[vtkusers] Conversion from ITK to VTK Coordinate spaces
david.gobbi at gmail.com
Tue May 9 13:06:40 EDT 2017
The question that you are asking does not have a straightforward answer,
because VTK doesn't actually define its coordinate space... in VTK you can
use whatever coordinate you want.
Since I work with DICOM images, I define my VTK world coordinate system to
be the DICOM patient coordinate system (that is, I follow the convention
that the +x direction is towards the left hand, the +z direction is towards
the head, and +y is towards the back). This simplifies moving data between
ITK and VTK, because ITK uses the same coordinate system.
However, this means that whenever I read images into VTK, I must put the
images into this coordinate system. So, for example, I wrote a class
called vtkDICOMToRAS (
convert images that are read by the vtkNIFTIImageReader from their native
coordinate system to the DICOM coordinate system.
Most of the VTK image readers assume that the user wants a RAS-like
coordinate system, but as I said, you are free to use whatever coordinate
system you desire inside VTK. A lot of people prefer RAS because it means
they don't have to convert coordinate systems whenever they read an image.
I prefer using a DICOM coordinate system because I work a lot with DICOM
images, and it improves interoperability with ITK.
To kind-of sort-of answer your question, the general transformation between
an ITK (DICOM-like) coord system and a RAS coordinate system is as follows:
-1 0 0 0
0 -1 0 0
0 0 1 0
0 0 0 1
However, it's rarely that straightforward, because it really depends on how
the data was read into VTK.
On Tue, May 9, 2017 at 9:46 AM, <sganesan at cusa.canon.com> wrote:
> I'm trying to write a function that can reasonably convert between ITK and
> VTK coordinate spaces. I have a given ITK transform with my 12 parameters,
> (3 fixed).
> a b c x
> d e f y
> g h i z
> l m n 1
> where (a,b,c,d,e,f,g,h,i) represent my ITK matrix, (x,y,z) is my center,
> and (l,m,n) is my translation.
> How do I convert that to VTK format
> a b c o
> d e f p
> g h i q
> 0 0 0 1
> where (a,b,c,d,e,f,g,h,i) represent my VTK matrix,, and (o,p,q) is my
> Is there a relatively straightforward way of doing this?
> Kind Regards,
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