17.1 Animated Camera Views🔗ℹ

Every scene and timeline clip stores one immutable camera. The camera can be sampled independently from the Visual state, and both tracks are rendered at the same absolute time.

A scene may begin with an explicit camera:

(define initial-camera

(make-camera #:world-width 14

#:center origin))

(define scene

(make-scene #:camera initial-camera))

Camera and Visual requests can share one play clip:

(scene-play scene

(move-to 'marker (vec2 3 1))

(camera-pan-to (vec2 2 1))

(camera-zoom-by 2)

#:duration 2)

Camera requests use the same composition vocabulary as Visual requests:

(scene-play scene

(succession

(camera-pan-to (vec2 -4 0))

(animation-group

(camera-follow 'marker)

(camera-zoom-by 2)))

#:duration 4)

Here pan occupies the first half. Follow and zoom occupy the second half in parallel because they write separate camera components. A top-level timed camera request uses literal local seconds; inside a composition, it follows the same duration-scaling rules as a timed Visual request. Each camera leaf compiles from the exact view at its local start. A local camera-follow tracks the sampled world-space target only during its own interval and then retains that endpoint view.

The camera center and visible world width are independent components. Pan and follow requests change the center. Zoom requests change visible width. A fit request changes both components. Requests for disjoint components may run together; overlapping requests that reserve the same component raise an exception. Relative pan and zoom requests compile from the camera at the start of their own local interval.

camera-zoom-by uses magnification rather than a width multiplier. A factor of two divides the visible world width by two and zooms in. A factor of one half doubles the visible width and zooms out. Visible world width is interpolated linearly between the clip-start and target widths.

scene-camera-at samples arbitrary times using the same half-open clip selection and closed total-duration interval as scene-sample. A wait clip holds both state and camera. scene-set-camera replaces only the current endpoint camera and appends no clip.

scene->pict, scene-frame->bitmap, and render-frames! use the sampled scene camera when their #:camera argument is #f. Supplying a camera is a deliberate static override and ignores every camera request for that render.

The camera’s pixel dimensions, aspect ratio, and background stay fixed while its center or visible width changes. Renderer-aware layout continues to accept a static camera value. To measure for one sampled view, pass (scene-camera-at scene time) explicitly. Layout does not reflow automatically on every frame.

This version does not animate camera rotation, pixel dimensions, or background style.

Render the pan-and-zoom example with:

17.2 Automatic Camera Framing and Following🔗ℹ

Camera fitting uses renderer-aware measurements, while camera following tracks a target for one clip. Both use ordinary requests accepted by scene-play.

17.3 Fixed-in-Frame Overlays and Callouts🔗ℹ

Frame space is an origin-centered coordinate domain for presentation content that must remain stable while the world camera moves. The semantic wrapper snapshots only a visible frame width; Pict rendering remains in the adapter.

Freeze a title at an explicit frame position:

(define title

(fixed-in-frame

(plain-text "Fixed title"

#:id 'title

#:font-size 1/2)

#:camera initial-camera

#:at (vec2 0 3)))

Frame-space Visuals remain ordinary affine and opacity animation targets. Later world pan and zoom do not alter their screen position or local render scale. A compound overlay is made by grouping ordinary content first and wrapping the complete group; frame-space wrappers themselves remain top-level.

Add a fixed annotation whose leader tracks a moving world Visual:

(define note

(callout

(plain-text "moving point" #:id 'note #:font-size 2/5)

marker

#:camera initial-camera

#:at (vec2 4 2)

#:connector-stroke "navy"

#:connector-width 2))

(scene-play

(scene-add (make-scene #:camera initial-camera) marker title note)

(move-to marker destination)

(camera-pan-by (vec2 2 1))

(camera-zoom-by 2)

#:duration 2)

The callout stores a Visual target as its stable identity. Complete scene rendering resolves that identity against each sampled top-level state, draws the leader from the target’s current world pixel position to the fixed annotation box, and then places the annotation. A literal vec2 target instead represents one fixed world point.

Renderer-aware relative layout measures frame-space Visuals with their captured frame scale. World and frame domains cannot be mixed in one layout calculation, and frame Visuals combined together must share the same captured frame width. Camera fit and follow remain world-only.

The canonical movie moves its marker along the same piecewise-linear sample edges that define the displayed quadratic graph. The traversal uses one move-along-path clip while the camera keeps the original continuous linear pan and zoom trajectory over the three seconds.

Render the canonical fixed-overlay example with: