Proposals:Calculators Architecture

Proposal

Decide on the best architecture for how to implement ITK Calculators. Should they inherit from Object and be separate from the pipeline, or should they inherit from ProcessObject and be part of the pipeline?

Current situation

There are currently two different types of Calculators implemented in ITK, one that inherits from Object and is not part of the pipeline, and one that inherits from ProcessObject and is part of the pipeline.

• Originally, all calculators had the following properties:
  • Inherit from Object
  • Are not part of the pipeline
  • Were designed to be fast, separate from pipeline structure, and easy to write avoiding the code overhead of filters.
  • Must include the following methods: Compute
  • Must include context specific Set/Get methods such as SetImage and GetThreshold
  • Examples: All calculators except those that inherit from itkHistogramThresholdCalculator, which inherits from ProcessObject. This includes, for example, OtsuMultipleThresholdsCalculator.

• A new group of calculators having the following properties:
  • Inherit from ProcessObject
  • Are part of the pipeline
  • Suffer speed and complexity overhead from belonging to the pipeline
  • Must include the following methods: Update, SetInput, GetOutput
  • Provide a Compute() method that simply calls Update()
  • Examples: All calculators that inherit from itkHistogramThresholdCalculator. This includes, for example, OtsuThresholdCalculator.

Existing limitations

• How should a developer write a new calculator? Should it inherit from Object or ProcessObject?
• Having two separate implementations, both with the name "Calculator", that inherit from entirely different trees and with different methods is inconsistent and confusing.
• Another drawback of having two different inheritance trees is that the calculators can't be hot-swapped since they have different methods.

Here is a very troublesome example: Example: Otsu.

The OtsuMultipleThresholdsCalculator, on the other hand, correctly inherits from Object (nor ProcessObject). But it implements a GenerateData() method without a pipeline! It implements an Update() method directly that explicitly calls the GenerateData method!

• We cannot take advantage of inheritance, so even though both algorithms compute nearly identical values, they

Solution (unfortunate):

• The only option is to instantiate one inside of the other.
• All methods need to be explicitly called on the one being instantiated.
• If a new method is added to the one being called, the caller needs to explicitly call that method as well (wouldn't be needed using inheritance)
• See the following patch to implement this: http://review.source.kitware.com/#/c/11935/

Proposed changes

A decision should be made on which framework should be chosen going forward. Then the current calculators should be refactored to all have the same inheritance tree and consistent methods. Here are some suggestions, none of which is great.

• Change the name of the calculators that inherit from itkHistogramThresholdCalculator to have "HistogramFilter" in the name instead of "Calculator"?
• Take guts out of the new calculators and put them in non-pipelined calculators that are called from the new calculators?
• Are there other better suggestions?

If the original framework (non-pipelined calculators) is chosen, methods like Compute() should be moved to a layer between Object and the calculator so that all calculators inherit them automatically to yield a consistent API. Such a layer could consist of classes that have consistent methods for inputs and outputs. For example:

• HistogramThresholdCalculator class with methods: Compute, SetInputHistogram, GetOutputThreshold
• ImageCalculator class with methods: Compute, SetInputImage

Current Status

Discussion phase...

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