[vtkusers] How to use backface culling with polylines
Donny
donnyz at charter.net
Sat Nov 20 13:00:48 EST 2010
THANKS JIM!!! That worked. I had to change one more bug, the element number
of earthcenter was element 0 for each dimension. Here is what I have now:
double altitude = distance - vtkGeoMath::EarthRadiusMeters();
double clipdist = vtkGeoMath::EarthRadiusMeters() - altitude;
double pox = earthcenter[0]+(clipdist*nx);
double poy = earthcenter[1]+(clipdist*ny);
double poz = earthcenter[2]+(clipdist*nz);
You are correct in that the dolly out zoom in affect was happening
previously. It just appeared at first glance that it was inverse.
It is amazing how precise the horizon clips now. It is right on the money.
You made my day. I can't thank all of you enough for your help.
Maybe this could be incorporated into vtk. I just don't know where.
Thanks.
-----Original Message-----
From: Jim Peterson [mailto:jimcp at cox.net]
Sent: Saturday, November 20, 2010 11:18 AM
To: Donny
Cc: 'David Gobbi'; 'Aashish Chaudhary'; 'Sebastien Jourdain';
vtkusers at vtk.org
Subject: Re: [vtkusers] How to use backface culling with polylines
Sorry, I did say untested.... this
double pox = earthcenter[0]+(clipdist*nx);
double poy = earthcenter[0]+(altitude*ny);
double poz = earthcenter[0]+(altitude*nz);
should have been:
double pox = earthcenter[0]+(clipdist*nx);
double poy = earthcenter[0]+(clipdist*ny);
double poz = earthcenter[0]+(clipdist*nz);
I am not sure I understand going higher shortens the horizon. This is
contrary to the real geometry like
http://en.wikipedia.org/wiki/Horizon#Approximate_formulas explains the
calculations for the real world. It sounds like you want to change the
camera viewangle in proportion to the altitude to "zoom in" in the
target feature as you move away. I think I would call that a
dolly-out-zoom-in video effect. Camera angle would not affect the
visible horizon distance, just how much of it is in the field of view,
in my opinion.
Hope that helped,
Jim
Donny wrote:
> Ok, I am close. The plane is acting just opposite of what I want though.
The
> higher up on the horizon I go the less of the horizon I see, and the lower
> on the horizon I go the more I see (including past the horizon).
>
> Here is the code:
>
> double dx = earthcenter[0] - camerapos [0];
> double dy = earthcenter[1] - camerapos [1];
> double dz = earthcenter[2] - camerapos [2];
>
> double distance = sqrt(dx*dx + dy*dy + dz*dz);
>
> double nx = -(dx/distance);
> double ny = -(dy/distance);
> double nz = -(dz/distance);
>
> double altitude = distance - vtkGeoMath::EarthRadiusMeters();
> double clipdist = vtkGeoMath::EarthRadiusMeters() - altitude;
> double pox = earthcenter[0]+(clipdist*nx);
> double poy = earthcenter[0]+(altitude*ny);
> double poz = earthcenter[0]+(altitude*nz);
>
> m_horizonplane->SetOrigin(pox,poy,poz);
> m_horizonplane->SetNormal(nx,ny,nz);
>
> Thanks.
>
> -----Original Message-----
> From: Jim Peterson [mailto:jimcp at cox.net]
> Sent: Saturday, November 20, 2010 10:52 AM
> To: Donny
> Cc: 'David Gobbi'; 'Aashish Chaudhary'; 'Sebastien Jourdain';
> vtkusers at vtk.org
> Subject: Re: [vtkusers] How to use backface culling with polylines
>
> Donny,
> assuming all your coordinates are in meters,
> What I was trying to say was project the altitude of the camera into the
> earth sphere to locate the clipping plane.
> so with the normal calculation you have :
>
> altitude = distance - vtkGeoMath::EarthRadiusMeters();
> clipdist = vtkGeoEarth::EarthRadiusMeters() - altitude;
> pox = earthcenter[0]+(clipdist*nx);
> poy = earthcenter[0]+(altitude*ny);
> poz = earthcenter[0]+(altitude*nz);
>
> Untested of course....
>
> Jim
>
> Donny wrote:
>
>> Does this look correct to calculate the horizon clipping plane?
>>
>> double dx = earthcenter[0] - camerapos[0];
>> double dy = earthcenter[1] - camerapos[1];
>> double dz = earthcenter[2] - camerapos[2];
>>
>> double distance = sqrt(dx*dx + dy*dy + dz*dz);
>>
>> double nx = -(dx/distance);
>> double ny = -(dy/distance);
>> double nz = -(dz/distance);
>>
>> //(2 (abs(camera position - globeposition) minus globe radius))
>> double pox = 2.0 * (abs(camerapos [0] - earthcenter [0]) -
>> vtkGeoMath::EarthRadiusMeters());
>>
>> double poy = 2.0 * (abs(camerapos [1] - earthcenter [1]) -
>> vtkGeoMath::EarthRadiusMeters());
>>
>> double poz = 2.0 * (abs(camerapos [2] - earthcenter [2]) -
>> vtkGeoMath::EarthRadiusMeters());
>>
>> m_horizonplane->SetOrigin(pox,poy,poz);
>> m_horizonplane->SetNormal(nx,ny,nz);
>>
>> I am not getting the results I expected (the clipping plane is way off).
>>
>> -----Original Message-----
>> From: Jim Peterson [mailto:jimcp at cox.net]
>> Sent: Saturday, November 20, 2010 9:03 AM
>> To: Donny
>> Cc: 'David Gobbi'; 'Aashish Chaudhary'; vtkusers at vtk.org
>> Subject: Re: [vtkusers] How to use backface culling with polylines
>>
>> 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:
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>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> | 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
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>
>
>
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