Video editing#

Every graphical machine has a video editor, each the platform's own and each an interface over one deterministic engine, crates/video (cw-video):

The interface code is crates/applications/src/apps/video/; its controls are listed in action-families.md.

Media formats#

Everything the editors read and write is a public format another program opens.

Movies are Animated PNG (APNG, the acTL/fcTL/fdAT extension registered in PNG Third Edition). The decoder (cw_video::apng) walks the chunks itself, checks every CRC, decodes each frame's compressed data with the png crate as a standalone image and composes frames with the file's own dispose_op and blend_op, so any conforming APNG plays as its author made it; frame durations accumulate as exact fractions, so a movie at 12 frames a second lasts exactly its frame count divided by 12. The encoder writes every frame at full size with APNG_BLEND_OP_SOURCE and a constant delay of 1/fps, 8-bit RGBA, looping forever. A browser plays an exported movie; a plain PNG viewer shows its first frame.

Sound is RIFF WAVE with linear PCM. Reading accepts WAVE_FORMAT_PCM (or WAVE_FORMAT_EXTENSIBLE carrying PCM) of 8, 16, 24 or 32 bits in any number of channels and reduces it to mono 16-bit, averaging the channels; it is resampled to the project rate (22 050 Hz) by exact integer linear interpolation. Writing produces canonical 16-bit mono PCM with a 44-byte header.

Stills are PNG (any colour type) and baseline or progressive JPEG (decoded with the renderer's scalar jpeg-decoder configuration). A still lasts as long as its clip.

Imported media is kept in the editor's state, so a snapshot carries it and a restored world shows the same frames: movie frames as QOI images (cw_video::qoi, the Quite OK Image format 1.0 — lossless and decodable in one pass, so the monitor can pull any frame at random), audio as little-endian PCM, both serialized as base64 behind shared reference counts. Import also stores up to eight 36-pixel-high thumbnails and a waveform of one peak per hundredth of a second, so a timeline draws without decoding anything. Pictures larger than 1280 pixels on the long edge are reduced on import, as an editor's optimized copy is; the file on disk is untouched.

The sample media#

Each machine's media folder is seeded (by scripts/build-live-world.mjs) with five files the engine generates deterministically in cw_video::samples and checks in under worlds/company-2026/files: Countdown.apng (a 3 s film-leader countdown), Color Bars.apng (2 s of 75% bars with a moving marker), Sunset.apng (3 s), Countdown Beeps.wav (a beep each second) and Music Bed.wav (4 s of chords), all 320×180 at 12 fps and 11 025 Hz mono. crates/video/tests/samples.rs fails if a checked-in file is not byte-for-byte what the generators write; regenerate with CW_UPDATE_SAMPLES=1 cargo test -p cw-video --test samples.

Project format#

A project is saved as <name>.cwvideo in the media folder: UTF-8 JSON, the serialization of cw_video::Project. Media is referenced by path, not embedded; opening a project reads every referenced file again and marks the ones that are gone as missing (their clips paint red and the monitor says "Missing media").

{
  "format": "computerworld-video-project",
  "version": 1,
  "name": "Trailer",
  "width": 320, "height": 180, "fps": 24, "sample_rate": 22050,
  "background": "000000",
  "media": [{"id": 3, "path": "Movies/Countdown.apng", "kind": "video"}],
  "tracks": [{"id": 1, "kind": "video", "name": "V1", "muted": false, "hidden": false, "locked": false}],
  "clips": [{
    "id": 7, "track": 1, "source": {"type": "media", "media": 3},
    "start": 0, "length": 72, "offset": 0, "speed": 100, "reverse": false,
    "opacity": {"value": 1000, "keys": [{"frame": 0, "value": 0, "ease": "ease"}, {"frame": 23, "value": 1000}]},
    "x": {"value": 0}, "y": {"value": 0}, "scale": {"value": 1000}, "rotation": {"value": 0},
    "volume": {"value": 100},
    "crop": {"left": 0, "top": 0, "right": 0, "bottom": 0},
    "grade": {"brightness": 0, "contrast": 0, "saturation": 0, "temperature": 0},
    "fade_in": 0, "fade_out": 12, "name": "Countdown.apng"
  }],
  "transitions": [{"id": 9, "left": 7, "right": 8, "kind": "cross_dissolve", "frames": 24}],
  "next_id": 10
}

