<code class="hljs avrasm">function segmentsIntr(a, b, c, d){  /** 1 解線性方程組, 求線段交點. **/ // 如果分母為0 則平行或共線, 不相交  var denominator = (b.y - a.y)*(d.x - c.x) - (a.x - b.x)*(c.y - d.y);  if (denominator==0) {  return false;  }  // 線段所在直線的交點坐標 (x , y)  var x = ( (b.x - a.x) * (d.x - c.x) * (c.y - a.y)   + (b.y - a.y) * (d.x - c.x) * a.x   - (d.y - c.y) * (b.x - a.x) * c.x ) / denominator ;  var y = -( (b.y - a.y) * (d.y - c.y) * (c.x - a.x)   + (b.x - a.x) * (d.y - c.y) * a.y   - (d.x - c.x) * (b.y - a.y) * c.y ) / denominator;  /** 2 判斷交點是否在兩條線段上 **/  if (  // 交點在線段1上  (x - a.x) * (x - b.x) <= 0 && (y - a.y) * (y - b.y) <= 0  // 且交點也在線段2上   && (x - c.x) * (x - d.x) <= 0 && (y - c.y) * (y - d.y) <= 0  ){   // 返回交點p  return {   x : x,   y : y   }  }  //否則不相交  return false  } </code>

var nx=b.y - a.y,  ny=a.x - b.x; var normalLine = { x: nx, y: ny };

var dist= normalLine.x*c.x + normalLine.y*c.y; 

function segmentsIntr(a, b, c, d){   //線段ab的法線N1  var nx1 = (b.y - a.y), ny1 = (a.x - b.x);   //線段cd的法線N2  var nx2 = (d.y - c.y), ny2 = (c.x - d.x);   //兩條法線做叉乘, 如果結果為0, 說明線段ab和線段cd平行或共線,不相交  var denominator = nx1*ny2 - ny1*nx2;  if (denominator==0) {  return false;  }   //在法線N2上的投影  var distC_N2=nx2 * c.x + ny2 * c.y;  var distA_N2=nx2 * a.x + ny2 * a.y-distC_N2;  var distB_N2=nx2 * b.x + ny2 * b.y-distC_N2;   // 點a投影和點b投影在點c投影同側 (對點在線段上的情況,本例當作不相交處理);  if ( distA_N2*distB_N2>=0 ) {  return false;  }   //  //判斷點c點d 和線段ab的關系, 原理同上  //  //在法線N1上的投影  var distA_N1=nx1 * a.x + ny1 * a.y;  var distC_N1=nx1 * c.x + ny1 * c.y-distA_N1;  var distD_N1=nx1 * d.x + ny1 * d.y-distA_N1;  if ( distC_N1*distD_N1>=0 ) {  return false;  }   //計算交點坐標  var fraction= distA_N2 / denominator;  var dx= fraction * ny1,  dy= -fraction * nx1;  return { x: a.x + dx , y: a.y + dy }; }