oxideav-scene

A time-based composition model for oxideav: a Scene is a canvas populated with Objects (images, videos, text, shapes, audio cues) animated over a timeline. Scenes are the foundation for three distinct workloads:

1. Document layout — a PDF page is a single-frame scene with text, vector shapes, and image objects laid out in their native coordinate system. Edits (adding a watermark, moving an image, rewrapping a paragraph) happen on the scene, not on rasterised pixels, so text stays selectable and vectors stay crisp on re-export.
2. Live streaming compositor — a long-running scene fed by external operations (AddObject, MoveObject, FadeOut). Intended to sit behind an RTMP server so a remote control plane can drive a per-viewer overlay: add a lower-third during a goal, slide a logo in, trigger a sound effect.
3. Non-linear video editor (NLE) timeline — Premiere/Resolve-style multi-track editing. Tracks are ordered groups of scene objects, transitions are keyframed cross-fades / wipes, effects are filter chains attached to a single object.

Zero C dependencies — pure Rust, same rules as the rest of oxideav.

Status

Scaffold. This crate ships the type model + public-API shape for all three use cases and a placeholder SceneRenderer trait. No real rendering, encoding, or file-format I/O yet — those land as follow-ups.

• Scene, SceneObject, ObjectKind, Transform, Animation, Keyframe, Easing, AudioCue types are in place.
• SceneRenderer + SceneSampler traits are defined but return Error::Unsupported on every call.
• No oxideav-codec or container integration yet — that comes after the render pipeline is real.

Data model

Scene

pub struct Scene {
    pub canvas: Canvas,               // pixel dims OR a vector-coord PDF page
    pub duration: SceneDuration,      // Finite(dur) | Indefinite (streaming)
    pub time_base: TimeBase,          // rational — matches oxideav-core
    pub sample_rate: u32,             // audio rate for the mix bus
    pub background: Background,       // solid colour / image / gradient / transparent
    pub objects: Vec<SceneObject>,    // z-ordered painter's algorithm
    pub audio: Vec<AudioCue>,         // triggered by timeline position
    pub metadata: Metadata,           // author / title / colour-space hints
}

A scene is addressed in its own time_base — same rational type oxideav uses everywhere. SceneDuration::Indefinite signals a streaming scene: no end, no rewinding, the composition is driven forward by wall-clock time + operation messages.

Canvas

pub enum Canvas {
    /// Pixel-based raster canvas. NLE + streaming compositor use this.
    Raster { width: u32, height: u32, pixel_format: PixelFormat },
    /// Unit-agnostic vector canvas. PDF pages use this — the unit is
    /// whatever the producer declared (pt, mm, px). All coordinates
    /// inside the scene live in this unit; rasterisation happens at
    /// export time.
    Vector { width: f32, height: f32, unit: LengthUnit },
}

Keeping both raster and vector under one type lets the same SceneObject/Animation/Transform primitives drive PDFs, compositor streams, and NLE timelines without forking the API.

SceneObject

pub struct SceneObject {
    pub id: ObjectId,                 // stable across edits/operations
    pub kind: ObjectKind,             // what it IS
    pub transform: Transform,         // where it is, right now (base state)
    pub lifetime: Lifetime,           // [start, end) in scene time
    pub animations: Vec<Animation>,   // per-property keyframe tracks
    pub z_order: i32,                 // painter's algorithm tie-break
    pub opacity: f32,                 // 0.0..=1.0 base opacity
    pub blend_mode: BlendMode,        // normal, multiply, screen, …
    pub effects: Vec<Effect>,         // filter chain (blur, colour shift, …)
    pub clip: Option<ClipRect>,       // geometric clipping region
}

ObjectKind

pub enum ObjectKind {
    /// Static bitmap — PNG/JPEG/raw, decoded upstream into a VideoFrame.
    Image(ImageSource),

    /// Video stream — consumed as a `Packet` iterator + decoder. The
    /// scene's clock drives the stream's PTS; seeking is handled by
    /// the underlying demuxer if it supports it.
    Video(VideoSource),

    /// Styled text run. Preserves font / size / weight / colour
    /// metadata so PDF export can emit real text strings and NLE /
    /// compositor rasterise through a text-shaping backend.
    Text(TextRun),

    /// Vector shape — rect, rounded rect, polygon, bezier path.
    Shape(Shape),

    /// Container object. Applies its own `Transform` before children.
    Group(Vec<ObjectId>),

    /// Live feed from an external source (RTMP input, camera, etc.).
    /// Packets arrive asynchronously; the compositor uses the most
    /// recent frame available at render time.
    Live(LiveStreamHandle),
}

ImageSource / VideoSource / LiveStreamHandle own the heavy resources — e.g. a VideoSource holds a demuxer + decoder pair, so copying a SceneObject is cheap but cloning the underlying pixels requires Arc-shared frame storage (managed by oxideav-core).

