[vtkusers] How to create triangle strips?
David Gobbi
david.gobbi at gmail.com
Thu Nov 25 14:01:55 EST 2010
Are you sure the problem isn't just that X and Y are not scaled properly? I
think you should do something like this when you set your points:
xmax = 1000.0
ymax = 1000.0
points.SetPoint(index, i*xmax/(nx-1), j*ymax/(ny-1), z);
David
On Thu, Nov 25, 2010 at 11:15 AM, Hugo Valdebenito <hugo at maptek.cl> wrote:
>
> David
>
> It works now, for some rare reason wen I did try to create a test with nx=3
> and ny=3, it show strange lines, I don't know why. but with a larger mesh
> (1000x1000) it works fine :S.
>
> It could be useful to include that as a example.
>
> Thanks a lot!!!
>
> Hugo
>
>
> [image: Maptek] Hugo Valdebenito H.
> Ingeniero de Software, Sudamérica
> 2 Norte 465, Viña del Mar, Chile
> Tel: +56-32 2690683 (Anexo 144) | Fax: +56-32 269 06 28
> hugo at maptek.cl | *Skype:* hugo.maptek | www.maptek.cl
>
> ------------------------------
> Recuerda revisar los cursos que Maptek tiene disponible<http://www.maptek.cl/products/training_sa.html>
> 2010/11/25 David Gobbi <david.gobbi at gmail.com>
>
> Hugo,
>>
>> I can't see anything wrong with your code, but like Jim said, I don't know
>> what kind of surface you are trying to create. Just try adding this:
>>
>> Actor.GetProperty().SetAmbient(1.0);
>>
>> Depending on the lighting, sometimes surfaces can be so dark you cannot
>> see them. By setting ambient lighting to 1.0, you make all surfaces appear
>> bright regardless of the light position or the surface orientation.
>>
>> David
>>
>>
>> On Thu, Nov 25, 2010 at 9:05 AM, Jim Peterson <jimcp at cox.net> wrote:
>>
>>> //set ids
>>>> for (int i = 0; i < nx - 1; i++)
>>>> {
>>>> *cells.InsertNextCell(ny * 2);*
>>>> for (int j = 0; j < ny; j++)
>>>> {
>>>> *cells.InsertCellPoint(i + j * nx);*
>>>> *cells.InsertCellPoint(i + j * nx + 1);*
>>>> }
>>>> }
>>>>
>>> I suggest you add a comment with the expected result and comment the
>>> lines, Unless I am mistaken, you are created sets of overlapping cells, at a
>>> minimum, I would expect the numbering to be i*nx instead of j*nx. that would
>>> result in sequential cell definitions.
>>>
>>> Hope that helps,
>>> Jim
>>>
>>> Hugo,
>>>
>>> What kind of topology are you expecting this loop to create?
>>>
>>>
>>> Hugo Valdebenito wrote:
>>>
>>>> Hi David, thanks for your useful help!
>>>>
>>>> I tried with you suddestion, but didn't get it work :(. please, can you
>>>> check my code?
>>>>
>>>> public void* GenerateTest*(int nx, int ny)
>>>> {
>>>>
>>>> vtkPoints points = vtkPoints.New();
>>>> vtkCellArray cells = new vtkCellArray();
>>>> //vtkTriangleStrip triangleStrip;
>>>>
>>>> int index = 0;
>>>> double z,zmax = 50.0;
>>>> points.SetNumberOfPoints(nx * ny);
>>>> for (int j = 0; j < ny; j++)
>>>> {
>>>> for (int i = 0; i < nx; i++)
>>>> {
>>>> z = zmax * Math.Sin(4 * 3.1 * i / (float)nx) + 4 * j /
>>>> (float)(ny);
>>>> //insert point and increment index
>>>> points.SetPoint(index++,(double)i, (double)j, (double)z);
>>>> }
>>>> }
>>>> //set ids
>>>> for (int i = 0; i < nx - 1; i++)
>>>> {
>>>> *cells.InsertNextCell(ny * 2);*
>>>> for (int j = 0; j < ny; j++)
>>>> {
>>>> *cells.InsertCellPoint(i + j * nx);*
>>>> *cells.InsertCellPoint(i + j * nx + 1);*
>>>> }
>>>> }
>>>> vtkPolyData polydata = new vtkPolyData();
>>>> polydata.SetPoints(points);
>>>> polydata.SetStrips(cells);
>>>> // Create an actor and mapper
>>>> vtkDataSetMapper mapper = new vtkDataSetMapper();
>>>> mapper.SetInput(polydata);
>>>> vtkActor Actor = new vtkActor();
>>>> Actor.SetMapper(mapper);
>>>> Actor.GetProperty().SetRepresentationToWireframe();
>>>> //Add the actors to the scene
>>>> _renderer.AddActor(Actor);
>>>>
>>>>
>>>> Hugo.
