MIDI Piano Roll Renderer

When to Use

• A MIDI file must be turned into a piano-roll picture for reading, teaching, or sharing.
• The user wants to see a song's structure: melody vs. accompaniment, chord progression, key, tempo.
• Choosing between a vertical falling-notes layout (default — easiest to play along, time flows down, inspired by Synthesia) and a horizontal DAW-style roll (option for wide screens / long phrases).

Turns any MIDI file into a polished piano-roll PNG: colored note blocks with note letters inside, a large chord track, automatic key + tempo detection, and a chromatic color legend. Two orientations are supported: vertical (time ↓ down, Synthesia-style falling notes — default) and horizontal (time → right, classic DAW style).

Requirements

• Python 3.8+ with mido and Pillow (pip install mido pillow)
• A bold monospace TTF for labels — the scripts use consolab.ttf (Windows Consolas Bold) and silently fall back to PIL's default font elsewhere

Usage

# vertical roll (time ↓ down, portrait) — DEFAULT, best for playing along
python scripts/render_vertical.py song.mid song_vertical.png

# horizontal roll (time → right, DAW-style) — option for wide screens / long phrases
python scripts/render_any.py song.mid song_roll.png

Common options (both scripts):

Example:

# Skip a sparse 10-bar intro, start at the main groove, force track roles
python scripts/render_any.py mario.mid mario_roll.png --mel 1 --acc 6 --start 10

What it does automatically

• Track picking — melody = the note-rich track with the highest average pitch; accompaniment = the note-richest remaining track. Override with --mel / --acc when a MIDI has odd instrumentation (e.g., a percussion track with a higher average pitch).
• Key detection — correlates the duration-weighted pitch-class profile against major/minor templates, then annotates the key signature accidental count, e.g. Key: Gm (bb), Key: C# (#######).
• Chord labels — per-bar weighted pitch-class template matching (triads, 7ths, maj7, sus2/sus4). Consecutive identical chords are merged into one label. Labels are anchored at the chord's entry beat, not centered over the merged span.
• Bar count (vertical script) — measures the shortest note in the window and guarantees it at least ~30 px tall by stretching the canvas; never renders fewer than 4 bars. With an explicit --bars, the canvas keeps a 4:3-derived height instead.
• Note letters — every melody note gets its name (C, D#, …) inside the block; very short notes get an auto-shrunk font so the letter always fits.
• Adaptive legend — the 13-swatch chromatic legend and the metadata line auto-shrink to fit narrow canvases.

Rendering conventions (dark theme)

• One bright palette color per pitch class (12 colors), accompaniment in muted steel blue.
• Black-key lanes darker than white-key lanes; thin dark lines separate every semitone row; bold white lines mark octaves (labeled C2…C7) and bar lines.
• Legend strip is always drawn below the grid, never overlapping it.
• Chord labels in bold yellow, large (110–150 px): top banner (horizontal) or left column (vertical).

Known limitations

• The chord/key heuristics are simple template matches: fast runs and chromatic passing tones can produce wrong labels (e.g., Emaj7 for a plain C major passage) and enharmonic oddities (A#m with 7 sharps instead of the conventional Bbm). Labels are advisory — hand-correct for publication.
• Percussion tracks can win the melody pick on sparse files; check the printed melody=<idx> line and override if needed.
• MIDI files with very long silent intros render nearly empty — use --start to jump into the music.

Files

• scripts/render_vertical.py — vertical renderer (default; falling-notes style, best for playing)
• scripts/render_any.py — horizontal renderer (DAW-style alternative)

Both are standalone (stdlib + mido + Pillow) and safe to copy into any project.

Example images

• assets/tetris_vertical.png — vertical: Tetris (Korobeiniki) theme, 4 bars, A minor 80 BPM
• assets/september_horizontal.png — horizontal: Wake Me Up When September Ends, 8 bars, G major 104 BPM