Transform + Animation

pub struct Transform {
    pub position: (f32, f32),   // canvas units
    pub scale: (f32, f32),      // 1.0 = natural size
    pub rotation: f32,          // radians, around anchor
    pub anchor: (f32, f32),     // 0.0..=1.0 normalised pivot
    pub skew: (f32, f32),       // radians (Premiere-style)
}

pub struct Animation {
    pub property: AnimatedProperty,
    pub keyframes: Vec<Keyframe>,   // time-sorted
    pub easing: Easing,             // segment-level default
    pub repeat: Repeat,             // once / loop / ping-pong
}

pub enum AnimatedProperty {
    Position, Scale, Rotation, Opacity, Skew, Anchor,
    EffectParam { effect_idx: usize, param: &'static str },
    Custom(String),  // SceneObjectContent defines semantics
}

pub enum Easing {
    Linear, EaseIn, EaseOut, EaseInOut,
    CubicBezier(f32, f32, f32, f32),  // CSS / AE compatible
    Step(usize),                      // N stepped frames
    Hold,                             // no interpolation — discrete
}

Keyframe values are typed per property (Vec2, f32, colour, etc.) via a KeyframeValue enum that interpolate(a, b, t, easing) acts on.

AudioCue

pub struct AudioCue {
    pub trigger: TimeStamp,          // when playback starts in scene time
    pub source: AudioSource,         // file / clip / generator
    pub volume: Animation,           // animated 0.0..=1.0
    pub duck: Vec<DuckBus>,          // other cues to attenuate while playing
}

Audio cues mix into a single output bus per scene. The render pass produces (VideoFrame, AudioBuffer) at each timestamp; the audio buffer spans the interval [last_render_time, this_render_time) at the scene's sample_rate.

Rendering pipeline

Scene + t  →  SceneSampler.sample_at(t)  →  RenderedFrame {
    video: Option<VideoFrame>,   // None for audio-only intervals
    audio: AudioBuffer,          // always valid, may be silence
    operations: Vec<ExportOp>,   // e.g. for PDF export: emit text run X
}

A SceneRenderer walks the SceneObject list in z-order, evaluating transforms + animations at t, clipping against the canvas, and compositing via the BlendMode. The renderer delegates per-object content fetching to each ObjectKind's own sampler:

• Image samplers hold a cached decoded VideoFrame.
• Video samplers advance their demuxer/decoder to the requested PTS and return the most recent frame.
• Text samplers shape glyphs via a pluggable TextShaper trait (default: a minimal monospace fallback; real layout engines land as separate crates).
• Shape samplers rasterise on demand via a pure-Rust vector rasteriser (planned as oxideav-rasterise, another follow-up).

Use cases in detail

PDF pages

Each page becomes a Scene with Canvas::Vector { unit: Pt, width, height } and one SceneObject per glyph run, image, and vector path. The scene's duration is Finite(1 frame). Edits (redact a region, drop a watermark, rewrap a column) happen on the scene graph. When the user re-exports:

• PDF out — the SceneRenderer walks the tree and emits PDF operators (Tj for text, Do for images, f/S for vectors), preserving structure. Text remains selectable, hyperlinks survive, bookmarks stay intact.
• PNG / JPEG out — the renderer rasterises at a requested DPI.

Streaming compositor (RTMP server)

A daemon holds one Scene per live channel with duration: Indefinite. A control-plane protocol (JSON over WebSocket, say) surfaces:

{"op": "add_object", "id": "lower-third", "kind": {"image": "...base64..."}, "transform": {...}}
{"op": "animate", "id": "lower-third", "property": "position", "from": [0, 0], "to": [200, 0], "duration_ms": 800, "easing": "ease_out"}
{"op": "remove_object", "id": "lower-third", "delay_ms": 5000}

The compositor renders the scene into a VP9/AV1/H.264 encoder fed to an RTMP muxer. Viewers receive a normal stream; the producer only sees the DSL.

NLE timeline (Premiere / Resolve style)

Tracks are SceneObject::Group children with a shared z-order band. Transitions between clips are implemented as opacity / position animations that overlap two Video objects. Effects are the effects: Vec<Effect> vector on each object. Scrubbing + preview works by driving the SceneSampler at arbitrary timestamps; export renders the entire duration at the target framerate.

Crate layout (scaffold today)

src/
├── lib.rs           — module exports + Scene / Canvas root types
├── object.rs        — SceneObject + ObjectKind + Transform + BlendMode
├── animation.rs     — Animation + Keyframe + Easing + interpolation
├── audio.rs         — AudioCue + AudioSource
├── render.rs        — SceneRenderer + SceneSampler traits
├── duration.rs      — SceneDuration + Lifetime
├── id.rs            — ObjectId (stable, editable)
└── ops.rs           — Operation enum for the streaming compositor

Everything is pub and #[non_exhaustive] on public enums so new variants can land without an SemVer break.

Non-goals (for now)

• Not a vector rasteriser. Shape rendering ships as a separate crate (oxideav-rasterise) pending.
• Not a text shaper. The TextShaper trait is pluggable; a real shaper lands in oxideav-text (pending).
• Not an NLE UI. This crate is the data model + renderer core; the UI is downstream.
• Not a document parser. PDF / SVG ingest land in oxideav-pdf / oxideav-svg (both pending) and produce Scenes.

License

MIT — same as the rest of oxideav.