How to pull Position and Focal Point data from default scene camera for camera angle extension

Development
1

Hi folks,

I’m trying to create an extension which will calculate the angle of the default scene camera w.r.t. the subject in the slicer 3D scene. This would be analogous determining the C-Arm LAO/RAO and CRA/CAU angles for proper image orientation.

The calculation is pretty straightforward and can be done with nothing but the camera position and focal point, but I can’t seem to obtain these values using GetPosition and GetFocalPoint. Here are some code snips:

>>> defaultCam = slicer.mrmlScene.GetNodeByID('vtkMRMLCameraNode1')
>>> defaultCamPosition = defaultCam.GetPosition()
>>> print(defaultCamPosition)
_000001eb844fabd8_p_void

On the other hand, GetCameraAngle does function as expected:

>>> viewAngle1 = defaultCam.GetViewAngle()
>>> print(viewAngle1)
30.0

Additionally, both SetPosition and SetFocalPoint function as expected. In fact, all of these are listed as valid member functions for the vtkMRMLCameraNode Class:

Finally, if I browse to this node in the slicer data module, all of these values are listed in the node information.

image
image560×778 28 KB

Any ideas on how to programmatically obtain the default scene camera position and focal point values?

Thanks for the help.

2

You can use GetPosition(double*) variant:

>>> defaultCam = slicer.mrmlScene.GetNodeByID('vtkMRMLCameraNode1')
>>> pos=[0,0,0]
>>> defaultCam.GetPosition(pos)
>>> pos
[54.62253179056516, 118.36363864842811, 61.73342989137638]
1 Like
3

Thanks Andras, I suppose I misunderstood the *double variant of this function. It’s a void function which assigns ‘None’ to the output, but changes the input argument to the position. For example:

>>> pos = [0,0,0]
>>> cameraPosition = defaultCam.GetPosition(pos)
>>> print(cameraPosition)
None
>>> pos
[3.0, 3.0, 3.0]