Musa Mahmood
e0eaf5642b
- A recording killed mid-way had wrong timestamps in players: its fragmented MP4's header gives a length of 0, and one cut short never gets the index a clean stop writes. modules/Screen_Record's record_remux now copies every recording's packets, as they are, into a regular MP4 (one header first with the length and every frame's index; faststart) before it goes into the library; should that fail, the fragmented MP4 goes in as it was. - Keyframes were every 60 frames, which, with frames written only on change, could span minutes of time, and a crash could lose a fragment that long. One is now forced every 2 s of time (forced-idr for NVENC and libx264). - A crash's leftover no longer claims a region at 0, 0 in its capture.json. - record-probe -remux IN OUT. Tests: a recording cut at its last fragment, as a crash cuts it, goes in as a regular MP4 of the time before the cut. - Checked on the recording that showed it: its MP4, cut mid-fragment, rewrites to 76 frames (5.2 s) with positive frame durations. A 12 s still recording now fragments every ~2 s, and the app killed 10 s into a recording leaves 8.1 s.
286 lines
16 KiB
Markdown
286 lines
16 KiB
Markdown
# The .mcap file
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One capture in one file: a fixed magic, a file version stamp, the capture's data as
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[Capture Text](capture_text.md), what OCR read in it, what other models added (chunks), a small preview,
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then the original and rendered image, then any large chunks. There is no ZIP and no compression around
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it; the images and videos keep their own compression.
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A capture is a Snagit capture (from a `.snagx`), an imported image (a photo or screenshot in any image
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format), an imported video (the mcap app's Import window), or a screen capture or recording (the mcap app's
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PrintScreen and Ctrl+PrintScreen).
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The parts a library browser needs (header, text, OCR, front chunks and preview) come first, so it reads
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the start of each file and never touches the images; a search, which needs no preview, reads less still.
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## Layout, version 4
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All integers are little-endian. Sizes count bytes.
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| Field | Type | |
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| --- | --- | --- |
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| 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. |
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| file version | u32 | `4`. Versions 1 to 3 are still read; see below. |
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| text size, text | u32, bytes | Capture Text, UTF-8: every JSON file of the capture, as `[[file]]` sections. |
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| OCR size, OCR | u32, bytes | OCR text, UTF-8 (below): what each OCR model read. Size 0 when none has. |
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| front chunks | chunk list | Small things models added, such as labels and an image vector (below). |
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| preview width, height | u16, u16 | In pixels. |
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| preview size, preview | u32, bytes | PNG. Size 0 when there is none. |
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| original name size, name | u16, bytes | The original's name: in the .snagx, such as `{GUID}.png` (the page's `CaptureBackgroundImage` names it too), or an imported file's name, such as `IMG_0001.HEIC` or `IMG_4022.MOV`. |
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| original size, original | u64, bytes | The capture as taken, exactly as it was: PNG from a .snagx; an imported image or video in its own format (JPEG, HEIC, WebP, RAW, MP4, MOV, MKV ...), told by its first bytes, not its name. |
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| rendered kind | u8 | 0: none. 1: the same as the original (stored once). 2: its own, below. |
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| 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. A video's is a frame from it, as PNG (below). |
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| back chunks | chunk list | Large things models added, such as segmentation masks: after the images, so reading the front never reaches them. |
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The file ends there: anything after the last part is a mistake.
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The magic borrows PNG's: the first byte has its high bit set, so a channel that strips it shows; `\r\n`
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shows line endings being converted; and `\x1A` stops `type` on Windows.
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A **chunk list** is a u32 count, then each chunk: a 4-byte tag of ASCII letters saying what it is
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(`labl`, `embd`), a u32 size, and that many bytes: a u16 key size, the key (whose it is: a model's
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name), and the chunk's own data. A file holds at most one chunk per tag and key.
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**Version 3** is version 4 with the original's size a u32, so an original of at most 4 GB; **version 2**
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is version 3 without the two chunk lists; **version 1** is version 2 without the OCR size and OCR. A
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reader takes all four; anything written is version 4, so an older file becomes version 4 the first time
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something is stored in it.
