[vtkusers] Correctly apply camera matrix

[Ken Martin]

The best approach in VTK is to save the key variables, camera position,
focal point, vup, dop, view angle. This is because the interaction methods
directly adjust those parameters, not the computed matrix.

On Mon, Jul 16, 2018 at 4:13 AM, andyjk <andrewkeeling at hotmail.com> wrote:

> I am trying to save a particular camera view, then reapply it later (ie be
> able to reset the the view to a previous position)
>
> I see from here (
> http://vtk.1045678.n5.nabble.com/How-to-restore-a-camera-
> view-td5728916.html
> ) that I can do this by saving  position, focal_point, View-up and
> view_angle,
>
> but I am trying to do this more neatly by saving just the 4x4
> transformation
> matrix of the camera (and assuming view angle is unchanged).
>
> How can I grab the current camera transformation matrix, and later re-apply
> it ?
>
> I have tried :
>
> 1) GRAB
> vtkSmartPointer<vtkMatrix4x4> mat;
>                 mat =
> iren->GetRenderWindow()->GetRenderers()->GetFirstRenderer()->
> GetActiveCamera()->GetModelViewTransformMatrix();
>
>                 saveVTKMat4x4(mat, "VTKCamera.vmx");
>
> 2) LOAD and APPLY
> loadVTKMat4x4(mat, "VTKCamera.vmx");
>                 mat->Print(std::cout);
>
>
>                 vtkSmartPointer<vtkMatrix4x4> mat_inverse =
> vtkSmartPointer<vtkMatrix4x4>::New();
>                 vtkMatrix4x4::Invert(mat, mat_inverse);
>                 vtkSmartPointer<vtkTransform> transform =
> vtkSmartPointer<vtkTransform>::New();
>                 transform->SetMatrix(mat);
>                 transform->Update();
>
>
>                 vtkSmartPointer<vtkMatrix4x4> current_mat;
>                 current_mat = custom_camera->GetViewTransformMatrix();
>
>                 vtkSmartPointer<vtkMatrix4x4> current_mat_inverse =
> vtkSmartPointer<vtkMatrix4x4>::New();
>                 vtkMatrix4x4::Invert(current_mat, current_mat_inverse);
>
>                 vtkSmartPointer<vtkTransform> transformInv =
> vtkSmartPointer<vtkTransform>::New();
>                 transformInv->SetMatrix(current_mat_inverse);
>                 transformInv->Update();
>
>
>                 custom_camera->ApplyTransform(transformInv);
>                 custom_camera->Modified();
>                 custom_camera->ApplyTransform(transform);
>
>
> [Where the save/load functions have been verified using vtk::print and are
> working fine]
>
> I presume when loading, I have to move the camera back to identity (via an
> inverse transform) before applying the new transform because I think
> ApplyTransform concatenates.
>
> I find that if I don't move the camera at all between saving and loading,
> it
> works.
> If I nudge the camera a tiny bit between loading and saving, the new view
> is
> displaced a bit more when loading, so I think I must be getting some
> inverse
> transforms muddled up?
>
> Is there a neat way to the current camera matrix then reapply it later ?
>
>
>
>
>
>
> --
> Sent from: http://vtk.1045678.n5.nabble.com/VTK-Users-f1224199.html
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-- 
Ken Martin PhD
Distinguished Engineer
Kitware Inc.
101 East Weaver Street
Carrboro, North Carolina
27510 USA

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