# The .mcap file

One capture in one file: a fixed magic, a file version stamp, the capture's data as
[Capture Text](capture_text.md), what OCR read in it, what other models added (chunks), a small preview,
then the images, then any large chunks. There is no ZIP and no compression around it; the images keep
their own PNG compression.

The parts a library browser needs (header, text, OCR, front chunks and preview) come first, so it reads
the start of each file and never touches the images; a search, which needs no preview, reads less still.

## Layout, version 3

All integers are little-endian. Sizes count bytes.

| Field | Type | |
| --- | --- | --- |
| magic | 8 bytes | `89 4D 43 41 50 0D 0A 1A`, which is `\x89MCAP\r\n\x1A`. Says this is an .mcap, in every version. |
| file version | u32 | `3`. Versions 1 and 2 are still read; see below. |
| text size, text | u32, bytes | Capture Text, UTF-8: every JSON file of the capture, as `[[file]]` sections. |
| OCR size, OCR | u32, bytes | OCR text, UTF-8 (below): what each OCR model read. Size 0 when none has. |
| front chunks | chunk list | Small things models added, such as labels and an image vector (below). |
| preview width, height | u16, u16 | In pixels. |
| preview size, preview | u32, bytes | PNG. Size 0 when there is none. |
| original name size, name | u16, bytes | The original image's name in the .snagx, such as `{GUID}.png`; the page's `CaptureBackgroundImage` names it too. |
| original size, original | u32, bytes | The capture as taken: PNG, exactly as it was in the .snagx. |
| rendered kind | u8 | 0: none. 1: the same as the original (stored once). 2: its own, below. |
| rendered size, rendered | u32, bytes | Only for kind 2. The capture with its annotations drawn in (Snagit's `thumbnail.png`): PNG, exactly as it was. |
| back chunks | chunk list | Large things models added, such as segmentation masks: after the images, so reading the front never reaches them. |

The file ends there: anything after the last part is a mistake.

The magic borrows PNG's: the first byte has its high bit set, so a channel that strips it shows; `\r\n`
shows line endings being converted; and `\x1A` stops `type` on Windows.

A **chunk list** is a u32 count, then each chunk: a 4-byte tag of ASCII letters saying what it is
(`labl`, `embd`), a u32 size, and that many bytes: a u16 key size, the key (whose it is: a model's
name), and the chunk's own data. A file holds at most one chunk per tag and key.

**Version 2** is version 3 without the two chunk lists; **version 1** is version 2 without the OCR size
and OCR. A reader takes all three; anything written is version 3, so an older file becomes version 3
the first time something is stored in it.

## The parts

- **Text.** Everything the capture's JSON held: `index.json`, `metadata.json`, the page's `{GUID}.json`,
  and `{GUID}.backup.json` when there is one.
- **OCR.** One section per OCR model, so a capture read by several models keeps every reading;
  reading it again with a model replaces only that model's section. See *OCR text* below.
- **Chunks.** What other models add, each keyed by the model's name, so models do not overwrite each
  other; storing a chunk again replaces only the one with its tag and key. A chunk a reader does not
  know is kept as it is when the file is rewritten, so a later build can add kinds without a new file
  version. See *Label chunks* below.
- **Preview.** The rendered image (or the original, when there is no rendered one) scaled so its longest
  side is at most 320 pixels, as a PNG. An image already that small is used as it is, never scaled up.
- **Original and rendered.** A capture with no annotations has a rendered image identical to the original,
  byte for byte; it is then stored once, as kind 1.
- **One page.** An .mcap holds one page, as every capture so far does; a .snagx with more is not converted.
- **Left out:** `{GUID}.backup.png`, Snagit's image from before an edit. Its `{GUID}.backup.json` is kept,
  as plenty of captures have one without the image.

## Versions

A reader checks the magic, then the file version. It refuses a version above its own, never writing
over a file from a later version, and reads every version up to its own. Version 0 does not exist.

## Converting

`snagit-converter mcap` converts .snagx files (or extracted captures) to .mcap, and `snagit-converter
snagx` converts .mcap files back to a .snagx Snagit can open. See the README for what each checks.

Back in a .snagx, the JSON is laid out as Snagit writes it, the images are the same bytes, and only the
backup image is missing. JSON numbers come back as values: `3.0` is written `3`.

## OCR text

In the style of the Ex4 text formats:

```
@version 2

[ocr ppocr-v5-mobile]
engine: "onnxruntime 1.30.0, CUDA"
image:  rendered 1920 1080
date:   2026-10-08T21:04:11Z
line: 412 96 640 18 412 96 1052 96 1052 114 412 114 0.993 "Assign Footprints - KiCad"
line: 20 140 220 14 20 140 240 140 240 154 20 154 0.981 "Symbol : Footprint Assignments"
line: 1700 300 20 160 1720 300 1720 460 1700 460 1700 300 0.952 "Read downwards"
```

| Key | Value |
| --- | --- |
| `[ocr NAME]` | A section: the model's name, as in its `models.config`. |
| `engine` | What ran it, quoted. |
| `image` | Which image was read (`rendered` or `original`) and its width and height; the lines' boxes are in its pixels. |
| `date` | When it was read, UTC. |
| `line` | A text line: x, y, width and height, then its four corners as x y pairs, its confidence (0 to 1), and its text, quoted with `""` for a quote. A control character in the text is written as a space. |

The corners go clockwise from the text's top left: top left, top right, bottom right, bottom left, as
the text reads. Level text has the box's corners; a turned line's say which way it reads (the third
line above reads downwards, so its top left is the box's top right). x, y, width and height bound them.

A line that cannot be read is skipped, as are keys this version does not know. **Version 1** OCR text
had no corners (`line: x y width height confidence "text"`); it is still read, with the box's corners.
The OCR text carries its own version, so this did not change the .mcap file version. `mcap_store_ocr`
(`modules/Capture_File/ocr_text.jai`) stores a section in a file.

## Label chunks

A label model (an image-text encoder such as SigLIP 2) adds two front chunks, keyed by its name.

**`labl`**: the capture's best labels from a label list, as text in the style above:

```
@version 1
engine: "onnxruntime 1.30.0, CUDA"
date:   2026-10-08T21:04:11Z
list:   9c4f0b2d81e3a675
label: 0.6720 "Settings"
label: 0.0951 "Installer"
```

| Key | Value |
| --- | --- |
| `engine`, `date` | As in OCR text. |
| `list` | Which label list they were scored against: a 64-bit FNV-1a hash of its names and prompts. Labels from another list are out of date. |
| `label` | A score, 0 to 1 (the model's own sigmoid; small, but their order is what counts), and the label's name, quoted with `""` for a quote. Best first. |

**`embd`**: the image's vector, binary: a u32 length, a u8 format (1: 16-bit floats), then the values,
little-endian. It is a unit vector, so any phrase's vector (from the same model's text encoder) scores
against it with a dot product: labels from a new list, or a search by meaning, need no new pass over
the images. 768 values, 1.5 KB, for SigLIP 2 base.

`mcap_store` (`modules/Capture_File/module.jai`) stores an OCR section and chunks in one rewrite;
`label_chunks.jai` reads and writes these two.