[Insight-developers] VectorImage: and contiguous memory assumption

Tue Apr 12 12:43:36 EDT 2011

Thanks Karthik. I'm making a test that will test this behavior, so if it changes unexpectedly in the future we'll know for our use cases.

Cheers,
Michael 

  _____  

From: Karthik Krishnan [mailto:karthik.krishnan at kitware.com] 
Sent: Monday, April 11, 2011 11:31 PM
To: M Stauffer (V)
Cc: Luis Ibanez; brian avants; Insight Developers
Subject: Re: [Insight-developers] VectorImage: and contiguous memory assumption

Yes. You can (currently) access the block like that and you will have the following memory organization, for the following images 

��x1 y1 x2 y2 x3 y3 x4 y4.... xN yN

�� Image< Vector< T, 2 >, 3 >

and

�� VectorImage< T, 3 >
�� vimage->SetVectorLength(2);

where N is the number of pixels.

Yes, itk::Vector does not have any mmeber vars other than data�

thx
--
karthik

On Tue, Apr 12, 2011 at 2:59 AM, M Stauffer (V) <mstauff at verizon.net> wrote:

Hi Luis,

How do we access the memory of an Image< Vector, Dimension > object as a
contiguous block?

When I use
Image<Vector,Dimension>::GetPixelContainer::GetBufferPointer(), I get an
array of Vector pointers, not a pointer to a block of pixel elements.

In a simple test I can get a contiguous block of properly-ordered pixel
elements like this:

Image<Vector<ValueType, VecDim>, Dim>::PixelContainer container =
image->GetPixelContainer();
ValueType* buffer =
reinterpret_cast<ValueType*>(container->GetBufferPointer() );

But is this safe to rely on? That the memory will always be contiguous
and ordered? I take it that itkVector doesn't have any member variables
other than the data, otherwise this wouldn't work.

Thanks.

Cheers,
Michael

>Hi Brian,
>
>
>The memory layout of the image type
>
> � � � � itk::Image< Vector , Dimension >
>
>for
>
> � � � � �Vector = itk::Vector< float, 2 >
>
>is:
>
> � � �x1 y1 x2 y2 x3 y3 x4 y4.... xN yN
>
>
>Where N is the number of pixels.
>
>
>So,
>the components of a pixel are grouped
>contiguously in memory.
>
>
> � � �Luis
>
>
>--------------------------
>On Tue, Mar 29, 2011 at 4:09 PM, brian avants <stnava at gmail.com> wrote:
>> luis
>>
>> do you by chance know the memory layout for the
>>
>> itk::Image< Vector , type > �?
>>
>> is it
>>
>> x1 x2 x3 y1 y2 y3
>>
>> or
>>
>> x1 y1 x2 y2 x3 y3
>>
>> ?
>>
>> brian
>>
>>
>>
>>
>> On Wed, Mar 23, 2011 at 8:49 PM, Luis Ibanez
><luis.ibanez at kitware.com> wrote:
>>> Brad, Nick,
>>>
>>> It turned out that I was wrong about the internal
>>> memory organization of the VectorImage class.
>>>
>>> Looking closely at the code, particularly the
>>> GetPixel, SetPixel methods of this class,
>>> and after checking with Karthik (who actually
>>> implemented this class), it turns out that the
>>> memory organization is actually
>>>
>>>
>>> 1) All the components of pixel 1, followed by
>>> 2) All the components of pixel 2, followed by
>>> 3) All the components of pixel 3, followed by
>>> 4) All the components of pixel 4, followed by
>>>
>>> and so on...
>>>
>>>
>>> Instead of what I wrongly thought:
>>>
>>>
>>> 1) All the pixels of component 1, followed by
>>> 2) All the pixels of component 2, followed by
>>> 3) All the pixels of component 3, followed by
>>>
>>> and so on...
>>>
>>>
>>>
>>> So, it makes me wonder if there is a case
>>> to be made for introducing an image class
>>> with the second type of memory organization...
>>>
>>>
>>> � � Luis
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>>
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