>>>>
>>>> 2010/11/23 David Gobbi <david.gobbi at gmail.com <mailto:
>>>> david.gobbi at gmail.com>>
>>>>
>>>>
>>>> Hi Hugo,
>>>>
>>>> Go with my method. Remove the vtkTriangleStrip class from your
>>>> code, it isn't needed and it just makes the code less efficient
>>>> (and IMHO more confusing). The vtkCellArray has many
>>>> InsertNextCell methods, and the one that takes a vtkCell object is
>>>> the least efficient of all of them. For you, I think the best way
>>>> to add cells is this:
>>>>
>>>> cells.InsertNextCell(number_of_ids_in_strip)
>>>> cells.InsertCellPoint(first_id_in_strip)
>>>> cells.InsertCellPoint(second_id_in_srip)
>>>> (repeat InsertCellPoint for all IDs)
>>>>
>>>> Repeat all of the above for each strip that you want to add, then
>>>> call SetStrips() to add the whole cell array to the
>>>> polydata. Look at the example code that I linked to in my
>>>> previous email for more info.
>>>>
>>>> David
>>>>
>>>>
>>>> On Tue, Nov 23, 2010 at 4:30 AM, Hugo Valdebenito <hugo at maptek.cl
>>>> <mailto:hugo at maptek.cl>> wrote:
>>>>
>>>> Hi David.
>>>>
>>>> Thanks for you help, I'm trying to undestand, I have some
>>>> doubts yet, Which is the difference between both methods?
>>>> Wich is better?.
>>>>
>>>> Hugo.
>>>>
>>>> 2010/11/22 David Gobbi <david.gobbi at gmail.com
>>>> <mailto:david.gobbi at gmail.com>>
>>>>
>>>>
>>>> Hi Hugo,
>>>>
>>>> Your code is a mix of the method that I suggested and the
>>>> method that David Doria suggested. The SetStrips() method
>>>> is a low-level method for building a vtkPolyData (as per
>>>> my suggestion). The InsertNextCell() method is a
>>>> high-level method for building a vtkDataSet (as per David
>>>> Doria's suggestion). You have to use one method or the
>>>> other, not both. I apologize for the confusion.
>>>>
>>>> David
>>>>
>>>> On Mon, Nov 22, 2010 at 11:10 AM, Hugo Valdebenito
>>>> <hugo at maptek.cl <mailto:hugo at maptek.cl>> wrote:
>>>>
>>>> Hi David.
