13: intro to scripting in vrml

scripting languages

• the vrml spec leaves language support up to the browser implementor but recommends javascript and java
• in practice, the url field of Script{} node supports inline / external javascript, and external java code
  • javascript: simpler, smaller, inlineable (fewer trips to server)
  • java: more powerful, faster, thread-based (asynchronous)
  • java uses external authoring interface (eai) to communicate with vrml scene
  • java / eai out of scope for this class, so we'll talk about ecmascript

ECMAscript

syntax: 'c-like'

• netscape's javascript 1.2 standardized through the european computer manufacturers association = ecmascript
• ecmascript reference (ecma-262) [»]
• javascript 1.3 reference easier to skim [»]

key tokens

• braces enclose code blocks

  function foo(bar) {
  }
  
  if (true) {
  }

• lines end with semicolon

  var sum = a + b;

• comments

  // single line

  /*
     multi-line
  */

datatypes

• scalar
  • boolean (SFBool) : true, false, TRUE, FALSE
  • numeric (SFInt32, SFTime, SFFloat) : integer or floating point numbers, stored in 64 bits (double precision)
• object
  • rest of vrml types (SFVec3f, SFColor, etc..) have object representations [»]
  • MF fields (multi-valued fields) are ordered collection of values which conceptually map to the ecmascript array object:
    • bracket access: myMultiVal[2]
    • length property: myMultiVal.length

variables

• implicit declaration : x (no typing like c's int x)
• explicit declaration (better) : var x (keyword var helps with garbage collection & scoping, required for recursion)

operators

• assignment =, +=, -=, *=, /=, %=
• comparison ==, !=, <, <=, >, >=
• arithmetic +, -, *, /, % (float division of integers)
• increment, decrement ++, --
• negation -
• bitwise &, |, ^, ~, <<, >>, >>>
• logical &&, ||, !
• string concatenation +
• conditional ?:

assignment

• dynamic typing: variable assumes type and value of assignee
• scalar (bool and numeric) are passed by value, objects are passed by reference
• SF <--> MF assignments copy value (pass by value)

flow control

• branching : if, if / else, switch
  examples
  

  if (some_boolean_condition) {
     // code executes when condition is true
  }
  
  if (some_boolean_condition) {
     // code executes when condition is true
  } else {
     // alternate code executes when condition is false
  } 
  
  switch (some_variable) {
     case (value_1) : // code for when some_variable = value_1
     case (value_2) : // code for when some_variable = value_2
     case (value_n) : // code for when some_variable = value_n
     default : // code for when no conditions met
  }

• repeating : while, for
  examples
  

  while (some_boolean_condition_remains_true) {
     // some code
  }
     
  for (var i = some_initial_val; i < some_limit; i++) {
     // some code
  }

objects

• similar to c, java classes, objects encapsulate data (properties) and functionality (methods)
• quick vrml object reference [»]
• global objects are always available:
  • Browser : getCurrentFramerate(), createVrmlFromString(), etc.
  • Math : PI, SQRT2, E, abs(), cos(), sin(), etc.
  • Number : MAX_VALUE, NEGATIVE_INFINITY, etc.
• others need to be instantiated
  • Array, Boolean, Date, Function, MFobjects, SFobjects, String, VrmlMatrix
  • instantiate with keyword new
new objectName ([initial_value(s)])
    ex.: var red = new SFColor(1, 0, 0)

functions

• writing
  • use keyword function
    

    function addemup(aVal, bVal) {
       return aVal + bVal;
    }

  • calling
    • use name of function, and place parameters within parenthesis
      

      var sum = addemup(5, 3);

  • ecmascript makes no special distinctions between functions and procedures

  using the script node

  script I/O

  • Script{}
  • eventIn: incoming value and timestamp are passed to function of same name
  • eventOut: assigning to an eventOut posts the value via routing (to any connected eventIns)
  • field: static value storage
  • no exposedFields! (browser implementation issues)
  • directOutput
    • set true to post to eventIns without routing (reading always ok)
    • better performance when false (scene can be optimized for things that can't change)
  • mustEvaluate
    • set true for createVrmlFromX
    • set false when ok to queue events until script result needed (better performance)
  • reserved functions
    • initialize() : called before world is rendered and before any events fired
    • shutDown() : called just prior to deleting script node from scene graph or unloading world
    • eventsProcessed() : called less frequently than per every event
      • good optimization when you only care about the latest state of the world
      • bad when you need the complete result history
    • print() : good for debugging; writes to console [»]
  • scopeless, always operational (so avoid continuous calculations when not affecting visible world)

  common scripting tasks

  range mapping

  • general technique: get to unit space, then map from there
    ex.: want to map: [-m,n] -> [0,x], or [-y,x]
    
    • shift range to start at 0 (add m): [0,n+m]
      
    • squash range to [0,1] (divide by n+m)
      
    • scale range to extent of choice:
         multiply by x: [0,x]
         multiply by x+y, subtract y: [-y,x]
  • inversion [0,1] -> [1,0]
    • ascending vs. descending intensities [»]
  • harnessing sine and cosine [-1,1] -> [0,1]
    • soft pulse [»]
    • smooth bounce [»]
    • adjusting interpolator inputs [»]

  type conversion

  • SFBool -> SFColor (highlighting with color change) [»]
  • SFVec3f -> SFColor (position -> sphere color) [»]

  sub-component extraction

  • planesensor x value -> light intensity [»]
  • cylinder rotation angle -> texture frame number [»]

  advanced topics

  • procedural geo
  • behaviors
  • ai / alife

  task06 [»]