[vtkusers] How to use backface culling with polylines
Jim Peterson
jimcp at cox.net
Sat Nov 20 10:02:41 EST 2010
Donny,
I haven't really tried to get this thought into code, but in a soid
geometry kind of sense, the horizon from a point of view is generally
described by a clip somewhere between the observer and the center of the
sphere, depending on the altitude. the direction of view should not be a
factor, and how close the clip plane is to the center of the globe
depends on the altitude. if the observer is at a height of 100' the
horizon (Excluding other terrain elevation) is 12.25 miles along the
surface of the sphere. the clip throught the center of the sphere would
make the horizon at about 6000 miles.
I would expect the clip normal to be the normalized difference between
the camera position and the globe position, and the approximate clip
plane position to be the point that is (2 (abs(camera position - globe
position) minus globe radius)) from the camera toward the globe position.
If the globe happens to be at 0,0,0 that would simplify the calculation
to just the globe radius and the camera position, but Copernicus changed
all that....
I haven't really studied the GEOVIS classes, but I would have thought
visible horizon clipping would be one of the tools. Maybe this should
evolve into a contribution.
Sounds like an interesting project.
Jim
Donny wrote:
> Hello again. I decided to try David's approach with clipping planes. Mainly
> because I will have quite a few lines when including the county lines in
> addition to the state lines and I did not want to create that many
> triangles. I was afraid it would slow down the performance.
>
> I have centered the clipping plane at the center of the earth and the normal
> is always the view normal. This works great when the focus point is on the
> surface of the earth and I rotate around it. The problem occurs when I pan.
> This moves the camera focal point and when it is no longer on the surface of
> the earth the clipping plane will show some details on the other side of the
> horizon. How can I force a pan operation to pan the focal point on the
> surface of the earth?
>
> Here is the pan code, I am using .NET delegates because of the static
> function pointer limitation for AddObserver:
>
> void OnVtkMiddleMouseDown(unsigned long eventId, System::Object^ clientData)
> {
> this->m_interactorstyle->StartPan();
> }
>
> void OnVtkMiddleMouseUp(unsigned long eventId, System::Object^ clientData)
> {
> this->m_interactorstyle->EndPan();
> }
>
> void OnVtkMouseMove(unsigned long eventId, System::Object^ clientData)
> {
> switch (this->m_interactorstyle->GetState())
> {
> case 1: //VTKIS_ROTATE
> this->Rotate();
> break;
>
> case 2: //VTKIS_PAN
> this->Pan();
> break;
>
> case 4: //VTKIS_DOLLY
> this->Dolly();
> break;
> }
> }
>
> void Pan()
> {
> // Get the vector of motion
>
> double focalPoint[3];
> double v[3];
> double p1[4];
> double p2[4];
>
>
> double* pos = m_camera->GetPosition();
> double* fp = m_camera->GetFocalPoint();
>
>
> vtkInteractorStyleUser::ComputeWorldToDisplay(m_backgroundrenderer,
> fp[0], fp[1], fp[2], focalPoint);
>
>
> vtkInteractorStyleUser::ComputeDisplayToWorld(m_backgroundrenderer,
> m_iren->GetEventPosition()[0], m_iren->GetEventPosition()[1],
> focalPoint[2], p1);
>
>
> vtkInteractorStyleUser::ComputeDisplayToWorld(m_backgroundrenderer,
> m_iren->GetLastEventPosition()[0], m_iren->GetLastEventPosition()[1],
> focalPoint[2], p2);
>
> double npos[3];
> double nfp[3];
>
> for (int i = 0; i < 3; i++)
> {
> v[i] = p2[i] - p1[i];
> npos[i] = pos[i] + v[i];
> nfp[i] = fp[i] + v[i];
> }
>
> m_camera->SetPosition(npos);
> m_camera->SetFocalPoint(nfp);
>
> this->m_iren->Render();
>
> }
>
> Thanks.
>
> -----Original Message-----
> From: David Gobbi [mailto:david.gobbi at gmail.com]
> Sent: Thursday, November 18, 2010 9:07 PM
> To: Donny
> Cc: vtkusers at vtk.org
> Subject: Re: [vtkusers] How to use backface culling with polylines
>
> You could also add a clipping plane to your vtk mapper that cuts the
> globe in half, back to front. By adding this plane only to the mapper
> that renders the lines, you can remove any lines that are past the
> horizon.
>
> David
>
>
> On Thu, Nov 18, 2010 at 7:13 PM, Jim Peterson <jimcp at cox.net> wrote:
>
>> Donny,
>> I think I would run the state lines through a ribbon filter making them
>>
> into
>
>> a triangle strip of some width. the strip would have a backface.
>>
>> Jim
>>
>> Donny wrote:
>>
>>> Thanks guys for the feedback. I have attached two images to visualize
>>>
> what
>
>>> follows. I hope I can explain this clearly.
