{ (define LIBNAME "Animated Images, Simulating Worlds")
  (include "head.tinc") }

<p>The teachpack provides the tools to write simple animations and
 interactive games. The first and simplest approach is to set up a
 simulation: 

<blockquote>
<code>{(idx run-simulation)} : Nat Nat Number [Nat -> Scene] -> true</code><br>
 <code>(run-simulation width height r create-image)</code>
   creates and shows a <code>width</code> x <code>height</code> canvas, 
   starts a clock, ticking every <code>r</code> (usually fractional) seconds, 
   and, every time the clock ticks, it applies <code>create-image</code> to
    the number of ticks passed since this function call. 

<p>In addition, the function pops up a frame and displays the pictures that
  <code>create-image</code> generates. The result is a simple animation.</p>

<p>Optional: the function consumes an optional fifth argument, a
 boolean. If this argument is <code>false</code> or missing,
 <code>run-simulation</code> acts as described; if it is present and
 <code>true</code>, the function can create an animated GIF of the
 simulation after you stop it.</p>

<p><b>Example:</b>
<pre>
<code>
(define (create-UFO-scene height)
  (place-image UFO 50 height (empty-scene 100 100)))

(define UFO (overlay (circle 10 'solid 'green) (rectangle 40 4 'solid 'green)))

(run-simulation 100 100 (/ 1 28) create-UFO-scene)
</code>
</pre>
</p>
</blockquote>

<p>For animated worlds and games, using the teachpack requires that you
 provide a data definition for <code>World</code>. In principle, there are
 no constraints on this data definition. </p>

<p>The teachpack works with two basic forms of data for visualizing the world: 
<ul>
<li><code>Image</code>, which are either inserted into DrScheme via the
  "Special" menu or constructed from the functions below; and </li>
<li><code>Scene</code>, which are created from <code>Image</code>s. 
 Specifically, a scene is an image whose pinhole is at position (0,0).</li>
</ul>
</p>

Given a data definition for worlds, the following functions create worlds,
visualize it, make the clock tick, and provide feedback about the mouse and
keyboard actions that the program's users perform:

<ol>
<li><code>{(idx big-bang)} : Nat Nat Number World -> true</code><br>
    <code>(big-bang width height n w)</code>
   creates and shows a <code>width</code> x <code>height</code> canvas,
   starts the clock, 
   makes it tick every n seconds, 
   and makes w the first world

<li><code>{(idx big-bang)} : Nat Nat Number World Boolean -> true</code><br>
  <code>big-bang</code> takes an optional fifth argument. If it is
  <code>true</code>, the world allows the generation of images from the
  animation, including an animated GIF image 

<li><code>{(idx on-tick-event)} : (World -> World) -> true</code><br>
   <code>(on-tick-event tock)</code> means that DrScheme must call <code>tock</code>
   on the current world every time the clock ticks; it uses the result as the next world 

<li><code>{(idx on-key-event)}  : (World KeyEvent -> World) -> true</code><br>
   <code>(on-key-event change)</code> means that DrScheme must call
   <code>change</code> 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

<pre>
<code>
   ;; A KeyEvent is one of: 
   ;; -- Char (char?)
   ;; -- Symbol (symbol?)
</code>
   When the Keyevent is a char, the programmer (user of the computer) has hit an
   alphanumeric key. Symbols such as <code>'left</code>, <code>'right</code>,
   <code>'up</code>, <code>'down</code>, <code>'release</code> denote arrow keys
   or the events of releasing a key on the keypad. 
</pre>

<li><code>{(idx on-mouse-event)} : (World Nat Nat MouseEvent ->
   World) -> true</code><br> <code>(on-mouse-event clack)</code> means that
   DrScheme must call <code>clack</code> on the current world, the current
   <code>x</code> and <code>y</code> coordinates of the mouse, and and a
   (representation of the) mouse event for every action of the mouse the
   programmer (user of the computer) makes; it uses the result as the next
   world

<pre>
<code>
  ;; A MouseEvent is one of:
  ;; - 'button-down
  ;; - 'button-up
  ;; - 'drag
  ;; - 'move
  ;; - 'enter
  ;; - 'leave
</code>
   The symbols denote the appropriate action with the mouse and (any of)
   its button(s).
</pre>

<li><code>{(idx on-redraw)} : (World -> Scene) -> true</code><br>
   <code>(on-redraw world->scene)</code> means that DrScheme calls
   <code>world->image</code> whenever the canvas must be redrawn (usually
   after a tick event/a keyboard event/a mouse event has occurred); 
   the function consumes the current world and produces a scene, which is
   then displayed in the teachpack's canvas 

<li><code>{(idx end-of-time)} : String u Symbol -> World</code><br> When DrScheme
   evaluates <code>(end-of-time "the end")</code>, 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.  
</ol></p>


<p>For the creation of scenes from the world, use the following functions
plus the functions on images below: 
<ol>
<li><code>{(idx nw:rectangle)}   : Nat Nat Mode Color -> Image</code><br>
   <code>(nw:rectangle width height mode color)</code>
   creates a width x height rectangle, solid or outlined, 
   with its anchor in the NW corner

<li><code>{(idx empty-scene)} : Nat Nat -> Scene</code><br>
   <code>(empty-scene width height)</code>
   creates a width x height "scene" (frame with origin in NW)

<li><code>{(idx place-image)} : Image Number Number Scene -> Scene</code><br>
   <code>(place-image image x y scene)</code>
   places image at (x,y) into scene; (x,y) are comp. graph. coordinates
   
<li><code>{(idx scene+line)} : Scene Number Number Number Number Color -> Scene</code><br>
   <code>(scene+line scene x0 y0 x1 y1 c)</code>
   places a line of color <code>c</code> from <code>(x0,y0)</code> to
   <code>(x1,y1)</code> into <code>scene</code>;  
   <code>(x,y)</code> are comp. graph. coordinates; 
   in contrast to the {(idx image.ss)} <code>add-line</code> function, this
   one cuts off those portions of the line that go beyond the boundaries of
   the given <code>Scene.</code>
   
<li><code>{(idx run-movie)}   : (Listof Image) -> true </code><br>
   <code>(run-movie loi)</code>
   shows the list of images in loi in a time-delayed manner; 
   assume: all images are of the same size 
</ol></p>

<hr />

<h3>Image Manipulation</h3>

<p>Finally, the teachpack provides all the functions that image.ss
provides. For completeness, the documentation of this teackpack is
included here: 

{(include "image-content.tinc")}

</p>

{(include "foot.tinc")}