• Time. Timeline positions (start, length, keyframe frames, fades, transition frames) are whole frames at fps. A clip's place in its source, offset, is in centiframes — hundredths of a timeline frame — so at any speed from 25% to 400% a clip advances a whole number of them per frame, and every trim, split and speed change is exact. The source shown at clip-local frame l is offset + l × speed, or offset + (length − 1 − l) × speed reversed.
• Sources are {"type": "media", "media": <id>}, {"type": "color", "color": "rrggbb"} or a title {"type": "title", "text", "size", "bold", "color", "background", "position": "top|center|lower|bottom"}.
• Properties. opacity and scale are thousandths (scale is of the size that fits the frame), x/y canvas pixels from the centre, rotation hundredths of a degree clockwise, volume percent (0–200). Each is a Param: a value, or keys with linear or ease (smoothstep) interpolation from each key to the next, in 16.16 fixed point. Crop is thousandths from each edge; the grade values are −100…100.
• Tracks composite bottom-up: the first video track is the bottom layer. Clips on a track never overlap; the loader refuses a file where they do, and any file whose ids, ranges or keyframes are inconsistent.
• Transitions sit on a cut between two clips that touch, centred on it: the outgoing clip plays on past its end and the incoming one starts early, for half the duration each (the nearest frame holds where the media runs out).

Rendering#

cw_video::Compositor::frame(f) is the only way a picture is made: the program monitor draws it and export encodes it, so the preview is the export. Per clip, the source frame is cropped, colour-corrected with the raster engine's adjustments (brightness/contrast, saturation via hue/saturation, temperature), fitted to the canvas, scaled, rotated and positioned by cw_raster::transform::affine (bilinear on premultiplied colour, 16.16 fixed point; an unscaled upright clip is copied pixel for pixel), and its opacity and fades applied. Tracks composite with source-over. Transitions mix the two clips' layers: cross dissolve in premultiplied colour, dip to black or white through the solid at the midpoint, a wipe revealing the incoming clip from the left, a slide bringing it in from the right. Titles are drawn from glyph coverage the renderer rasterises in the platform's own font (AppEffect::RasterText), so a title is exactly the renderer's text; a build without the renderer says so on the monitor rather than showing a blank.

Audio (cw_video::audio) sums every unmuted audio track sample for sample: a clip's samples are read at its speed (faster raises the pitch, as tape would), backwards when reversed, scaled by its volume keyframes and its fades, and clipped to 16 bits.

All arithmetic is integer or the raster engine's correctly rounded IEEE basics, so a native and a Wasm build produce identical frames, samples and files.

Playback and export#

Playback is driven by the world clock, never the host's: pressing play records the clock and the frame; the playhead at any later moment is from + elapsed × fps × rate, clamped to the programme, and it stops at either end. Time passes when the world's does — a sleep in a shell, network activity — so sleep 1.5 after Space plays 36 frames at 24 fps. J/K/L change the rate in steps of 1×, 2× and 4× either way.

Export writes <name>.apng and <name>.wav to the media folder at 90p, 180p or 360p and 12, 24 or 30 fps. It is encoded between actions: after every environment step, each application with background work (NativeApp::busy) gets one step (NativeApp::background), in which an export encodes up to four 320×180 frames' worth of pixels. Progress is the count of frames really encoded, so it is a function of steps taken and replays exactly. When the last frame is done the files are written; a re-imported export holds exactly the frames that were composited.

Feature matrix#

What each interface shows (the engine supports all of it; a product only offers what the real application does, and refuses the rest):

Known limits#

• Clips carry either pictures or sound: an APNG has no audio track, so a movie's sound is a separate WAV clip, not linked audio.
• Audio is mono; there is no pan, and speed changes the pitch (no time-stretching).
• The canvas is 320×180 at 24 fps; exports scale it to the chosen size.
• iMovie's volume keyframes, Clipchamp's filters and effects, and Kdenlive's full effect catalogue are not reproduced; each product shows only what is listed above.