1.1 A few useful words🔗ℹ

A Visual is an object that can be drawn: a circle, some text, a formula, or a group of other objects. Its name, called an ID, lets an animation refer to it later.

A Scene describes the animation built so far. It includes its objects, camera, and timed changes. It is not just one frame. A scene state is what you get when you ask for that Scene at a particular time.

A camera chooses the visible area and output size. Sampling means asking what should be visible at a time. Rendering means drawing that result. Encoding means turning the rendered frames into a video file.

One Scene produces all five pictures. A Visual describes the circle; a sampled state places it at one time. The complete program is in Quick Start: a moving circle.

1.2 Two ways to arrange a video🔗ℹ

Use ordinary Scenes when you want direct control over objects and their motion. Use animate/slides when named regions such as title, explanation, and figure are a better starting point. A layout places those regions for you.

Slides are not a separate renderer. slide->pict gives a Racket Pict; slide->scene gives an ordinary Animate Scene. See Slides, builds, and continuity.

1.3 Preparation does the work that must happen first🔗ℹ

Some content needs work before it can be drawn. Text must be measured. A formula may need TeX. A construction needs fixed points and label positions. A recording needs a measured duration. Animate calls this preparation.

For slides, prepare once and then sample the result at different times. Workers can share prepared math and geometry data without repeating their layout work. This does not mean that every possible Racket value is transferable.

1.4 Where to put your code🔗ℹ

Keep descriptions in normal modules. Put preview and file-writing commands in an explicit runner or a module+ main block. Then loading a lesson to inspect it does not unexpectedly open a window or render a video.

The Modules to require chapter shows which module provides each group of operations.