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An original can be larger than memory, so a file is read and rewritten with its original left on disk:
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`mcap_read_without_original` reads everything else and says where the original lies, and
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`mcap_write_streamed` copies it across a few MB at a time. Storing what a scan adds works that way.
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## The parts
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- **Text.** Everything the capture's JSON held: `index.json`, `metadata.json`, the page's `{GUID}.json`,
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and `{GUID}.backup.json` when there is one. An imported image has two files instead (below).
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- **OCR.** One section per OCR model, so a capture read by several models keeps every reading;
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reading it again with a model replaces only that model's section. See *OCR text* below.
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- **Chunks.** What other models add, each keyed by the model's name, so models do not overwrite each
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other; storing a chunk again replaces only the one with its tag and key. A chunk a reader does not
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know is kept as it is when the file is rewritten, so a later build can add kinds without a new file
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version. See *Label chunks* below.
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- **Preview.** The rendered image (or the original, when there is no rendered one) scaled so its longest
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side is at most 320 pixels, as a PNG. A Snagit PNG already that small is used as it is, never scaled
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up. An imported image's preview is always a PNG made from it, turned the right way up by its EXIF
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orientation, which a reader applies to the original too.
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- **Original and rendered.** A capture with no annotations has a rendered image identical to the original,
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byte for byte; it is then stored once, as kind 1.
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- **One page.** An .mcap holds one page, as every capture so far does; a .snagx with more is not converted.
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- **Left out:** `{GUID}.backup.png`, Snagit's image from before an edit. Its `{GUID}.backup.json` is kept,
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as plenty of captures have one without the image.
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## Imported images
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An image imported from a folder has no rendered image (kind 0), and its Capture Text holds two files:
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```
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[[file "metadata.json"]]
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AppName: "Apple iPhone 15 Pro" the camera or phone, from EXIF; "" when unknown
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CaptureDate: "2024-05-12 10:15:30" when it was taken, local time, as Snagit writes it
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WindowName: "IMG_4021.HEIC" the file's name
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[[file "import.json"]]
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SourcePath: "E:/Phone/DCIM/IMG_4021.HEIC" where it was imported from
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SourceSize: 2994394
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SourceModified: "2024-05-12 10:15:31"
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ContentHash: "d3fa07db58597cec" 64-bit FNV-1a of the original's bytes
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Format: "HEIF"
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Width: 3024 as shown: after the orientation is applied
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Height: 4032
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DateFrom: "EXIF" or "file name", or "modified time"
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ImportDate: "2026-10-08 22:28:17"
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Orientation: 6 and, when the EXIF has them: CameraMake, CameraModel,
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Software, TimeZoneOffset, Latitude, Longitude, Altitude
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```
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So the parts a library reads from Snagit's `metadata.json` (date, title, app) work for photos too.
