Implement a title with CJK characters on a VTK widget
July 26, 2024
4
minutes
Sometimes a 2D text widget is in need like the title of a VTK widget. So What requirements do we have?
Q: Does the text widget have a fixed relative position to the VTK widget?
A: Yes.
Q: Does the content of the text widget change dynamically?
A: Yes.
Q: Is the text widget selectable or draggable?
A: No.
Q: What’s the position and the size of the text widget?
A: It lays on the middle top of the VTK widget and it has 10% of the VTK widget’s size.
Q: What kind of characters do we need to support?
A: English characters and CJK characters
VTK support for special characters
VTK does support English characters, and it has fonts like Courier, Arial, and Times. What if we want to support other characters, e.g., CJK characters? Right, we could useVTK_FONT_FILE, which means users must provide a specific font file. If code runs on the Windows platform, Microsoft Yahei is a good choice. It has a full package of characters, and you don’t need to worry about the lack of some characters.
vtkSmartPointer<vtkTextActor> textActor = vtkSmartPointer<vtkTextActor>::New();
textActor->GetTextProperty()->SetFontFamily(VTK_FONT_FILE);
textActor->GetTextProperty()->SetFontFile("C:/Windows/Fonts/msyh.ttc");
First, I thought about vtkTextWidget to implement the function. And the code looks like that. (AI helps a lot.)
//Initialize necessary components for a VTKWidget
vtkNew<vtkRenderer> renderer;
vtkNew<vtkRenderWindow> renderWindow;
renderWindow->AddRenderer(renderer);
vtkNew<vtkRenderWindowInteractor> renderWindowInteractor;
renderWindow->SetInteractor(renderWindowInteractor);
//Create a vtkTextActor
vtkSmartPointer<vtkTextActor> textActor = vtkSmartPointer<vtkTextActor>::New();
textActor->GetTextProperty()->SetFontFamily(VTK_FONT_FILE);
textActor->GetTextProperty()->SetFontFile("C:/Windows/Fonts/msyh.ttc");
textActor->SetInput("Hello, world!"); //the characters can be changed to CJK characters
//Create a vtkTextRepresentation
vtkSmartPointer<vtkTextRepresentation> textRepresentation = vtkSmartPointer<vtkTextRepresentation>::New();
textRepresentation->GetPositionCoordinate()->SetCoordinateSystemToNormalizedViewport();
textRepresentation->GetPositionCoordinate()->SetValue(0.5, 0.9); // on the middle top
textRepresentation->GetPosition2Coordinate()->SetCoordinateSystemToNormalizedViewport();
textRepresentation->GetPosition2Coordinate()->SetValue(0.1, 0.1);
textRepresentation->SetShowBorderToOff(); //The widget has no frame border
//Create a vtkTextWidget and start it
vtkSmartPointer<vtkTextWidget> textWidget = vtkSmartPointer<vtkTextWidget>::New();
textWidget->SetRepresentation(textRepresentation);
textWidget->SetInteractor(renderWindowInteractor);
textWidget->SetTextActor(textActor);
textWidget->SelectableOff();
textWidget->On();
//Render
renderWindowInteractor->Start();
renderWindow->Render();
It works well. However, it has a fatal con. The vtkTextWidget can’t change its content dynamically since the renderWindowInteractor->Start() . That means, its On() function must be triggered in the front of the start function mentioned before, and its content is immutable once On() function is triggered. That doesn’t fit into our requirement.
So how about using the vtkTextActor directly?
//Initialize necessary components for a VTKWidget and start interactor
...
//Create a vtkTextActor
vtkSmartPointer<vtkTextActor> textActor = vtkSmartPointer<vtkTextActor>::New();
textActor->SetInput("Hello, world!");
//configure the vtkTextActor's position
textActor->GetTextProperty()->SetFontSize(12);
textActor->GetPositionCoordinate()->SetCoordinateSystemToNormalizedViewport();
textActor->GetPositionCoordinate()->SetValue(0.5, 0.9); // on the middle top
textActor->GetPosition2Coordinate()->SetCoordinateSystemToNormalizedViewport();
textActor->GetPosition2Coordinate()->SetValue(0.1, 0.1);
renderer->AddActor2D(textActor);
renderWindow->Render();
That looks better. The text represents on the middle top of the VTK widget no matter how the VTK widget zooms in and out. But there is another problem, the textActor doesn’t scale along with the VTK widget resizing. Even though it has a function SetTextScaleModeToViewport() , but it zooms out but doesn’t zooms in back. In consideration with the requirement, it’s ok to keep a fixed size.
Reference
VTK doc
VTK pipeline and an example
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