[vtkusers] Uniformly subdividing a 3D space with points

[Bryn Lloyd]

David Doria wrote:
> On Sun, Jan 17, 2010 at 10:08 AM, Bryn Lloyd <blloyd at vision.ee.ethz.ch> wrote:
>> Hi David,
>>
>>
>> You could try the following:
>>
>> 1. put your regular grid (e.g. vtkImageData) into a vtkCellLocator.
>>
>> 2. query for each of your points in the point cloud:
>> vtkCellLocator::FindCell(...)
>>
>> 3. store this information in a way suitable for you, e.g. vtkDataArray for
>> the point cloud.
>>
>>
>> BTW. vtkDataSet (and all sub-classes) has its own "FindCell" method, so you
>> could skip the vtkCellLocator.
>>
>>
>> Cheers
>> Bryn
>>
>>
>>
>>
>>
>> David Doria wrote:
>>> I have a point cloud. I would like to break it down into a uniform
>>> grid of cubic voxels (axis aligned is fine). That is, I would like to
>>> be able to ask "which points are in voxel (i,j,k)?". The only way I
>>> know how to do this is to actually create a cube to represent each
>>> voxel and then, for each cube, use a vtkSelectEnclosedPoints filter.
>>> This seems very "manual" - I'd need to come up with a data structure
>>> to store the cubes in this (i,j,k) type format, etc. It seems like
>>> there should be a way to divide the space into a grid of cubes all in
>>> one shot.
>>>
>>> Does anyone know of a filter that does this efficiently?
>>>
>>> Thanks,
>>>
>>> David
>>> _______________________________________________
> 
> Ok, seems like we're headed in the right direction. I wanted to make
> sure I was using vtkCellLocator correctly, so I tried to use the
> GenerateRepresentation function to visualize it. However, GetLevel()
> seems to return -1?
> 
> http://www.cmake.org/Wiki/VTK/Examples/CellLocator
> 
> Assuming it was working, I started building the code for my original question:
> http://www.cmake.org/Wiki/VTK/Examples/GridPointCloud
> 
> It seems to kind of work... except it always says point 0 is in cell
> -1. There shouldn't be negative cell indices, right? So you're
> recommending asking which cell every point is in, then storing the
> cellId as an array in the PointData? Then I would somehow (how?) query
> this PointData array to get all of the point indices that are in a
> particular cell?
> 
> Also, then I could traverse the cells in linear order, but is there a
> way to access them in (i,j,k) notation without simply writing my own
> (linear index)->(i,j,k) function?
> 
> Thanks for your help,
> 
> David


If FindCell returns -1, then it could not find a cell which contains the 
points. The point might be outside the regular grid (image)...


BTW. I just checked vtkImageData again. You might be interested in the 
function "ComputeStructuredCoordinates". It will compute the (i,j,k) 
coordinates of the cell from your 3D point (x,y,z).