Animated Images

world.ss

The teachpack provides two kinds of functions. The first four allow students to simulate a small world of animated drawings and games:

big-bang : Number Number Number World -> true
(big-bang width height n w) creates and shows a width x height canvas, starts the clock, makes it tick every n seconds, and makes w the first world

on-tick-event : (World -> World) -> true
(on-tick-event tock) means that DrScheme must call tock on the current world every time the clock ticks; it uses the result as the next world

on-key-event : (World KeyEvent -> World) -> true
(on-key-event change) means that DrScheme must call change on the current world and a (representation of the) keyevent for every keystroke the programmer (user of the computer) makes; it uses the result as the next world


   ;; A KeyEvent is one of: 
   ;; -- Char (char?)
   ;; -- Symbol (symbol?)

   When the Keyevent is a char, the programmer (user of the computer) has hit an
   alphanumeric key. Symbols such as 'left, 'right,
   'up, 'down, 'release denote arrow keys
   or the events of releasing a key on the keypad.

on-redraw : (World -> Image) -> true
(on-tick-event world->image) means that DrScheme calls world->image whenever the canvas must be redrawn; the result is displayed in the teachpack's canvas

end-of-time : String u Symbol -> World
When DrScheme evaluates (end-of-time), it stops the clock and displays the given string or symbol; no further tick events, key events, or redraw events take place until the world is created again.

The rest are functions for creating scenes:

nw:rectangle : Number Number Mode Color -> Image
(nw:rectangle width height mode color) creates a width x height rectangle, solid or outlined, with its anchor in the NW corner

empty-scene : Number Number -> Scene
(empty-scene width height) creates a width x height "scene" (frame with origin in NW)

place-image : Image Number Number Scence -> Scene
(place-image image x y scene) places image at (x,y) into scene; (x,y) are comp. graph. coordinates

add-line : Scene Number Number Number Number Color -> Scene
(add-line scene x0 y0 x1 y1 c) places a line of color c from (x0,y0) to (x1,y1) into scene; (x,y) are comp. graph. coordinates

run-movie : (Listof Image) -> true
(run-movie loi) shows the list of images in loi, time-delayed

Finally, the teachpack provides all the functions that image.ss provides.