[Insight-users] Problems with image corruption : Reusing filters

[Luis Ibanez]

Hi Radhika,

  - Code reuse is a good thing
  - Variable names reuse is a bad, very bad thing

  - Filter reconection is a dangerous practice

Nothing good can result from saving a couple of bytes
in one extra pointer in your program.

When you reused "extracter2D" you were probably
thinking in terms of procedural programming.  e.g.
invoking functions for performing processing on data.

The data flow pipeline is a different paradigm.
It is like connecting circuits: When you need a
second switch, you don't take it out from somewhere
else in the circuit... you just add a new one  :-)

When you connect the filter with

         a->SetInput( b->GetOutput() )

Those connections are intended to be used in a
permanent way by running data through the filters
multiple times.

If you need a second extract filter, you should
(as you discovered by yourself) create a new extract
filter object and connect it in the pipeline in the
new appropriate place.

It is a "pipeline": you put the tubes, the elbows,
the tees, the valves,... as many as you need, and
once you install them, you don't take them out.
Especially not just before letting water run through
the pipe.


Think "plumber" and you will be ITK-enlightened.



Regards,


    Luis


--------------------------------
Radhika Sivaramakrishna wrote:

> Hi Luis,
> Thanks for your suggestion. I will try this also, but I am not sure if this
> is the problem though it is a good suggestion. After I sent off that piece
> of code to you, I decided to do some more experimenting. In the code I sent
> you I reused extracter2D (ExtractImageFilter Pointer) twice once to extract
> a slice from the greyscale image, and then a second time to extract a slice
> off the mask image. I decided to use two different pointers (one
> extracter2Dimage and another extracter2Dmask) and lo and behold, things
> started working and things were written out correctly. I dont understand why
> this should happen? Could you explain?
> 
> Radhika
> 
> 
> ----- Original Message ----- 
> From: "Luis Ibanez" <luis . ibanez at kitware . com>
> To: "Radhika Sivaramakrishna" <radshashi at earthlink . net>
> Cc: <insight-users at itk . org>
> Sent: Wednesday, December 17, 2003 11:19 AM
> Subject: Re: [Insight-users] Problems with image corruption : Shouldn't use
> LargestPossibleRegion for Iterators.
> 
> 
> 
>>Hi Radhika,
>>
>>It seems that the error is related to the use of
>>"GetLargestPossibleRegion()" in order to define
>>the iterators in line 230 of your code:
>>
>>
>>
>>>    ImageType2D::RegionType region = binimg->GetLargestPossibleRegion();
>>>    ConstIteratorType maskit (mask, region);
>>>    ConstIteratorType binimgit( binimg, region );
>>>    IteratorType newmaskit(newmask,region);
>>>
>>>   maskit.GoToBegin();
>>>   binimgit.GoToBegin();
>>>   newmaskit.GoToBegin();
>>>   while( ! maskit.IsAtEnd() && ! binimgit.IsAtEnd() && !
> 
> newmaskit.IsAtEnd())
> 
>>>     {
>>>newmaskit.Set(binimgit.Get());
>>>       if( maskit.Get() == 0 )   // if mask pixel is off
>>>         {
>>>newmaskit.Set(0);
>>>         }
>>>       ++maskit;
>>>++binimgit;
>>>++newmaskit;
>>>     }
>>
>>In general you shouldn't use "LargestPossibleRegion"
>>since there is no guarranty that all this data is
>>going to be in memory.  The safe region to use is the
>>"Buffered" region.
>>
>>Just to make sure that this is the problem, please
>>insert the line:
>>
>>            binimg->Print( std::cout )
>>
>>in line 229, run the program and look at the printed
>>values of the image regions. There will be: Requested,
>>Buffered and LargestPossible region.
>>
>>My guess is that the largest possible region is
>>larger than the buffered region and your iterators
>>are stepping out of the image buffers, therefore
>>corrupting the memory.
>>
>>BTW the process that you are attempting with the
>>iterators is done by the MaskImageFilter
>>http://www . itk . org/Insight/Doxygen/html/classitk_1_1MaskImageFilter . html
>>
>>You may simply replace this while loop with a
>>MaskImageFilter.
>>
>>
>>
>>Please let us know what you find.
>>
>>
>>Thanks
>>
>>
>>   Luis
>>
>>
>>
>>------------------------------------
>>Radhika Sivaramakrishna wrote:
>>
>>>Hi Luis,
>>>I am having some problems with my program. I have enclosed a portion of
>>>my code here where the problem is. I am not able to figure it out.
>>>Basically, "binimg" in my code is the output of the relabel filter. At
>>>that point when I write it out, it is correct. However, further down the
>>>line, when I try to write out the output of the relabel filter, it is
>>>corrupted. In fact, all are corrupted and they all have the same values
>>>which is that of the result of the mask filter. Even "gs" the output of
>>>the extractimagefilter is wrong at the point where I am writing it out.
>>>Could you help me figure out the problem? Maybe there is something wrong
>>>in the way I am setting the region of iteration for each or is there a
>>>problem in reusing the extractimagefilter for first extracting a
>>>greyscale slice and then using the same thing to extract a mask?
>>>
>>>Thanks
>>>Radhika
>>>
>>
>>
>>
> 
> 
>