Point's coordinates between two registered images

Development
1

Hi!

I have two images registered together and I would like to, given the IJK coordinates of a point in the first image, get its IJK coordinates in the second one.
To do so, I did this:

 firstVolume->GetIJKToRASMatrix(ijkToRasMatrix);
 secondVolume->GetRASToIJKMatrix(rasToIjkMatrix);

 double ijkCoordinates1[3] = {x,y,z};
 double rasCoordinates[3];
 ijkToRasMatrix->MultiplyPoint(ijkCorrdinates1,rasCoordinates);

 double ijkCoordinates2[3];
 rasToIjkMatrix->MultiplyPoint(rasCoordinates,ijkCoordinates2);

But the point I obtain in the second image doesn’t match the point in the first one.
Is this the correct way to do want I want to do ?

Thanks for your help,

Sandra.

2

If the two volume nodes have different parent transforms, then you need to take into account the transform between them:

firstVolumeToSecondVolumeTransformMatrix = vtk.vtkMatrix4x4()
slicer.vtkMRMLTransformNode.GetMatrixTransformBetweenNodes(firstVolume.GetParentTransformNode(), secondVolume.GetParentTransformNode(), firstVolumeToSecondVolumeTransformMatrix)
3

Ok,
but when I do firstVolume.GetParentTransformNode() I get a null node (same thing for the second volume) so the transform matrix between these nodes is identity

4

That is good then, they are in the same coordinate system.

However, there is a problem with your coordinates. Homogeneous point coordinates have 4 components, not just 3. The value of the 4th component must be 1.

For example:

double ijkCoordinates1[4] = {x,y,z,1};
double rasCoordinates[4] = {0,0,0,1};
5

That works perfectly, thank you very much !
I thought I was missing some kind of transformation between the two images and I wouldn’t have thought about this.