REAPER JSFX Core
Foundation skill for writing JSFX audio plugins in REAPER. This skill covers the EEL2 language, code structure, and performance optimization patterns essential for any JSFX plugin.
Always load this skill when working with JSFX. Other JSFX skills (audio, synth, midi, ui) build upon this foundation.
Rules
| Rule | Description |
|---|---|
| language | EEL2 syntax, operators, math functions, loops, strings, user-defined functions |
| sections | Code sections (@init, @slider, @block, @sample, @gfx, @serialize) and execution flow |
| variables | Special variables, slider definitions, transport state, PDC, shared memory |
| performance | CPU optimization, denormal handling, branchless patterns, memory efficiency |
| libraries | Creating and using .jsfx-inc include files, namespacing, code organization |
Official Documentation
Key Principles
1. Performance First
JSFX runs in real-time audio context. The @sample section executes thousands of times per second (e.g., 44,100 times/sec at 44.1kHz). Every operation matters:
// BAD: Branch in sample loop
@sample
condition ? (
spl0 *= gain1;
) : (
spl0 *= gain2;
);
// GOOD: Branchless - compute gain once in @slider
@slider
gain = condition ? gain1 : gain2;
@sample
spl0 *= gain;
2. Precompute in Appropriate Sections
Move calculations to the earliest possible section:
| Section | Runs | Use For |
|---|---|---|
@init | Once on load/reset | Memory allocation, lookup tables, constants |
@slider | On parameter change | Coefficient calculation, derived values |
@block | Per audio block (~1-10ms) | Tempo sync, MIDI preprocessing |
@sample | Per sample (44100+/sec) | Only essential sample processing |
3. Variables Are Global by Default
EEL2 variables don't need declaration and are global. Use meaningful names and conventions:
// Slider-derived values: prefix with slider name
slider1:volume_db=0<-60,12,0.1>Volume (dB)
@slider
volume_db_lin = 10^(volume_db/20); // Linear gain from dB
// State variables: descriptive names
filter_z1 = 0; // Filter state (z^-1)
env_current = 0; // Envelope current value
// Constants: UPPER_CASE or prefix
TWO_PI = 2 * $pi;
MAX_DELAY_SAMPLES = 192000;
4. Use Memory for Buffers, Variables for State
@init
// Buffers in memory (addressable array)
delay_buffer = 0; // Start at memory offset 0
delay_length = 48000;
freembuf(delay_buffer + delay_length); // Inform memory manager
// State in named variables (faster access)
delay_write_pos = 0;
delay_read_pos = 0;
@sample
// Memory access for buffer
delay_buffer[delay_write_pos] = spl0;
// Variable access for state
delay_write_pos += 1;
delay_write_pos >= delay_length ? delay_write_pos = 0;
Quick Reference
| Task | Rule |
|---|---|
| Understanding EEL2 syntax | language |
| Structuring plugin code | sections |
| Using sliders and special vars | variables |
| Optimizing CPU performance | performance |
| Creating reusable code | libraries |
Minimal Plugin Template
desc:My Effect
tags:effect utility
slider1:gain_db=0<-24,24,0.1>Gain (dB)
in_pin:left input
in_pin:right input
out_pin:left output
out_pin:right output
@init
// Initialize state here
@slider
// Recalculate coefficients when sliders change
gain_lin = 10^(gain_db/20);
@sample
// Process audio
spl0 *= gain_lin;
spl1 *= gain_lin;