The fluid family of opcodes wraps Peter Hannape's SoundFont 2 player, FluidSynth: fluidEngine for instantiating a FluidSynth engine, fluidSetInterpMethod for setting interpolation method for a channel in a FluidSynth engine, fluidLoad for loading SoundFonts, fluidProgramSelect for assigning presets from a SoundFont to a FluidSynth engine's MIDI channel, fluidNote for playing a note on a FluidSynth engine's MIDI channel, fluidCCi for sending a controller message at i-time to a FluidSynth engine's MIDI channel, fluidCCk for sending a controller message at k-rate to a FluidSynth engine's MIDI channel. fluidControl for playing and controlling loaded Soundfonts (using 'raw' MIDI messages), fluidOut for receiving audio from a single FluidSynth engine, and fluidAllOut for receiving audio from all FluidSynth engines.

These opcodes can also use soundfonts to generate sound. sfplay etc. were created for one purpose -- to use the samples in SoundFonts. The fluid opcodes were created for another purpose -- to use SoundFonts more or less the way they were designed to be used, i.e. using keyboard mappings, layers, internal processing, etc.