[vtkusers] vtkDataSet::FindPoint, vtkExtractVOI and world coordinates?

Wed Sep 7 15:06:37 EDT 2016

On Wed, Sep 7, 2016 at 12:58 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> Hi Elvis,
>
> Let's break this down into two parts:
>
> 1) Going from VTK world coords to VTK data coords is achieved via the prop
> matrix (where a "prop" is a vtkActor, a vtkVolume, a vtkImageSlice, etc):
>
> For data-to-world, apply the matrix to the point.
> For world-to-data, apply the inverse of the matrix.
>
> 2) Going from VTK data coords (x,y,z) to point indices (i,j,k) is done
> with these equations:
>
> x = origin[0] + i * spacing[0]
> y = origin[1] + j * spacing[1]
> z = origin[2] + k * spacing[2]
>
> i = round((x - origin[0])/spacing[0])
> j = round((y - origin[1])/spacing[1])
> k = round((z - origin[2])/spacing[2])
>
> Personally, I avoid using any VTK APIs to do these computations, for two
> reasons:
>
> a) I prefer mathematical abstractions over programatic abstractions.  If I
> have the equations, then I can combine them and simplify them and often
> achieve nice performance gains as a result.
>

I totally agree David, I would even arrange transformation (2) into a
matrix with a scaling diagonal (spacing) and a translation vector (origin)
and  apply (2) similarly to how (1) is applied.

>
> On Wed, Sep 7, 2016 at 9:55 AM, Elvis Stansvik <
> elvis.stansvik at orexplore.com> wrote:
>
>> Den 7 sep. 2016 3:32 em skrev "Cory Quammen" <cory.quammen at kitware.com>:
>> >
>> > Elvis,
>> >
>> > Have a look at vtkImageData::FindPoint(double x, double y, double z).
>> >
>> > It takes into account the image spacing and origin, but will not
>> > correct for any modifications to the position of the vtkVolume you
>> > seem to be using to represent it in the scene.
>>
>> Thanks Cory,
>>
>> I did initially find FindPoint, but I was unsure how I can translate the
>> vtkIdType it returns (some sort of index?) into i,j,k voxel coordinate. Do
>> you know?
>>
>> Since then I've also found  vtkImageData::ComputeStructuredCoordinates,
>> which looks similar, but does seem to produce a ijk voxel coordinate.
>> Perhaps that is the function I'm looking for? It's doubly interesting since
>> it has a static variant where I can specify the origin, spacing etc
>> manually, which I might have a need for actually, if I want to do the same
>> computation but don't have access to an actual vtkImageData instance, only
>> its parameters so to speak.
>>
>> Elvis
>>
>> >
>> > HTH,
>> > Cory
>> >
>> >
>> >
>> > On Wed, Sep 7, 2016 at 3:10 AM, Elvis Stansvik
>> > <elvis.stansvik at orexplore.com> wrote:
>> > > Den 4 sep. 2016 2:25 em skrev "Elvis Stansvik"
>> > > <elvis.stansvik at orexplore.com>:
>> > >>
>> > >> Hi all,
>> > >>
>> > >> I have multiple volumes in a renderer, positioned above/below each
>> other
>> > >> (like a "string" of volumes) along the Z axis, none of them are
>> > >> intersecting. As the user clicks+drags the mouse in the viewport, I
>> need to
>> > >> capture the mouse position at the start/end positions of the drag
>> operation
>> > >> and convert them to world coordinates. I think I have these parts
>> figured
>> > >> out.
>> > >>
>> > >> Then, I need to search through the list of my volumes (sorted by
>> their end
>> > >> positions in world coordinates), and construct a series of
>> vtkExtractVOI
>> > >> that will extract the selected segments. For volumes that are
>> completely
>> > >> inside the selected area, the vtkExtractVOI will extract the full
>> volumes,
>> > >> but for the start/end volume (which may be the same volume), it will
>> only
>> > >> extract a portion of it.
>> > >>
>> > >> I looked at the vtkDataSet::FindPoint, which I think will give me a
>> linear
>> > >> point ID (not familiar with that from before) for a point given in
>> world
>> > >> coordinates. But how can I convert that to a structured point
>> position that
>> > >> I can use when setting up the extent to use for my vtkExtractVOI?
>> > >>
>> > >> Is the static vtkStructuredData::ComputePointStructuredCoords
>> helper the
>> > >> function I'm looking for? Or is there some other easier way to
>> extract an
>> > >> area of a volume when the area is expressed in world coordinates?
>> > >> (specifically, as a start/end Z position in world coordinates).
>> > >>
>> > >> Here's an ASCII drawing that perhaps explains better what I want to
>> do,
>> > >> the rectangular areas are my volumes:
>> > >>
>> > >> ^  +-----+
>> > >> |  |     |
>> > >> |  | P1  |  <--- User clicks here (world coordinate X, Y, Z)..
>> > >> |  |     |
>> > >> Z  +-----+
>> > >>    +-----+
>> > >>    |     |
>> > >>    |     |
>> > >>    +-----+
>> > >>    +-----+
>> > >>    |     |
>> > >>    |     |
>> > >>    |  P2 |  <--- ..and drags to here (world coordinate A, B, C).
>> > >>    |     |
>> > >>    |     |
>> > >>    |     |
>> > >>    +-----+
>> > >>
>> > >> From this I want to create three vtkExtractVOI, that when combined
>> gives a
>> > >> list of vtkImageData that represents the selected area along the Z
>> axis (in
>> > >> the X and Y direction, everything from the volumes should be
>> included).
>> > >>
>> > >> Thanks in advance,
>> > >> Elvis
>> > >
>> > > I realized my mail above is a little long-winded and unclear. Sorry
>> about
>> > > that. In essence, what I'm asking is how to map an x,y,x (double)
>> point in
>> > > world coordinates to a i,j,k (int) grid position (voxel coordinate)
>> of a
>> > > given volume. I don't want the nearest point, but to return "null" or
>> > > equivalent if the world point does not fall inside any voxel.
>> > >
>> > > I'd also be interested how to do the same but against a plain
>> vtkImageData
>> > > instead of a vtkVolume. For this I assume I'd have to provide some
>> info
>> > > about where in the world the vtkImageData belongs (where/how I intend
>> to map
>> > > it into the scene).
>> > >
>> > > Anyone know what the VTK API for this is?
>> > >
>> > > Thanks in advance,
>> > > Elvis
>>
>
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Kitware, Inc.
Senior R&D Engineer
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