[vtkusers] vtkPolyData to be used with vtkImageStencil.

[Dean Inglis]

Anja,

sorry, I didn't look closely at your points: I thought
they formed a square/rectangle.  So the code for vtkCellArray
really should be as you show below.  The z offset is
necessary for the vtkPolyDataToImageStencil to work:
it does not work with polylines, but needs surfaces,
so the extrusion makes a "ribbon".  If you pass the polyData
output of the extrusion filter to a vtkPolyDataMapper/vtkActor
and visualize it you will see what I mean.  This will also
help with unserstanding possible errors: make sure your image
actor can be manipulated with a interactorStyleTrackballCamera
and not interactorStyleImage.  Also, I would change the background color
of your renerwindow to something other than black. If your 2D (?) image
has z-thickness, you may have to change the z offset and
extrusion scale.  Also, if the triangle is too acute relative
to the pixel dimensions, or if your closed polydata loops back on
itself or has "knots", you can get erroneous results.

Dean


-----Original Message-----
From: Anja Ende [mailto:anja.ende at googlemail.com]
Sent: Wednesday, October 25, 2006 9:16 AM
To: Dean's business account
Cc: vtkusers at vtk.org
Subject: Re: [vtkusers] vtkPolyData to be used with vtkImageStencil.


Another question that I wanted to ask is why do we set the z offset?
Clearly, this is needed as the output is completely different... But what
does it actually do?

Thanks,
Anja


On 25/10/06, Anja Ende <anja.ende at googlemail.com> wrote:
Hi Dean,

Thanks for the reply and code fragmant. This really helps!

Your suggestiosn were completely on the money (as usual!). It was the
problem with not closing the loop and the extrusion vector just being copied
and pasted from example code without bothering to understand what it meant!

However, there seems to be a problem with the stencil output:

The points I have chosen are:

float pts[4][3] = {{-55.7812, 9.375, -0.5},
                {-18.2812, 49.6875, -0.5},
                { 21.0938, 14.0625, -0.5},
                {-55.7812, 9.375, -0.5}};

which should form a triangle.

However, the output looks very weird. The triangle is too small and very
jagged. Also, the dimensions of the triangle seem wrong. There is a
screenshot attached with this message. Please mail me if you cannot see it
on the website and I will send you the screenshot immediately.

Also, here is the latest code now:

float pts[4][3] = {{-55.7812, 9.375, -0.5},
                {-18.2812, 49.6875, -0.5},
                {21.0938, 14.0625, -0.5},
                {-55.7812, 9.375 , - 0.5}};

vtkFloatArray* pcoords = vtkFloatArray::New();
pcoords->SetNumberOfComponents(3);
pcoords->SetNumberOfTuples(4);
for (int i=0; i<4; i++)
{
     pcoords->SetTuple(i, pts[i]);
}
// Create vtkPoints and assign pcoords as the internal data array.
vtkPoints* points = vtkPoints::New();
points->SetData(pcoords);
vtkPolyData* polydata = vtkPolyData::New();
polydata->SetPoints(points);


 vtkCellArray* strips = vtkCellArray::New();
 strips->InsertNextCell(4);
 strips->InsertCellPoint(0);
 strips->InsertCellPoint(1);
 strips->InsertCellPoint(2);
 strips->InsertCellPoint(3); // same coordinates as the first one..

polydata->SetStrips(strips);

vtkLinearExtrusionFilter* extrude = vtkLinearExtrusionFilter::New();
extrude->SetInput(polydata);
extrude->SetScaleFactor(1);
extrude->SetExtrusionTypeToNormalExtrusion();
extrude->SetVector(0, 0, 1);

vtkPolyDataToImageStencil* dataToStencil = vtkPolyDataToImageStencil::New();
dataToStencil->SetInput(extrude->GetOutput());

vtkImageStencil* stencil = vtkImageStencil::New();
stencil->SetInput(m_viewer->GetOutput());
stencil->SetStencil(dataToStencil->GetOutput());
stencil->ReverseStencilOn();
stencil->SetBackgroundValue(0);

stencil->Update();

Also, when I add more points, so instead of a triangle have a quadrilateral.
I see a triangle shape being stencilled and the outside the loop some lines
through the image... but they are completely outside the loop. However, I
can see that the shape is completely closed....

Thanks for all the help!

Anja





--
Cheers,

Anja