>>>
>>> I have three renderers that I am adding to the render window.
>>>
>>> The first is to contain a textured vtkGlobeSource actor and is called the
>>> background renderer.
>>>
>>> The second contains several vtkPolyData actors and are lines (State and
>>> county boundaries, roads, rivers…), this is called the map renderer.
>>>
>>> The Third contains an actor visualizing a weather radar volume, this is
>>> called the radar renderer.
>>>
>>> The background renderer is always set as layer 0 and so is always
>>>
> rendered
>
>>> first.
>>>
>>> The map renderer and radar renderer will alternate between the 1^st and
>>> 2^nd layers depending on the view angle of the camera.
>>>
>>> If the camera is more than 45 degrees above the horizon then the radar
>>> renderer is layer 1 and the map renderer is layer 2 so that the user can
>>>
> see
>
>>> the features below the radar volume.
>>>
>>> If the camera is less than or equal to 45 degrees above the horizon then
>>> the map renderer is layer 1 and the radar renderer is layer 2 so that the
>>> radar volume is always in front of the map features.
>>>
>>> This works fine until I draw all state lines for the entire US. Because
>>> the vtkGlobeSource simulates the curvature of the earth I see the
>>>
> backside
>
>>> of the state lines where they are over the view horizon.
>>>
>>> Is there a way I can put the map features and radar volume on the same
>>> renderer and accomplish the same thing?
>>>
>>> Thanks.
>>>
>>> -----Original Message-----
>>> *From:* Aashish Chaudhary [mailto:aashish.chaudhary at kitware.com]
>>> *Sent:* Thursday, November 18, 2010 8:37 AM
>>> *To:* David Gobbi
>>> *Cc:* vtkusers at vtk.org; Donny
>>> *Subject:* Re: [vtkusers] How to use backface culling with polylines
>>>
>>> Donny,
>>>
>>> Were you talking about hidden lines removal may be?
>>>
>>> Thanks,
>>>
>>> On Thu, Nov 18, 2010 at 9:03 AM, David Gobbi <david.gobbi at gmail.com
>>> <mailto:david.gobbi at gmail.com>> wrote:
>>>
>>> Jerome is correct, OpenGL culls faces according to the polygon
>>> winding. So wireframe polygons can be culled, but polylines cannot,
>>> even if they have normals assigned to them. There are some details in
>>> the OpenGL FAQ:
>>> http://www.opengl.org/resources/faq/technical/clipping.htm
>>>
>>> David
>>>
>>>
>>> On Wed, Nov 17, 2010 at 11:47 PM, Jérôme <jerome.velut at gmail.com
>>> <mailto:jerome.velut at gmail.com>> wrote:
>>>
>>>> Hi Donny,
>>>>
>>>> My feeling is that backface culling should not work with polylines
>>>> because they
>>>> have actually no face.
>>>>
>>>> Jerome
>>>>
>>>> 2010/11/18 Donny <donnyz at charter.net <mailto:donnyz at charter.net>>:
>>>>
>>>>> I am drawing polylines using vtkPolyData and cannot get backface
>>>>> culling to
>>>>> work with them.
>>>>>
>>>>>
>>>>>
>>>>> I ran into this subject
>>>>> http://public.kitware.com/pipermail/vtkusers/2003-January/065023.html
>>>>>
> ,
>
>>>>> but
>>>>> did not help.
>>>>>
>>>>>
>>>>>
>>>>> Any solutions?
>>>>>
>>>>>
>>>>>
>>>>> Donny Zimmerman
>>>>>
>>>>> donnyz at charter.net <mailto:donnyz at charter.net>
>>>>>
>>>>> 308-227-1756
>>>>>
>>> _______________________________________________
>>> Powered by www.kitware.com <http://www.kitware.com>
>>>
>>> Visit other Kitware open-source projects at
>>> http://www.kitware.com/opensource/opensource.html
>>>
>>> Please keep messages on-topic and check the VTK FAQ at:
>>> http://www.vtk.org/Wiki/VTK_FAQ
>>>
>>> Follow this link to subscribe/unsubscribe:
>>> http://www.vtk.org/mailman/listinfo/vtkusers
>>>
>>>
>>>
>>>
>>> --
>>> | Aashish Chaudhary
>>> | R&D Engineer
>>> | Kitware Inc.
>>> | www.kitware.com <http://www.kitware.com>
>>>
>>> ------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> Powered by www.kitware.com
>>>
>>> Visit other Kitware open-source projects at
>>> http://www.kitware.com/opensource/opensource.html
>>>
>>> Please keep messages on-topic and check the VTK FAQ at:
>>> http://www.vtk.org/Wiki/VTK_FAQ
>>>
>>> Follow this link to subscribe/unsubscribe:
>>> http://www.vtk.org/mailman/listinfo/vtkusers
>>>
>>>
>>
>
>
>
More information about the vtkusers
mailing list