<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(jié)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(cè) (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 }; 
}

<code class="hljs avrasm">var triArea=( (a.x - c.x) * (b.y - c.y) - (a.y - c.y) * (b.x - c.x) ) /2 ; </code>

function segmentsIntr(a, b, c, d){ 
 
 // 三角形abc 面積的2倍 
 var area_abc = (a.x - c.x) * (b.y - c.y) - (a.y - c.y) * (b.x - c.x); 
 
 // 三角形abd 面積的2倍 
 var area_abd = (a.x - d.x) * (b.y - d.y) - (a.y - d.y) * (b.x - d.x); 
 
 // 面積符號相同則兩點在線段同側(cè),不相交 (對點在線段上的情況,本例當作不相交處理); 
 if ( area_abc*area_abd>=0 ) { 
 return false; 
 } 
 
 // 三角形cda 面積的2倍 
 var area_cda = (c.x - a.x) * (d.y - a.y) - (c.y - a.y) * (d.x - a.x); 
 // 三角形cdb 面積的2倍 
 // 注意: 這里有一個小優(yōu)化.不需要再用公式計算面積,而是通過已知的三個面積加減得出. 
 var area_cdb = area_cda + area_abc - area_abd ; 
 if ( area_cda * area_cdb >= 0 ) { 
 return false; 
 } 
 
 //計算交點坐標 
 var t = area_cda / ( area_abd- area_abc ); 
 var dx= t*(b.x - a.x), 
 dy= t*(b.y - a.y); 
 return { x: a.x + dx , y: a.y + dy }; 
 
}