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This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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When you look down the "north pole" with the Terrain interactor style, you get a lot of warning messages like so:

Warning: In /opt/vtk_5_4_2/VTK/Rendering/vtkRenderer.cxx, line 1051 vtkOpenGLRenderer (0x9dff598): Resetting view-up since view plane normal is parallel

In vtkRenderer.cxx, there is this test: vup = this->ActiveCamera->GetViewUp(); if ( fabs(vtkMath::Dot(vup,vn)) > 0.999 ) { vtkWarningMacro(<<"Resetting view-up since view plane normal is parallel"); this->ActiveCamera->SetViewUp(-vup[2], vup[0], vup[1]); }

So, vtkRenderer.cxx starts to panic when the dot > 0.999, which I will show happens before the Terrain's limit of 179 degrees.

There is code in vtkInteractorStyleTerrain that, quote:

// Move the camera. // Make sure that we don't hit the north pole singularity.

but, it does not do the job well enough. This is the code that avoids the singularity:

double angle = vtkMath::DegreesFromRadians( acos(vtkMath::Dot( dop, vup) ) ); if ( ( angle + e ) > 179.0 || ( angle + e ) < 1.0 ) { e = 0.0; }

I added some printouts after this code: cout << "dot " << fabs(vtkMath::Dot( dop, vup)) << " angle+e = " << angle << " + " << e << " = " << angle+e << endl;

Note, the dot represents the past, angle+e represents the future.

I saw these messages as i rotated towards the north pole:

dot 0.997361 angle+e = 175.836 + 0.640569 = 176.477 dot 0.99811 angle+e = 176.477 + 0.640569 = 177.117 dot 0.998735 angle+e = 177.117 + 0.640569 = 177.758 Warning: In /opt/vtk_5_4_2/VTK/Rendering/vtkRenderer.cxx, line 1051 vtkOpenGLRenderer (0x9dff598): Resetting view-up since view plane normal is parallel dot 0.999235 angle+e = 177.758 + 0.640569 = 178.399

So, dot=0.999234 after 0.640569 was added to 177.117 to get angle=178.399

We have not reached 179 degrees and we have surpassed dot=0.999234.

So the solution appears to be either change vtkRenderer.cxx's limit to 0.9999 or high (keep adding 9's), OR to change the Terrain's test to 2 and 178 degrees.