25.1 Describe one object🔗ℹ

A Visual is an object Animate can draw. Here it is a circle. (vec2 x y) gives a position: x increases to the right and y increases upward. The units describe the drawing, not output pixels. The circle below has its center three units left of the origin. Its ID, 'moving-circle, is the name we will use to move it later.

(require animate)

(define disc

(circle #:id 'moving-circle

#:center (vec2 -3 0)

#:radius 3/4

#:fill "dodgerblue"

#:stroke "navy"

#:stroke-width 3))

In a source file, put #lang racket/base at the top before these forms.

25.2 Put it in a Scene🔗ℹ

A Scene describes an animation, including what happens over time. make-scene starts an empty one. scene-add puts our circle into it. Adding an object does not itself make time pass.

(define initial

(scene-add (make-scene) disc))

25.3 See the Scene🔗ℹ

Sampling asks for a Scene at one time. scene->pict samples and draws in one operation, producing a Racket Pict. The expression shown below is the expression you would use in DrRacket; the documentation build only scales and frames its resulting Pict for the page.

> (scene->pict initial 0)

The lower-level scene-sample operation returns a scene state. That state is the immutable collection of Visual values at the requested time; scene-state->pict can draw such a state explicitly.

25.4 Give it a movement🔗ℹ

move-to describes a requested change. scene-play adds that change to our Scene. #:duration 1 gives the movement one second. The result is a new Scene; initial still describes the unmoving starting point.

(define movement

(scene-play initial

(move-to 'moving-circle (vec2 3 0))

#:duration 1))

The duration claim is checked as part of the documentation build:

> (scene-duration movement)

1

The frames are evenly spaced in time. The circle moves at constant speed because we have not selected a different speed curve.

To inspect one exact time without making a movie, sample and draw the Scene directly:

> (scene->pict movement 1/2)

The midpoint is also available as semantic state, independently of the pixels:

> (visual-position    (scene-state-ref (scene-sample movement 1/2) 'moving-circle))

(vec2 0 0)

25.5 Keep the result visible🔗ℹ

scene-wait adds time without changing the picture. This adds half a second at the right-hand end, bringing the total to 1.5 seconds.

(define animation (scene-wait movement 1/2))

> (scene-duration animation)

3/2

A complete standalone version is "scribblings/examples/moving-circle.rkt". The checked-in frame strip is generated from that maintained example; the short expressions above are independently executed by Scribble so stale Guide code fails the manual build.

Continue with Save frames and make a video to save pictures and a movie, or Build a small scene to build a larger native Scene.