[vtkusers] using both SetResliceTransform and SetResliceAxes

[Clément D.]

Great David, I understand and it works now ! Thanks

2017-06-28 19:44 GMT+02:00 David Gobbi <david.gobbi at gmail.com>:

> Hi Clément,
>
> The basic formula for the transformation in vtkImageReslice is as follows:
>
>     p_in = T A p_out
>
> where T is ResliceTransform, A is ResliceAxes, p_out is "output data
> coords", and p_in is "input data coords".  In most cases, p_out is also
> "world coords".
>
> The documentation says "T is applied to the resampling grid that has been
> defined by A", which is just another way of saying that "T is applied after
> A".  As shown in the above equation, T is associated with p_in, not with
> p_out.
>
> In other words, you cannot use T to perform a transformation in world
> coords.
>
> Instead, the code would be something like this:
>
> # define the desired translation in output (i.e. world) coords
> t_world = (1.0, 0.0, 0.0, 0.0)
> # convert to data coords
> A = slice.GetResliceAxes()
> t_data = A.MultiplyPoint(t_world)
> # create a transformation for data coords
> T.Translate(t_data[0:3])
> slice.SetResliceTransform(T)
>
>  - David
>
>
> On Wed, Jun 28, 2017 at 10:12 AM, Clément D. <clement.douarre at gmail.com>
> wrote:
>
>> Hello everyone, new to VTK, etc etc.
>>
>> I wish to extract a  2D slice from a 3D volume, and then translate this
>> slicing in the volume, following the axis of the slice
>>
>> So say I declare my slice this way (code is in Python) :
>>
>> -------------------------------------------
>> oblique = vtk.vtkMatrix4x4()
>> #Some oblique matrix, vectors are in fact normalised but here I just put
>> random numbers for this example
>> oblique.DeepCopy((0.3, 0   , 0   , 10,
>>                   0  , 0.28, 0   , 10,
>>                   0.8, 0   , 0.97, 10,
>>                   0  , 0   , 0   , 1))
>>
>> slice=vtk.vtkImageReslice() #create the slice object
>> slice.SetOutputExtent(0, 5, 0, 5, 0, 0)
>> slice.SetOutputOrigin(-0.5 * 5, -0.5 * 5, 0)
>>
>> slice.SetResliceAxes(oblique) #set the resliceAxes matrix to it
>> -----------------------------------------------
>>
>> So far so good, I get a slice oriented along the oblique matrix. It gets
>> trickier when I want to translate it.
>> Bear in mind that I would like a translation *along the axis of the
>> oblique matrix* and not in world coordinates.
>> I.E. i would like to translate along vectors (0.3,0,0.8), (0,0.28,0) and
>> (0,0,0.97) (vectors of the oblique matrix)
>>
>> So if I do this :
>>
>> -------------------------------------------
>> transformation = vtk.vtkTransform();
>> slice.SetResliceTransform(transformation)
>> transformation.Translate(1,0,0)
>> slice.Update()
>> -------------------------------------------
>>
>> I would expect reading documentation on setResliceTransform that my slice
>> would move along one of these axis (the Z axis here).
>> Because my understanding of setResliceTransform and setResliceAxes being
>> used together is that the latter is used to set up a grid upon which the
>> former will then act (so translating will "slide" along the axes of
>> setResliceAxes)
>> However, I do not observe this behaviour (I don't really how the slice is
>> moving, but certainly not in the axes of oblique), did I get something
>> wrong ?
>>
>> Many thanks,
>> Clément
>>
>
>
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