>>>
>>> To iterator through an Image or VectorImage, use the Iterators as 
>>> described in the Software Guide [1]
>>>
>>> HTH,
>>> Matt
>>>
>>> [1] http://itk.org/ItkSoftwareGuide.pdf
>>>
>>>
>>> On Thu, Mar 6, 2014 at 8:23 AM, Jan Ehrhardt 
>>> <ehrhardt at imi.uni-luebeck.de <mailto:ehrhardt at imi.uni-luebeck.de>> 
>>> wrote:
>>>
>>>     Hi folks,
>>>
>>>     I want to iterate through a vector image without an iterator. My
>>>     approach was:
>>>
>>>         MultiChannelImageType::AccessorType accessor =
>>>     pImage->GetPixelAccessor();
>>>         MultiChannelImageType::InternalPixelType* pBuffer =
>>>     pImage->GetBufferPointer();
>>>
>>>         for( OffsetValueType i = from; i < to; ++i )
>>>         {
>>>             vecPixel = accessor.Get(*pBuffer, i);
>>>         }
>>>
>>>     but the results are not correct (see bug(?) below). If the
>>>     bug(?) in DefaultVectorPixelAccessor is corrected (
>>>     m_OffsetMultiplier = l ) the behaviour of standard itk iterators
>>>     change. I assume, I don't understand the parameters of
>>>     DefaultVectorPixelAccessor::Get() correctly.
>>>     How can I use the DefaultVectorPixelAccessor correctly?
>>>
>>>     best,
>>>     jan
>>>
>>>
>>>     On 03/06/2014 12:25 PM, Jan Ehrhardt wrote:
>>>
>>>         Hi folks,
>>>
>>>         there is a bug in DefaultVectorPixelAccessor (ITK 4.4.0):
>>>
>>>           void SetVectorLength(VectorLengthType l)
>>>           {
>>>             m_VectorLength = l;
>>>             m_OffsetMultiplier = ( l - 1 );
>>>           }
>>>
>>>         DefaultVectorPixelAccessor(VectorLengthType l)
>>>           {
>>>             m_VectorLength = l;
>>>             m_OffsetMultiplier = l - 1;
>>>           }
>>>
>>>         OffsetMultiplier should be m_OffsetMultiplier = l;
>>>
>>>         Best regards,
>>>         Jan
>>>
>>>
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>>>
>>>
>>
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