proc vector LLDIS(vector $la,vector $lab,vector $lc,vector $lcd){
vector $u = $la-$lc;
float $a = dot($lab,$lab);
float $b = dot($lab,$lcd);
float $c = dot($lcd,$lcd);
float $d = dot($lab,$u);
float $e = dot($lcd,$u);
//float $f = dot($u,$u);
//平行数字小 不平行数字越来越大
float $det = $a * $c - $b * $b;
float $sNum;
float $tNum;
float $sDenom;
float $tDenom;
float $s;
float $t;
    if($det < 0.001){
        $sNum= 0;
        $tNum=$e;
        $tDenom= $c;
        $sDenom = $det;
    }else{
        $sNum = $b*$e-$c*$d;
        $tNum = $a*$e-$b*$d;
    }

    $sDenom= $det;
    if($sNum < 0){
        $sNum =0;
        $tNum = $e;
        $tDenom = $c;
    }else if($sNum > $det){
        $sNum = $det;
        $tNum = $e + $b;
        $tDenom = $c;
    }else{
        $tDenom = $det;
    }

    if($tNum <0){
        $tNum =0;
        if( - $d < 0){
            $sNum =0;
        }else if (-$d >$a){
            $sNum = $sDenom;
        }else{
            $sNum = -$d;
            $sDenom = $a;
        }
    }else if ($tNum > $sDenom){
        $tNum = $tDenom;
        if((-$d+$b)<0){
            $sNum =0;

        }else if((-$d +$b) > $a){
            $sNum = $sDenom;

        }else{
            $sNum = -$d+$b;
            $sDenom = $a;
        }
    }

    if($sDenom != 0 ){
        $s = $sNum / $sDenom;

    }else{
        $s =0;
    }
    if($tDenom != 0){
        $t = $tNum / $tDenom;

    }else{
        $t =0;
    }


    vector $v =  $la + ($s * $lab) - $lc + ($t*$lcd);
    return ($v);

}