The effects included are designed to be as portable as possible, as well as making it easy to add additional effects.
To build the app, simply open the project in android studio and build. The app integrates Oboe via a git submodule. Make sure when cloning the repository to clone the submodule as well using git clone --recursive, or git submodule update --init --recursive.
To update the version of Oboe being used, descend into the Oboe repository (oboe location) and update from its remote. Then, call git submodule update in this repository. Alternatively git submodule update --recursive --remote will pull the latest version of Oboe from remote.
Although the CMake file requires android headers (to use Oboe), the effects themselves can be compiled with any C++17 compliant compiler.
The UI code (Kotlin) calls native code through the JNI bridge to query information about the various effects implemented, and how to render the effect information in the UI. This means that adding an effect only needs to be done on the native side. The JNI bridge passes information regarding implemented effects descriptions to the UI as well as functions called when the user modifies effects in the UI.
The DuplexEngine is responsible for managing and syncing the input and output Oboe streams for rendering audio with as low latency as possible. The FunctionList class contains the a vector of effects that correspond to the list of effects (and their parameters) that the user wants to use to process their audio. Effects (and the FunctionList) overload their function operator to take in two numeric iterator types. E.g <template iter_type> void operator() (iter_type begin, iter_type end), where the iter_types correspond to C++ iterators carrying audio data. To erase the type of different objects, the FunctionList holds objects of types std::function<void(iter_type, iter_type)> i.e. functions which operate on the range between two numeric iterators in place. The DuplexEnginesimply calls the FunctionList on every buffer of samples it receives.
The effects folder contains the classes of various implemented effects. It also contains Effects.h where a global tuple of all the Effect descriptions implemented lives. The description folder contains the description for all of the effects. Each description takes the form of a class with static methods providing information regarding the effect (including name, category, parameters, and a factory method).
To add an effect, simply add a Description class similar to the existing classes, and add the class to the tuple in Effects.h. The description must provide a way to build the effect by either constructing another class corresponding to an effect, or pointing to a standalone function. Adding new effects is welcome!
A instructional implemented effect to examine is the TremoloEffect.h (a modulating gain). Many of the effects in the delay category inherit from DelayLineEffect which provides a framework to easily implement many delay based effects, as well as the comb filter effects (FIR, IIR, allpass). The slides have a block diagram displaying the mathematical basis of the effect. The nonlinear effects (distortion and overdrive) are implemented using a standalone function (from SingleEffectFunctions.h. The gain effect is implemented by a simple lambda in its description class.