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## Screen captures
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A region of the screen taken with the mcap app's PrintScreen is stored as a PNG original with no
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rendered image (kind 0), and its Capture Text holds two files:
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```
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[[file "metadata.json"]]
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AppName: "Google Chrome" the program of the window under the region's middle:
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its description, else its file's name; "" when unknown
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CaptureDate: "2026-10-10 14:05:09" when it was taken, local time, as Snagit writes it
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WindowName: "Assign Footprints - KiCad" that window's title
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[[file "capture.json"]]
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Region: "selection" or "screen": a click took the whole screen
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Left: -1920 on the virtual desktop, whose origin is the primary
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Top: 40 screen's top left, so it can be negative
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Width: 700
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Height: 300
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AppPath: "C:/Program Files/Google/Chrome/Application/chrome.exe"
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ContentHash: "5c1e9a0d77b2f413" 64-bit FNV-1a of the PNG's bytes
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Format: "PNG"
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```
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## Screen recordings
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A recording made with the mcap app's Ctrl+PrintScreen is a video: its original is the H.264 MP4, and
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its rendered image (kind 2) a frame from it as a PNG, as for an imported video. It is recorded as a
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fragmented MP4, so a crash still leaves one that plays, and stored as a regular one, its frames copied
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as they are: one header first, with its length and the index of every frame. Its
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metadata.json is a still's. Its capture.json has a still's keys, with Format "MP4", and these:
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```
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Duration: 12.48 seconds
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FrameRate: 30 as the container says; frames are written only when the screen changes
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VideoCodec: "h264"
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PosterTime: 1.25 seconds into it of the rendered frame
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Encoder: "h264_nvenc" FFmpeg's encoder: h264_nvenc, h264_amf, h264_qsv, h264_mf or libx264
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Capture: "DXGI DuplicateOutput1 on NVIDIA GeForce RTX 5080 Laptop GPU"
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or "Windows.Graphics.Capture on ..., border off"
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DroppedFrames: 0 frames the encoder fell too far behind for
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Paused: 10.2 seconds paused, left out of the video; only when it was paused
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```
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A recording a crash cut short, put in the library at the next start, has no window, Encoder or
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Capture: its Width and Height are its frames', and its CaptureDate is from its file's name.
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A reader tells a recording from a still by VideoCodec, as it tells an imported video by its import.json.
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## Imported videos
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A video imported from a folder (MP4, MOV, MKV, WebM, AVI, WMV, MPEG; read with FFmpeg, on Windows) is
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stored as it was in the original. Its rendered image (kind 2) is a frame from it as a PNG, at its own
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size and turned the right way up: a tenth of the way in, at most 3 seconds, past the black or blurred
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frames phones often start with. The preview is made from that frame, and OCR and labels read it, as
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they read a Snagit capture's rendered image; nothing reads the video itself but the import.
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Its metadata.json is a photo's. Its import.json has the keys above (Format is the container: "MP4",
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"QuickTime", "WebM" ...; Width and Height as shown) and these:
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```
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Duration: 83.4 seconds
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FrameRate: 30
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VideoCodec: "h264" FFmpeg's name for it: "hevc", "vp9", "av1" ...
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Rotation: 90 degrees clockwise its frames are turned to be shown
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PosterTime: 3 seconds into it of the rendered frame
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Audio: true
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DateFrom: "metadata" Apple's creationdate (local time), else the file name, else the
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container's creation_time (UTC, as this computer's local time), else
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the modified time
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and, when the container has them: CameraMake, CameraModel, Software,
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TimeZoneOffset, Latitude, Longitude, Altitude
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```
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An animated GIF is imported as an image; its import.json adds `Frames` and `Duration` (seconds).
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## Versions
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A reader checks the magic, then the file version. It refuses a version above its own, never writing
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over a file from a later version, and reads every version up to its own. Version 0 does not exist.
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## Converting
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`snagit-converter mcap` converts .snagx files (or extracted captures) to .mcap, and `snagit-converter
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snagx` converts .mcap files back to a .snagx Snagit can open. See the README for what each checks.
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The mcap app's Import makes the same .mcap from a .snagx (modules/Snagx). Only captures from a .snagx
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convert back: an imported image has no Snagit JSON to make one from.
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Back in a .snagx, the JSON is laid out as Snagit writes it, the images are the same bytes, and only the
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backup image is missing. JSON numbers come back as values: `3.0` is written `3`.
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## OCR text
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In the style of the Ex4 text formats:
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```
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@version 2
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[ocr ppocr-v5-mobile]
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engine: "onnxruntime 1.30.0, CUDA"
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image: rendered 1920 1080
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date: 2026-10-08T21:04:11Z
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line: 412 96 640 18 412 96 1052 96 1052 114 412 114 0.993 "Assign Footprints - KiCad"
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line: 20 140 220 14 20 140 240 140 240 154 20 154 0.981 "Symbol : Footprint Assignments"
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line: 1700 300 20 160 1720 300 1720 460 1700 460 1700 300 0.952 "Read downwards"
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```
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| Key | Value |
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| `[ocr NAME]` | A section: the model's name, as in its `models.config`. |
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| `engine` | What ran it, quoted. |
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| `image` | Which image was read (`rendered` or `original`) and its width and height; the lines' boxes are in its pixels. |
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| `date` | When it was read, UTC. |
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| `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. |
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The corners go clockwise from the text's top left: top left, top right, bottom right, bottom left, as
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the text reads. Level text has the box's corners; a turned line's say which way it reads (the third
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line above reads downwards, so its top left is the box's top right). x, y, width and height bound them.
