Effect Development Example

[WIP] Mixxx Effect Development Example

This is a practical guide of how to develop a simple native effect for mixxx effects framework. In order to demonstrate actual steps needed, a simple utility effect is used as an example effect.

More analytical info on topic can be found in the Effects section of the Developer Guide.

Utility Effect

This effect comprises of two control elements, one knob that sets the level/gain of the input processing audio and one switch that mutes or unmutes the audio output.

Code developed for this effect can be reviewed in this pull request: Custom effect example #1284.

The actual effect implementation is done in the files utilityeffect.cpp and utilityeffect.h located at src/effects/native/ directory.

In header file, after importing all the needed files, a structure with the effect group state is declared. This structure will hold all the state variables that the effect will keep from one block processing to another.

Next the actual effect class is declared, with all the required methods. In order to be able the effect to work with the native effect backend,the following methods should be implemented: getId, getManifest and processChannel.

Implementation of these methods is done in utilityeffect.cpp

getManifest is responsible for connecting the control elements with the effect process. The two control elements are declared as pointers to EffectManifestParameter object, and their type and default/min/max values are set. Some extra info such as a description for hover text and name tags that will indicate the control elements by the skin are declared here as well.

Constructor of the effectobject is where all effect initialization shall take place.

Finally the process method is where all the action happens. process is called every numSamples/2 * sampleRate seconds, and it delivers stereo interleaved samples at pInput buffer. Output buffer should be completed in a similar interleaved way with the processed samples.

This Utility effect implements also a simple ramping function, by adding a controllable ramp envelope when effect state is mutually changed from muted to unmuted and from enabled to disabled.

In order for the native effect backend be aware of the new effect, it should be included and registered in src/effects/native/nativebackend.cpp and src/test/nativeeffects_test.cpp

Finally, add the new files to CMakeLists.txt so they are included in the cmake build.