>>>>
>>>> It works with one strip, but that doesn't work with 2
>>>> o more strips. that is my implementation (sorry, is
>>>> C#, I translated the c++ example)
>>>>
>>>> public void GenerateSurface(int nx, int ny)
>>>> {
>>>>
>>>> vtkPoints points = vtkPoints.New();
>>>> vtkCellArray cells = new vtkCellArray();
>>>> vtkTriangleStrip triangleStrip;
>>>>
>>>>
>>>> double z,zmax = 50.0;
>>>> points.SetNumberOfPoints(nx * ny);
>>>> //se points
>>>> int index = 0;
>>>> for (int j = 0; j < ny; j++)
>>>> {
>>>> for (int i = 0; i < nx; i++)
>>>> {
>>>> z = zmax * Math.Sin(4 * 3.1 * i /
>>>> (float)nx) + 4 * j / (float)(ny);
>>>> //insert point and increment index
>>>> *points.SetPoint(index++,(double)i,
>>>> (double)j, (double)z);*
>>>>
>>>>
>>>> }
>>>> }
>>>> //set ids
>>>> for (int i = 0; i < nx - 1; i++)
>>>> {
>>>> index = 0;
>>>> triangleStrip = new vtkTriangleStrip();
>>>>
>>>> triangleStrip.GetPointIds().SetNumberOfIds(ny*2);
>>>>
>>>> for (int j = 0; j < ny; j++)
>>>> {
>>>>
>>>> triangleStrip.GetPointIds().SetId(index++, i + j * nx);
>>>>
>>>> triangleStrip.GetPointIds().SetId(index++, i + j * nx
>>>> + 1);
>>>> }
>>>> *cells.InsertNextCell(triangleStrip);*
>>>> }
>>>> vtkPolyData polydata = new vtkPolyData();
>>>> polydata.SetPoints(points);
>>>> *polydata.SetStrips(cells);*
>>>> // Create an actor and
>>>> mapper
>>>> vtkDataSetMapper mapper = new
>>>> vtkDataSetMapper();
>>>> mapper.SetInput(polydata);
>>>> vtkActor Actor = new vtkActor();
>>>>
>>>> Actor.SetMapper(mapper);
>>>>
>>>> Actor.GetProperty().SetRepresentationToWireframe();
>>>> //Add the actors to the scene
>>>> _renderer.AddActor(Actor);
>>>> }
>>>>
>>>>
>>>> Hugo
>>>>
>>>>
>>>>
>>>> 2010/11/22 David Gobbi <david.gobbi at gmail.com
>>>> <mailto:david.gobbi at gmail.com>>
>>>>
>>>>
>>>> Hi Hugo,
>>>>
>>>> The vtkPolyData stores the strips in an array
>>>> called "Strips", that is
>>>> how it knows what cells are strips. And all
>>>> vtkDataSet objects have a
>>>> GetCellType(i) method that VTK can use to check
>>>> the type of cell "i".
>>>> I have some code here that generates several
>>>> shapes with triangle
>>>> strips:
>>>>
>>>> https://github.com/dgobbi/ToolCursor/blob/master/vtkGeometricCursorShapes.cxx
>>>>
>>>> David
>>>>
>>>>
>>>> On Mon, Nov 22, 2010 at 5:55 AM, Hugo Valdebenito
>>>> <hugo at maptek.cl <mailto:hugo at maptek.cl>> wrote:
>>>> >
>>>> > You should to see the triangles, maybe it's
>>>> wrong. with 4 ids you define 2
>>>> > triagles, with 5 --> 3 and so on (that structure
>>>> saves memory). How does you
>>>> > said to vtk that you are using Strips? I don't
>>>> know how to define that on
>>>> > VTK.
>>>> >
>>>> >
>>>> > Thanks!
>>>> >
>>>> >
>>>> >
>>>> > 2010/11/19 David Doria <daviddoria at gmail.com
>>>> <mailto:daviddoria at gmail.com>>
>>>>
>>>> >>
>>>> >> On Fri, Nov 19, 2010 at 3:22 PM, Hugo
>>>> Valdebenito <hugo at maptek.cl
>>>> <mailto:hugo at maptek.cl>> wrote:
>>>> >> > Thanks david!!
>>>> >> >
>>>> >> > vtkCellArray object is defined, but never
>>>> used....
>>>> >> >
>>>> >> > Anyone can suggest a fix?, I'm beginner on VTK
>>>> >> > Hugo
>>>> >>
>>>> >> Try it now. It doesn't show the line through
>>>> the middle of the square
>>>> >> (the center edge of the triangles), but maybe
>>>> that is expected with a
>>>> >> triangle strip?
>>>> >>
>>>> >>
>>>> >>
>>>>
>>>> http://www.vtk.org/Wiki/VTK/Examples/Cxx/Broken/GeometricObjects/TriangleStrip
>>>> >>
>>>> >> David
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>>
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>>>
>>>
>>
>
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