Modchart Editor FnF

The Modchart Editor FnF provides a domain-specific tool for designing, validating, and exporting rhythm charts for Friday Night Funkin' mods. Primary users are indie developers, modders, and educators prototyping rhythm level design. It addresses the need to define song tempo maps, note timing, and lane placement in a portable, game-ready data format, enabling rapid iteration without coding runtime logic. It targets charts that must be shared, version-controlled, and integrated into FnF mod pipelines.
The tool models a modchart as structured data with metadata, a tempo map, and a sequence of notes across lanes. Required features include tempo map, note definitions with beat and lane, boundary validation, and a clean JSON export. Optional/advanced features: tempo changes within a chart, hold notes, arcs and chords, import templates, lane presets, and schema validation against common FnF mod formats. It computes the chart duration from the tempo map and enforces timing consistency across changes. It supports standard 4-lane charts and can accommodate 3- or 5-lane variants.
Inputs include track length (seconds or beats), tempo map (list of tempo changes with beat and bpm), lane count, notes (beat, lane, type, duration), and optional metadata (title, author). Outputs are a JSON object containing the modchart, tempo map, notes, holds, and metadata. The export translates beat-based timings into timestamps using the tempo map and validates data integrity.
Core calculations convert beat positions to times using piecewise-constant BPM segments, handle tempo changes, and validate that notes lie within 0–duration. The engine sorts tempo events, clamps durations, and detects overlapping notes. If tempo map is missing, a default BPM is applied; if notes fall outside, they are flagged for correction.
Missing required inputs or malformed tempo maps produce export errors with codes. Out-of-range lanes, negative durations, zero BPM, or notes beyond chart duration trigger precise errors. External data unavailability returns clear failure signals.