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A line that cannot be read is skipped, as are keys this version does not know. **Version 1** OCR text
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had no corners (`line: x y width height confidence "text"`); it is still read, with the box's corners.
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The OCR text carries its own version, so this did not change the .mcap file version. `mcap_store_ocr`
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(`modules/Capture_File/ocr_text.jai`) stores a section in a file.
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## Label chunks
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A label model (an image-text encoder such as SigLIP 2) adds two front chunks, keyed by its name.
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**`labl`**: the capture's best labels from a label list, as text in the style above:
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```
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@version 1
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engine: "onnxruntime 1.30.0, CUDA"
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date: 2026-10-08T21:04:11Z
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list: 9c4f0b2d81e3a675
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label: 0.6720 "Settings"
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label: 0.0951 "Installer"
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```
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| Key | Value |
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| --- | --- |
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| `engine`, `date` | As in OCR text. |
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| `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. |
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| `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. |
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**`embd`**: the image's vector, binary: a u32 length, a u8 format (1: 16-bit floats), then the values,
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little-endian. It is a unit vector, so any phrase's vector (from the same model's text encoder) scores
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against it with a dot product: labels from a new list, or a search by meaning, need no new pass over
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the images. 768 values, 1.5 KB, for SigLIP 2 base.
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`mcap_store` (`modules/Capture_File/module.jai`) stores an OCR section and chunks in one rewrite;
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`label_chunks.jai` reads and writes these two.
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## Segmentation chunks
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A segmentation model (Mask2Former, `modules/Segment`) adds two chunks, keyed by its name, in one
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rewrite (`mcap_store` with `front` and `back`), so a file has both or neither. Images only: a video's
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frame is not segmented.
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**`segs`**, in front: what is in the image, as text in the style above. Search and Stats read it.
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```
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@version 1
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engine: "onnxruntime 1.30.0, CUDA"
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date: 2026-10-12T09:30:00Z
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image: rendered 4032 3024
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classes: 5c1e9a0d77b2f413
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segment: 1 0.981 0.2140 1210 380 900 2210 "person"
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segment: 4 0.993 0.3105 0 0 4032 1100 "sky"
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```
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| Key | Value |
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| --- | --- |
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| `engine`, `date` | As in OCR text. |
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| `image` | Which image was read (`rendered` or `original`) and its size, as shown (turned by its EXIF orientation). The boxes are in its pixels. |
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| `classes` | A 64-bit FNV-1a hash of the model's class list: segments from another list are out of date. |
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| `segment` | Its id in the mask, its score (0 to 1), its area (a fraction of the image), its box (x y width height), and its class as people search it ("wall", not COCO's "wall-other-merged"), quoted with `""` for a quote. Largest first. |
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**`mask`**, at the back: which segment each pixel is in, for drawing. Binary:
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| Field | Type | |
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| --- | --- | --- |
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| format | u8 | `1`: run-length encoded. |
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| width, height | u16, u16 | The map's: the image's shape, its longest side at most 1024. |
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| id bits | u8 | `8`. |
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| runs | | To the end of the chunk, row by row, wrapping from one row to the next: a u8 segment id (0: none) and its length, LEB128 (7 bits a byte, low bits first, the top bit set on every byte but the last). The runs cover the map exactly. |
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On 27 photos and frames, a mask was 0.8 to 6.4 KB and the `segs` text 200 to 400 bytes.
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`mcap_erase` with both tags takes them out together. `segment_chunks.jai` reads and writes them.
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