Documentation/Labs/VTKWidgets

Motivation

This page has the following purposes:

• It documents the user and developer features we wish to support in Slicer
• It documents the changes could be considered in the official VTK Widgets.
• It presents Slicer specific implementation acting as an alternative to VTK widgets.
• It evaluates the VTK widgets in terms of the appropriateness of their current design and implementation for efficiently addressing the needed use cases in Slicer.

Features Missing in 4.4

For Users

• Control L to make a new fiducial list
• Control I for persistent place toggle
• Control M to make a ruler between last two placed fiducials
• back tick and control/shift back tick to jump slices to fiducials
• delete or backspace (or 'd') to delete fiducial under mouse
• 'p' to place fiducial without having to click in window to grab focus
• Right click context menu (allow all operations currently supported in the table widget in the Markups GUI)
• modifier key to move fiducials between slices in 2D
• scaling issues (2D widget is scaled down by 300x compared to 3D)
• restrict to surface (found issues with the point placer wrt how it would be used in Slicer)
• rubber band placement for rulers

For Developers

• A start/end and moving event with seed number
• A start/end and hover event
• near widget vs near handle events
• A 2D version of the oriented handle widget
• consistent widget interfaces for click to place as well as programatic placement (vs them being added with zero size and having to be manipulated)
• consistent distribution of settings from the widget to all handles (process events on/off, visibility, display properties).
• expanded WidgetState settings that can be queried and maybe set

Design and Implementation

For the fiducials, we are currently using a vtkSeedWidget in both 2D and 3D. With the oriented polygonal handle representation being the only option to show a glyph and text together, and it being a 3D widget, it introduces some quirks in the 2D viewers. The Markups displayable managers have already been split into 2D and 3D versions to deal with this.

Some approaches:

• subclass the seed widget and representation and handles and create Slicer specific implementations
  • Cons:
    • not easy to feed back into VTK
  • Pros:
    • full control
    • did something similar already with the ROI widget
• modify the Slicer specific VTK github fork to implement the features we need in C++, then migrate them back via gerrit
  • Cons:
    • Still constrained by the VTK implementation as a starting point
  • Pros:
    • more freedom to experiment/customise
    • easy to feed back into VTK
• Port the Slicer3 Tcl implmentation of the 2D fiducials to Python for Slicer4
  • Cons:
    • issues with the reloadability of the scripted displayable managers
    • not easy to feed back into VTK
    • not quite as modular as we might end up with the widget type code embedded in the displayable manager
  • Pros:
    • full control
    • nominally easier as that version supported many of our missing use cases
• Port the Slicer3 Tcl implementation of the 2D fiducials into C++ for Slicer4
  • Cons:
    • Slightly more complex port
  • Pros:
    • full control
    • supports modularity (make a widget class first, then the displayable manager can be scripted)
    • can have a custom VTK class in our fork, easy to port back to VTK

References

• Interactive Contour Editing thread on slicer-dev (2015)
• ARRA grant widget notes (2010)
• Mouse and Keyboard Shortcuts (nightly)
• Slicer3 Tcl SWidget, SeedSWidget, FiducialsSWidget