[RFC] Filter Development

[lucienpeach]

Introduces an abstract Filter class from which discrete-time filter implementations can be derived. Also provides several Filter implementation classes corresponding to various commonly used filter types.

This is a new feature. Addresses #62.

Change List

• Provides Filter object, an abstract class for implementing discrete-time filtering.
• Provides LowPassFilter object (a child of Filter), which implements a discrete time low-pass filter.

Upcoming Changes

• Inclusion of DifferenceEquation object (name subject to change) which will determine a LCCDE from Transfer Function polynomials.
• Will provide additional common filter implementation, facilitated by DifferenceEquation, including:
  • HighPassFilter object implementing a high-pass filter
  • BandPassFilter object implementing a band-pass filter
  • BandStopFilter object implementing a band-stop filter
  • NotchFilter object implementing a notch filter
• Improvements to generalizability.

Potential (Long-Term) Future Changes

• Dynamic time step implementation.

Testing

This PR draft has yet to be tested. Testing will be conducted by implementing the various filter implementations in the SimpleControlIOExample example behavior.

Questions

Any feedback on how to improve or enhance the generalizability of this feature would be much appreciated.

[ethanmusser]

Seems like you're on the right track! One major structural change, but otherwise just some documentation/formatting updates. Looking forward to seeing the full implementation!

TODOs

• Make abstract filter and the provided child classes type-agnostic (at least for POD scalars, maybe also for scalar containers).
  • Make abstract Filter class into a type-agnostic class template.
  • Make implemented LowPassFilter class into a type-agnostic class template.
• Fix up the formatting per the style guide. Here are some general rules to note. I recommend you get an auto-formatter set up to use the default clang-format Google style (see this page for information on setting this up in VSCode).
  • All code should use two-space indents, and have a hard break at 80 characters (with a few, select exceptions). The only exception to the two-space intents are modifications to existing code which already uses four-space indents.
  • Opening brackets should follow on the same line as a function/object/namespace declaration.
  • Namespace contents should not be indented, per this section.
• Add documentation comments to all classes, members, and functions. Be sure to use Doxygen-friendly comments or comment blocks.

Resources

I recommend you reference the following pages as you write code.

• The Google C++ Style Guide for Drake is the style guide for all C++ code in this repository. All naming, formatting, and general structure should follow this.
• The Doxygen Manual outlines how to comment your code so that the documentation generates correctly.
• The ISO C++ Core Guidelines provide all kinds of guidelines for writing C++ code. I link to a few of these in my comments, but here is the full page if you want to check it out.

[ethanmusser]

[General] Filter should be a class template. At a minimum, it should be assumed that Filter takes in some kind of scalar type (e.g., int, float, etc.). Abstracting further, it could take in containers containing scalars (e.g., Eigen::Vector<float, ...>, std::array<double, ...>, std::vector<int>, etc.). Your current implementation only accepts Eigen::Vector3f data, which is not always the desired type.

For the scalar case, this might be

Suggested change

	class Filter {
	template<typename Scalar>
	class Filter {

Then, the data types inside would be Scalar instead of Eigen::Vector3f.

[ethanmusser]

[General] Same comment as in Filter. This should probably be a class template.
Note that, for this class, both the definition and implementation would have to live in the header file if you make this a class template. See here for more information.

[ethanmusser]

[General] Per this ISO C++ Standard item, you should use an initializer list here instead of assignment.