28.1 Start two movements together🔗ℹ

Requests in one ordinary scene-play run together. Both objects below move for two seconds. The Scene lasts two seconds, not four.

(define together

(scene-play initial

(move-to 'dot (vec2 3 1))

(move-to 'tile (vec2 3 -1))

#:duration 2))

> (scene-duration together)

2

28.2 Run one movement after another🔗ℹ

Pass the result of the first scene-play to the second. Each call appends a new interval. Here the circle moves for one second; then the square moves for one second. The total is again two seconds, but the picture tells a different story.

(define first-move

(scene-play initial

(move-to 'dot (vec2 3 1))

#:duration 1))

(define in-order

(scene-play first-move

(move-to 'tile (vec2 3 -1))

#:duration 1))

> (scene-duration in-order)

2

Two separate calls both starting from initial would create two alternative Scenes. They would not join automatically. Return values connect the timeline.

28.3 Delay the second start🔗ℹ

A timed wrapper gives a request its own start and duration. Used directly inside scene-play, its start is measured from that call’s play interval, not from the beginning of the whole video. Both requests must finish within the enclosing duration.

(define overlapping

(scene-play initial

(timed (move-to 'dot (vec2 3 1))

#:start 0 #:duration 2)

(timed (move-to 'tile (vec2 3 -1))

#:start 1/2 #:duration 2)

#:duration 5/2))

> (scene-duration overlapping)

5/2

The last finish is 2.5 seconds, so the outer duration is 5/2. Exact fractions are useful here: 1/2 means precisely half a second. A later play interval may use another #:start 0; that means its own start.

Try changing the square’s start to one second. Increase the outer duration to three seconds too. The circle still finishes at two seconds; the square finishes at three. Changing only the delay would put its endpoint outside the old interval.

More compact composition tools include succession, animation-group, and lagged-start. They are not needed for this example. Their allocation rules are in Animation requests; in particular, timing nested inside a composition can be scaled to the enclosing interval. Do not assume that the direct timed example describes every nested case.

28.4 Change the speed, not the schedule🔗ℹ

An easing function maps elapsed progress to movement progress. Halfway through a two-second interval, elapsed progress is 1/2. Without an explicit choice, these movements use constant-speed linear. (smooth) makes them start and finish gently without changing the duration.

(define eased

(scene-play initial

(move-to 'dot (vec2 3 1))

(move-to 'tile (vec2 3 -1))

#:duration 2

#:easing (smooth)))

> (scene-duration eased)

2

These sample times are the same as in the first strip. Compare the distance travelled during the first half-second. The middle and final positions agree; the positions between them need not. Write (smooth), with parentheses: it constructs the callable rate value that scene-play expects. The slide API later uses a different spelling, a named choice such as 'smooth.

28.5 Introduce an object🔗ℹ

Opacity controls how visible an object is: zero is transparent, one is fully visible. Presence says whether the object exists in the scene state at all. These are different questions.

fade-in introduces an absent object. Start with an empty Scene; do not add dot with scene-add first.

(define appearing

(scene-play (make-scene)

(fade-in dot)

#:duration 1))

The final state really contains the introduced object:

> (scene-state-has? (scene-sample appearing 1) 'dot)

#t

28.6 Hide an object, or remove it🔗ℹ

fade-to changes opacity but keeps the object present. fade-out changes opacity and removes the object at the end of the interval.

(define invisible

(scene-play (scene-add (make-scene) dot)

(fade-to 'dot 0)

#:duration 1))

(define removed

(scene-play (scene-add (make-scene) dot)

(fade-out 'dot)

#:duration 1))

The final pictures look alike. The scene states do not. These two statements are checked when the documentation is built:

> (scene-state-has? (scene-sample invisible 1) 'dot)

#t

> (scene-state-has? (scene-sample removed 1) 'dot)

#f

That difference determines how to bring the circle back:

(define visible-again

(scene-play invisible

(fade-to 'dot 1)

#:duration 1))

(define reintroduced

(scene-play removed

(fade-in dot)

#:duration 1))

> (scene-state-has? (scene-sample visible-again 2) 'dot)

#t

> (scene-state-has? (scene-sample reintroduced 2) 'dot)

#t

Use fade-to for the existing invisible object; use fade-in for the absent one. A zero-opacity object cannot be introduced a second time under the same ID. An absent object cannot be targeted as though it were still present.

28.7 Keep a last picture on screen🔗ℹ

A movie normally samples times before the exact final endpoint. Add scene-wait when viewers should see the completed state for a while. For example:

(define with-hold (scene-wait in-order 1))

> (scene-duration with-hold)

3

This gives the finished pair one extra second.

Continue with Draw a graph for a small mathematical diagram, or take the layout-based route at Make a slide and give it time.