Documentation/4.0/Developers/Style Guide

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For the latest Slicer developers documentation, visit the Nightly page.

Code style

Code that inherits from VTK classes should follow VTK coding conventions for naming, indentation, etc. See for details.


  • Acronyms should be written in all CAPS.
    • RASToSlice not RasToSlice
    • vtkMRML not vtkMrml
  • Words should be spelled out and not abreviated
    • GetWindow not GetWin


  • include extensive comments in the header files
  • keep the comments up to date as the code changes
  • use the keyword TODO to flag spots in the code that need to be revisited
  • do not leave blocks of commented out code in the source file -- if needed insert links to prior svn versions as comments

Language Specific

Library Dependencies

  • MRML classes should only depend on vtk and itk (not Slicer Logic or Qt)
  • Logic classes depend on MRML to store state
  • Logic classes should encapsulate vtk and itk pipelines to accomplish specific slicer tasks (such as resampling volumes for display)
  • GUI classes can depend on MRML and Logic and Qt

Development Practices

  • While developing code, enable VTK_DEBUG_LEAKS (ON by default) in your vtk build and be sure to clean up any leaks that arise from your contributions.

Coordinate Systems

  • World space for 3D Views is in RAS (Right Anterior Superior) space. See Coordinate systems.
  • All units are expressed in Millimeters (mm)


  • Use STL where you can, but follow the VTK guidelines


Commit message prefix

Subversion Commits to Slicer require commit type in the comment. Valid commit types are:

  BUG:   - a change made to fix a runtime issue
           (crash, segmentation fault, exception, or incorrect result,
  COMP:  - a fix for a compilation issue, error or warning,
  ENH:   - new functionality added to the project,
  PERF:  - a performance improvement,
  STYLE: - a change that does not impact the logic or execution of the code.
           (improve coding style, comments, documentation).

The Subversion command to commit the change is:

 svn commit -m "BUG: fixed core dump when passed float data" filename1[, filename2, ...]

By using the -m command line option, it's not possible to submit a message having multiple line. Submitting a mutli-line message can be achieved using the -f option:

 svn commit -f /path/to/message filename1[, filename2, ...]

It's also possible to set the environment variable SVN_EDITOR

Message content

A good commit message title (first line) should explain what the commit does for the user, not how it is done. How can be explained in the body of the commit message (if looking at the code of the commit is not self explanatory enough).


  • Bad: BUG: Check pointer validity before dereferencing -> implementation detail, self-explanatory (by looking at the code)
  • Good: BUG: Fix crash in Module X when clicking Apply button
  • Bad: ENH: More work in qSlicerXModuleWidget -> more work is too vague, qSlicerXModuleWidget is too low level
  • Good: ENH: Add float image outputs in module X
  • Bad: COMP: Typo in cmake variable -> implementation detail, self-explanatory
  • Good: COMP: Fix compilation error with Numpy on Visual Studio

If the commit is related to a mantis issue (bug or feature request), you can mention it in the commit message body:

BUG: Fix crash in Volume Rendering module when switching view layout

vtkSetAndObserveMRMLNodeEventsMacro can't be used for observing all types of vtkObjects,
only vtkMRMLNode is expected by vtkMRMLAbstractLogic::OnMRMLNodeModified(...) 
Closes #1641

Where 1641 refers to the issue number in mantis

Importing changes from external project/repository

When you update the git tag or svn revision of any external project, explicit in the commit message what the update does instead of just mentioning that an update in made.

This will avoid having a Slicer commit history made of ineligible messages:

r19180 - ENH: Update git tag
r19181 - BUG: Update svn revision
r19182 - ENH: revision updated

Ideally it should be the same message than the commit(s) in the external repository.

Example: ENH: Add feature A in module X instead of ENH: Update git tag


UI Design Guidelines

General guidelines


  • Elements in panels should be justified (use of a QFormLayout can simplify this task)
  • Sections should be organized in such a way that the workflow takes the user from top to bottom.
  • Capitalize the first letter in any text specified for a label or button
  • Try to use brief phrases when specifying text for a label or button rather than using sentences or sentence fragments ( use "Load volumes" instead of "Choose a volume to load")
  • Provide fully descriptive tool tips with each widget defined


  • Use the default values for the margins or 0. Default margins are automatically controlled by the Slicer custom style (see QStyle::PM_LayoutLeftMargin)
  • The minimum size hint of the top level module widget is used to determine the minimum width of the module. In Qt Designer, you can ensure it's sizing is correct by changing the QLayout::SizeConstraint to QLayout::SetMinimumSize.