[vtkusers] Plane polydata intersection

Mon Jun 24 11:05:39 EDT 2013

Hi Cory.

Indeed the plane I will use can change position and direction.

Please, could you explain what is custom shaders and how to use them to compute distance ?

Thank you.
-Laurent

On Jun 24, 2013, at 10:46 AM, Cory Quammen <cquammen at cs.unc.edu> wrote:

> Laurent,
> 
> If your plane doesn't change orientation, then you should be able to
> change your isolevel value as you move the plane along its normal,
> thereby avoiding the distance computation, e.g., compute the distance
> one time relative to the current plane, then add 5 to the isolevel if
> your plane moves 5 units along the plane normal, etc.
> 
> For a non-geometry based approach, you might use custom shaders to
> compute the distance of each vertex to your plane and discard
> fragments that aren't within some distance to the plane. This would be
> fast but may suffer from giving you a more-than-one-pixel-thick
> representation of the intersection in parts of the scene closest to
> the camera and perhaps missing some pixels in parts of the scene
> furthest from the camera.
> 
> Hope that helps,
> Cory
> 
> 
> 
> On Mon, Jun 24, 2013 at 10:20 AM, Laurent Chauvin
> <lchauvin at bwh.harvard.edu> wrote:
>> Hi Cory,
>> 
>> Thank you for your reply.
>> 
>> However, you said I have to compute distance from each point to my plane,
>> then use vtkContourFilter.
>> But I think it would be really heavy computation to compute distance from
>> each point to the plane every time I move the plane, isn't it ?
>> 
>> I would like to be able to display the intersection while the plane is
>> moving. If I have a polydata like a bone or a skull or something like that,
>> it would be really long to compute this distance, isn't it ?
>> 
>> Thank you.
>> -Laurent
>> 
>> 
>> 
>> On Jun 24, 2013, at 10:04 AM, Cory Quammen <cquammen at cs.unc.edu> wrote:
>> 
>> On Sun, Jun 23, 2013 at 12:41 PM, Laurent Chauvin
>> <lchauvin at bwh.harvard.edu> wrote:
>> 
>> Hi Cory,
>> 
>> Thank you very much for your reply.
>> However, I'm not sure to understand what you mean.
>> Should I create a new polydata with all points at the same z coordinates ?
>> 
>> 
>> Laurent,
>> 
>> You don't need to create a new polydata. Instead, you can add a new
>> vtkDataArray (likely a vtkFloatArray or vtkDataArray in this case) to
>> the point data of whatever polydata you already have by using
>> vtkPolyData::GetPointData()->AddArray(...).
>> 
>> For axis-aligned planes, this new array could simply contain the x, y,
>> or z coordinate of each of the points in your vtkPolyData. If you want
>> an intersection of your poly data with a plane spanning x and y, you
>> would add a point data array storing all the z values. You can then
>> use the vtkContourFilter to get the polydata consisting of line
>> segments that represents the intersection of your polydata with the
>> plane at a given z value by specifying that z value as the isolevel
>> for the contour filter. You have to make sure you tell the
>> vtkContourFilter to use your new coordinate array.
>> 
>> For an quick example of this concept, I've attached a simple ParaView
>> statefile showing how you can do this for a cylinder polydata. In this
>> example, I've specified my plane as spanning the x and y coordinates
>> with z value 0.2. In ParaView, I've used the Calculator to compute the
>> array consisting of the z coordinates of each point in the cylinder.
>> Forgive me if you don't have ParaView installed, we use it all the
>> time when prototyping visualizations.
>> 
>> Does this work with some oblique planes ( not parallel to x,y, or z axis ) ?
>> 
>> 
>> For the general case of an arbitrary plane, you would want to compute
>> the distance from each point your desired plan instead of creating a
>> new array containing the x, y, or z coordinates of each point. Then
>> you would set the vtkContourFilter isolevel to 0. The point plane
>> calculation is pretty simple:
>> http://mathworld.wolfram.com/Point-PlaneDistance.html
>> 
>> Does that make sense?
>> 
>> Thank you.
>> -Laurent
>> 
>> 
>> On Sun, Jun 23, 2013 at 12:31 PM, Cory Quammen <cquammen at cs.unc.edu> wrote:
>> 
>> 
>> Laurent,
>> 
>> For this special case of computing the intersection between a polydata
>> and a plane, you can use a little trick. First, you can compute a new
>> point data set for your polydata with,say, the z coordinates of each
>> point in the polydata. Then, to get the intersection with an xy plane,
>> you then use the vtkContourFilter operating on this coordinate point
>> data and specify the z value that corresponds to your plane. The
>> result should be equivalent to the intersection of the polydata with
>> that plane.
>> 
>> It should be relatively fast to compute the contour, so you can
>> probably do this interactively.
>> 
>> Hope that helps,
>> Cory
>> 
>> On Sat, Jun 22, 2013 at 2:46 PM, Laurent Chauvin
>> <lchauvin at bwh.harvard.edu> wrote:
>> 
>> Hello,
>> 
>> I'm working on Slicer, and I was trying to find a nice way to display
>> intersection between polydata and a plane.
>> I read we could use a vtkCutter, but the thing is, I would like to do
>> this
>> interactively. I want to be able to move the plane, and update the
>> intersection while moving the plane.
>> I'm not sure using vtkCutter for this purpose would work. I think
>> vtkCutter
>> will take some times to execute, then moving the plane will be slow.
>> 
>> I know this question has been asked before, but I would like if there
>> were
>> some update since then. New widgets or object that I could use for that
>> ?
>> 
>> I was thinking using vtkImageReslice, get polydata of the plane and
>> compute
>> the intersection between the 2 polydata, but I think it would also be to
>> computing intensive to be able to do it smoothly with an interactive
>> plane.
>> 
>> Thank you.
>> -Laurent
>> 
>> 
>> 
>> 
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>> 
>> 
>> --
>> Cory Quammen
>> Research Associate
>> Department of Computer Science
>> The University of North Carolina at Chapel Hill
>> 
>> 
>> 
>> 
>> 
>> --
>> Laurent Chauvin, MS
>> Surgical Navigation and Robotics Laboratory, Radiology
>> Brigham And Women's Hospital, Harvard Medical School
>> http://wiki.ncigt.org/index.php/User:Lchauvin
>> 
>> 
>> 
>> 
>> --
>> Cory Quammen
>> Research Associate
>> Department of Computer Science
>> The University of North Carolina at Chapel Hill
>> <PlaneIntersectionExample.pvsm>
>> 
>> 
> 
> 
> 
> -- 
> Cory Quammen
> Research Associate
> Department of Computer Science
> The University of North Carolina at Chapel Hill

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