3 Interactive Preview
animate/preview is the optional GUI surface. It uses exactly the normal scene sampling and Pict rendering semantics, at a lower pixel scale when requested. Start it from GRacket or through raco animate preview; see "examples/source-block-hot-reload.rkt" for the complete program.
(require animate/preview) (open-program-preview "derivative.rkt" 'derivative-video #:fps 30 #:pixel-scale 1/2)
The preview is an inspection workbench. An open-project-preview whose project requests audio monitoring prepares a cached PCM WAV proxy and waveform from its authored audio cues. Reuse requires both the normalized cue plan and the content hashes of its source files to match, so replacing narration at the same pathname does not play stale audio. When ffplay is available, playback follows the same absolute semantic timeline as the visual preview; without it, the waveform and visual preview remain usable. A module-backed project uses an isolated renderer process that can be restarted after a blocked render. Direct Scene previews remain in-process and therefore support cooperative—not forced—cancellation.
Every scheduled preview request has an explicit outcome: 'complete, 'canceled, 'superseded, 'failed, 'timed-out, or 'worker-restarted. Scrubbing normally makes the preceding request 'superseded; that is expected scheduler work, not an author error. A project-worker timeout is reported as 'timed-out and records a separate worker-restart diagnostic before the current frame is requested again. The last good bitmap stays visible through that recovery.
Clicking or dragging the playhead in the production timeline seeks to an exact semantic time. Shift-dragging marks a half-open review range. Play range reviews it once; Loop range stores it in the headless controller and repeats it from the same absolute semantic times. The block, section, and cue selectors jump to the corresponding authored location when that kind of declaration is present. The adjacent speed menu changes realtime playback and the optional audio monitor together. Save A and Save B retain two independent, dotted comparison ranges; Play A and Play B review either one without replacing the marked range or changing the loop. When a project has an available audio backend, the Mute control silences only that backend. The visual playhead, scene sampling, timeline lanes, and frame cache continue unchanged; unmuting restarts audio from the controller’s current absolute timeline time. The control is disabled when the preview has no audio monitor (for example, when ffplay is unavailable). Exact playback intentionally favors every rendered frame over wall-clock speed, so it is useful for visual review but not synchronized audio monitoring.
The same transport and cue operations are available without a GUI widget. This is useful for editor integrations and for testing an authored timeline:
(preview-scrub! preview 12.5) (preview-jump-to-section! preview 'explanation) (preview-jump-to-cue! preview 'definition) (preview-set-loop-range! preview 12 18) (preview-play! preview)
The cue selector is intentionally absent for a plain Scene, and the block selector is intentionally absent unless the preview was opened from a source program. These controls never invent sections, cues, or blocks from rendered pixels.
After a program reload, a current Visual selection is retained only when its identity is unambiguous: Animate tries its exact path, then a unique named formula part, then the same canonical formula source and mapped source span. The preview explains the outcome beside the selection; a repeated candidate is cleared rather than silently retargeted.
Selecting a mapped formula leaf promotes the inspector subject to its exact source-map unit. The preview then draws thin golden boxes around all mapped units and a crimson box around the selected one. These are inspector overlays, not Scene Visuals: they are measured from the existing sampled scene and never enter a final render or bitmap-cache key. Shared or unmapped leaves select the enclosing formula instead of guessing which source occurrence was intended. The Formula source panel also displays the canonical source directly. Clicking a mapped character selects that exact source unit and highlights every one of its mapped leaves, even when the unit corresponds to more than one leaf. Whitespace, unsafe ranges, and source with no visible mapped ink keep the formula panel selected and explicitly report that there is no rendered leaf; they never snap to a neighbouring character. The row-level Select mapped source unit command remains a convenient shortcut when the unit has exactly one rendered leaf, and deliberately stays inert for a multi-leaf unit rather than choosing a glyph by drawing order. Headless integrations can apply the same policy with scene-inspector-subject-at-path and then construct an immutable scene-inspector-document.
While a transform-matching-strings clip is active, the String matching inspector lists the retained planner decisions and draws its planned routes in gold. The routes come from the compiled formula transition plan, so they stay meaningful during an interior cross-fade whose temporary rendered layers deliberately have no source-map identities. Like every inspector overlay, they are preview chrome and never affect the rendered video.
Selecting a relation also exposes its declared phase, structure, cacheability, and dependencies. Visual and anchor dependencies receive thin purple inspection arrows from their measured current boxes to the relation; value dependencies remain textual because a scalar has no honest canvas location. These arrows are preview chrome only and are never added to the sampled scene.
Every inspector document also has a Camera section taken from the same immutable scene sample as the displayed frame. A source-program preview adds a Source block section only when the playhead lies in a retained compiled block; it reports the exact half-open interval, generation, branch mode, and source location rather than guessing from timeline pixels. When a render has published timing/cache data, a Render diagnostics section displays a copied snapshot. A plain Scene has no source block, and a frame that has not rendered yet has no render-diagnostic fields; the inspector reports neither as if it were known.
The optional Diagnostics control shows measured preview state, including the requested and displayed frames, semantic visual lag, render time, cache usage, canceled work, and worker cancellation capability. The same immutable snapshot is available headlessly through preview-session-diagnostics, so a bug-report or editor integration does not need to interrogate GUI widgets.