[vtkusers] How to use backface culling with polylines
Donny
donnyz at charter.net
Sat Nov 20 21:12:09 EST 2010
I tried using vtkGeoView. I am adding quite a few very different actors to
the scene such as textured polydata, untextured polydata, lines, 2d text, 3d
text, and volumes. I did not like the representation approach because it
seemed too limited. I also wanted to control the interactor at a deeper
level. Basically it was too high level for my purposes.
I am using vtkGlobeSource, not only for an actor, but also for the static
function vtkGlobeSource::ComputeGlobePoint. My GIS data is in lat,lon
coordinates so this function is priceless. Most of my atmospheric data is in
meters from a central point so it fits in perfectly with my scene.
I am not sure what the best approach for the new functionality would be
since my state and county line data is not associated with the
vtkGlobeSource actor at all. It appears that the vtkHorizonClippedGeoView
would be the best approach although it would not have been my first choice
to use. I would be in favor of a static vtkGlobeSource function that would
calculate the horizon clip plane origin and normal based on parameters
passed to it.
Thanks guys.
-----Original Message-----
From: Jim Peterson [mailto:jimcp at cox.net]
Sent: Saturday, November 20, 2010 7:55 PM
To: Aashish Chaudhary
Cc: Donny; David Gobbi; Sebastien Jourdain; vtkusers at vtk.org
Subject: Re: [vtkusers] How to use backface culling with polylines
Aashish,
I think I would support having some GeoView Clip Horizon function. I
don't really know how we feel about endless subclassing. I believe if we
extend the function of vtkGeoView we would need an attribute to control
whether to clip horizon or not, an attribute for the clipping plane, a
modified callback on the vtkCamera to recompute the clipping plane on
camera movement.
If we make a vtkHorizonClippedGeoView subclass, we would not need the
control attribute, use of the class would mean the behavior was desired.
Donny, what would be more useful to you? are you currently using
vtkGeoView?
Jim
Aashish Chaudhary wrote:
> Jim, Donny, Gobbi
>
> This is nice! I think we can put this inside the geovis as vtkGeoClip
> or something along the lines? I will talk to Jeff about it. Would you
> be interested in making this addition?
>
> Thanks,
>
>
> On Sat, Nov 20, 2010 at 1:31 PM, Jim Peterson <jimcp at cox.net
> <mailto:jimcp at cox.net>> wrote:
>
> Donny, just to be sure we have a true normalized vector for the
> rest of the calcs, I think I would make the clop position an
> array, and then use vtkMath::Normalize(clipPos); to be sure we
> have a normal that has a length 1. soSo I think the normal
> calculation should probably be change from:
>
>
> 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);
>
> to something like this so that all of the point locations and
> vectors keep their parts together:
>
> double dv[] = {0.,0.,0.,}
> dv[0] = camerapos[0] - earthcenter[0]; // subtract
> earthcenter from camera pos
> dv[1] = camerapos[1] - earthcenter[1];
> dv[2] = camerapos[2] - earthcenter[2];
> double distance = sqrt(dv[0]*dv[0] + dv[1]*dv[1] + dv[2]*dv[2]);
>
> double nv[] = {0.,0.,0.,};
> nv[0] = (dv[0]/distance); // no inversion with the above
> subtract
> nv[1] = (dv[1]/distance);
> nv[2] = (dv[2]/distance); vtkMath::Normalize(nv); //
> assert normalize to unit length
> Also, if Altitude is greater than earth radius, we would just use
> the earth center as the clip plane location.
>
> Glad I could help.
> Jim
>
>
> Donny wrote:
>
> 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
> <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 <mailto: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
> <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 <mailto: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
> <mailto:jimcp at cox.net>] Sent: Saturday, November 20,
> 2010 9:03 AM
> To: Donny
> Cc: 'David Gobbi'; 'Aashish Chaudhary';
> vtkusers at vtk.org <mailto: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
> <mailto:david.gobbi at gmail.com>] Sent: Thursday,
> November 18, 2010 9:07 PM
> To: Donny
> Cc: vtkusers at vtk.org <mailto: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 <mailto: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
> <mailto:aashish.chaudhary at kitware.com>]
> *Sent:* Thursday, November 18, 2010 8:37 AM
> *To:* David Gobbi
> *Cc:* vtkusers at vtk.org
> <mailto: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>
> <mailto: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>
> <mailto: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>
> <mailto: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>
> <mailto:donnyz at charter.net
> <mailto:donnyz at charter.net>>
>
> 308-227-1756
>
>
>
>
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>
> --
> | Aashish Chaudhary
> | R&D Engineer
> | Kitware Inc.
> | www.kitware.com <http://www.kitware.com>
> <http://www.kitware.com>
>
>
>
>
>
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>
>
>
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> | Aashish Chaudhary
> | R&D Engineer
> | Kitware Inc.
> | www.kitware.com <http://www.kitware.com>
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