lathe/lib/polygon2d.js

A discrete solid modeller using BSPs

define(['./plane2d', './world2d', './vertex2d'], function(Plane2D, world, Vertex2D) {

  'use strict';

  var Polygon2D = function(boundingPlanes) {
    this.boundingPlanes = boundingPlanes;
  };

  Polygon2D.world = new Polygon2D([world.xPlus, world.yPlus, world.xMinus, world.yMinus]);

  Polygon2D.prototype.toVertices = function() {
    var n = this.boundingPlanes.length;
    var that = this;
    return this.boundingPlanes.map(function(bi, i) {
      var bmin1 = that.boundingPlanes[(i+n-1) % n];
      return new Vertex2D(bmin1, bi);
    });
  };

  Polygon2D.prototype.reverse = function() {  
    return new Polygon2D(this.boundingPlanes.slice(0).reverse());
  };
  
  Polygon2D.prototype.splitBy = function(h) {
    var frontOutputPlanes = [];
    var backOutputPlanes = [];
    var n = this.boundingPlanes.length;

    function swapOrientation(orientation) {
      if (orientation === '-') {
        return '+';
      } else if (orientation === '+') {
        return '-';
      } else {
        return '0';
      }
    }

    for (var i = 0; i < n; ++i) {
      var bi_min1  = this.boundingPlanes[(i+n-1) % n];
      var bi     = this.boundingPlanes[i];
      var bi_plus1 = this.boundingPlanes[(i+1) % n];
      var frontLookupKey = [
        new Vertex2D(bi_min1, bi).orientationToPlane(h),
        new Vertex2D(bi, bi_plus1).orientationToPlane(h)
      ].join('');
      var backLookupKey = frontLookupKey.split('').map(swapOrientation).join('');

      var frontSignal = lookupTable[frontLookupKey];
      if (frontSignal === 'B') {
        frontOutputPlanes.push(bi);
      } else if (frontSignal === 'HB') {
        frontOutputPlanes.push(h);
        frontOutputPlanes.push(bi);
      }

      var backSignal = lookupTable[backLookupKey];
      if (backSignal === 'B') {
        backOutputPlanes.push(bi);
      } else if (backSignal === 'HB') {
        backOutputPlanes.push(h);
        backOutputPlanes.push(bi);
      }
    }
    return {
      front: frontOutputPlanes.length > 0 ? new Polygon2D(frontOutputPlanes) : undefined,
      back: backOutputPlanes.length > 0 ? new Polygon2D(backOutputPlanes) : undefined,
    };
  };

  var lookupTable = {
    '+-' : 'B',
    '--' : '0',
    '-+' : 'HB',
    '++' : 'B',
    '00' : '0',
    '+0' : 'B',
    '0+' : 'HB',  
    '-0' : '0',  
    '0-' : '0',  
  };

  return Polygon2D;

});