[Insight-users] Definition of Image Origin

[Didier Rajon]

Thanks Luis for your answer.

I do have a little concern about one filter.  That's the ImageFileWriter.
Since the index is not saved with the image in most cases, should the 
filter shift the origin to the first pixel of the dataset before writing 
the image so that the ImageFileReader can assume an image index of (0,0) 
when it reads the image.

If I do the following:
  1) read a vtk file using itk::ImageFileReader
  2) crop the image by one pixel using itk::ExtractImageFilter
  3) write the result into a vtk file using itk::ImageFileWriter
  4) read the file just writen in step 3) using itk::ImageFileReader
then, my image is shifted by one pixel.

I also use VTK and they do shift the image origin before they save the 
image into a file.

Thanks,

Didier.


Luis Ibanez wrote:

>
> Hi Didier,
>
> The origin is associated with the pixel of Index (0,0).
>
> However, as you pointed out, an image may actually start
> in an index (15,25), and therefore the origin may be
> "outside" of the sampling grid of the image.
>
> That's fine because at the end, the purpose of the origin
> is to be able to compute the coordinates of every pixel
> in the sampling grid of the image.
>
> Note that not all filters may be doing the right thing
> when managing images that have a Start index different
> from (0,0).  If you find any suspicious behavior please
> let us know, so we can check if this is the result of a
> bug in a particular filter.
>
>
>   Thanks
>
>
>      Luis
>
>
>
> ------------------
> Didier Rajon wrote:
>
>> Hello,
>> I'm a little confused about the definition of the Image Origin.
>> According to the definition that I found in the ITK Software Guide - 
>> Second Edition (August 30, 2005); p 40: "the image origin is 
>> associated with the coordinates of the first pixel of the image".
>> On page 224 of the same documentation I found Eq. (6.20) that 
>> suggests that the origin is associated with the coordinates of the 
>> pixel indexed by (0, 0).
>> On page 234 of the same documentation I found an expression to 
>> calculate the image center and that suggests that the origin is 
>> associated with the coordinates of the first pixel of the image, 
>> whatever the index of the image.
>> So, if my image has a LargestPossibleRegion().GetIndex() equal to 
>> (10,10), is my image origin the coordinates of the pixel indexed by 
>> (10,10) (my first pixel in this case) or the coordinates of the pixel 
>> indexed by (0,0) (which is out of my dataset in this case)?
>> Thanks,
>>
>> Didier.
>>
>>
>>
>
>
>

-- 
Didier Rajon
Department of Neurological Surgery
University of Florida
MBI, L2100 PO Box 100265
Gainesville, FL, 32610-0265
Tel: (352) 294 0144
Fax: (352) 392 8413