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Carry a re-engraved slice's notation in the bundle

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A replaced slice shipped as pixels only, so the LilyPond behind it died
with the project file — and a project is spent once exported. Correcting
one wrong note meant retyping the system.

Slices gain an optional `engraving` object: the language, key, time and
one entry per voice holding the notes and lyrics verbatim. Structured per
staff rather than one blob of source, because that is what both a later
edit and a MIDI render want; song-level key and time are resolved per
slice so reading one needs no context from its neighbours.

Additive and ignorable, so the format stays at version 1.
This commit is contained in:
Esa Kataja
2026-07-29 11:42:44 +03:00
parent 630541c0cd
commit cf1344c5bf
3 changed files with 135 additions and 3 deletions
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docs/bundle-format.md
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@@ -14,7 +14,8 @@ that writes or reads one.
**Slice** — one *system* of music: a single line spanning all voices, typically
four to twelve bars, with lyrics intact. A slice is the atomic unit of the
format. Slices are images and carry no notion of pitch, voice or beat.
format. A slice is presented as an image; a slice that was engraved rather than
scanned may also carry the notation it was engraved from.
**Marker** — a semantic annotation attached to a slice, describing a navigational
feature printed in the score: a rehearsal letter, a repeat, a jump.
@@ -112,6 +113,7 @@ Each entry of `slices` is an object:
|---|---|---|---|
| `file` | string | required | Name of the image entry in the archive. |
| `markers` | array | optional | Markers on this slice. Omitted when there are none. |
| `engraving` | object | optional | The notation this slice's image was engraved from, when it was engraved rather than scanned. See [Engraving](#engraving). |
**The array order is the reading order of the score.** It is the only ordering
the format defines. Filenames often sort into the same order, but a consumer
@@ -158,6 +160,75 @@ One specific hazard is worth naming, because it is silent: an image pipeline
that *discards* the alpha channel rather than compositing it will turn every
slice into a solid black rectangle, since the colour channels are all zero.
## Engraving
Most slices are photographs of print: an image and nothing more. A slice that
was *engraved* — set from notation rather than scanned — can carry the notation
it came from, in an `engraving` object.
```json
{
"file": "007.webp",
"engraving": {
"lang": "lilypond",
"key": "aes",
"time": "4/4",
"print_time": false,
"voices": [
{ "clef": "treble", "notes": "c4 des ees f | ees2. r4", "lyrics": "Kai -- paa -- va sy -- dän" },
{ "clef": "treble_8", "notes": "aes,4 aes aes aes | aes2. r4" },
{ "clef": "bass", "notes": "aes,4 ges f ees | aes2. r4" }
]
}
}
```
| Field | Type | | |
|---|---|---|---|
| `lang` | string | required | The notation language. `"lilypond"` is the only value defined by this version. |
| `voices` | array | required | One entry per staff, in the order they are printed top to bottom. At least one. |
| `key` | string | optional | Key signature, in `lang`'s spelling. For `lilypond`, the tonic of the major spelling: `"aes"`, `"c"`, `"fis"`. |
| `time` | string | optional | Time signature, as `"4/4"`. |
| `print_time` | boolean | optional | Whether the time signature is printed on this system. Default `false`. |
Each entry of `voices`:
| Field | Type | | |
|---|---|---|---|
| `notes` | string | required | The music for this staff, verbatim in `lang`. |
| `clef` | string | optional | `"treble"`, `"treble_8"`, `"alto"`, `"bass"`. Default `"treble"`. |
| `lyrics` | string | optional | The words under this staff, verbatim in `lang`. Omitted when the staff has none. |
Three properties make this worth carrying:
- **It is the source, not a transcription.** The image was engraved from exactly
these strings. A consumer that re-engraves them gets the same system back.
- **It is editable.** A wrong note can be corrected here and the slice engraved
again, which a raster image does not allow.
- **It is playable.** `voices` are separated per staff with pitches, durations
and a key, so the passage can be sounded — a practice track, a click, a
pitch reference — without anyone reading the image.
`notes` and `lyrics` are opaque to this format. They are whatever `lang` accepts,
including constructs the fields above say nothing about: slurs, dynamics,
tuplets, and the tie idioms that carry a note across a slice boundary. A consumer
that does not speak `lang` must pass them through unaltered or ignore them, never
attempt to repair them.
An `engraving` **describes the slice above it, not the whole song**. Each is
self-contained: `key` and `time` are stated per slice, so nothing has to be
inherited from a neighbour or from the bundle. Slices without an `engraving` are
scanned, and the two kinds mix freely within one score — re-engraving a single
ruined system is the ordinary case.
A consumer that only presents the score can ignore `engraving` entirely. The
image is always the authority on what the slice looks like; where an image and
its engraving disagree, the image is what the producer intended to be read.
Notation languages other than `lilypond` may be added by future versions. A
consumer should ignore an `engraving` whose `lang` it does not know, and present
the slice image as it would any other.
## Markers
A marker annotates the slice it appears on.
@@ -215,7 +286,9 @@ ignore markers it does not understand rather than reject the bundle.
`v` is an integer that increases when a change would break an existing consumer.
Additions that a consumer can safely ignore — new optional fields, new marker
types — do not increase it.
types, new `engraving` languages — do not increase it. `engraving` was added
this way: a bundle carrying one is still a version 1 bundle, and a consumer that
has never heard of it presents the score unchanged.
A consumer should refuse a bundle whose `v` it does not recognise rather than
attempt to interpret it.
@@ -228,6 +301,8 @@ A consumer is advised to check:
- `title` is present and non-empty; `slices` is a non-empty array.
- Every `file` names an entry present in the archive.
- Every `destination` is within the bounds of `slices`.
- Every `engraving` has a `lang` and a non-empty `voices`; unknown `lang` values
are ignored rather than rejected.
- Archive entry names contain no path separators, no `..`, and no absolute
paths, as with any archive from an untrusted source.
@@ -272,7 +347,12 @@ song.zip
{ "file": "004.webp" },
{ "file": "005.webp" },
{ "file": "006.webp" },
{ "file": "007.webp" },
{ "file": "007.webp", "engraving": { "lang": "lilypond", "key": "aes", "time": "4/4",
"voices": [ { "clef": "treble",
"notes": "c4 des ees f | ees2. r4",
"lyrics": "Kai -- paa -- va sy -- dän" },
{ "clef": "bass",
"notes": "aes,4 ges f ees | aes2. r4" } ] } },
{ "file": "008.webp", "markers": [ { "type": "coda" } ] },
{ "file": "009.webp" }
]