Draw tangent lines using js drawing_javascript skills

Sample: http://www.zhaojz.com.cn/demo/draw9.html

Copy code The code is as follows:

//Draw tangent line
//point a point outside the circle
//dot circle center
//r radius
function drawCircleTangent(point, dot, r){
//Draw auxiliary lines-start
var color = 'DarkRed'; //Color of tangent line
var color2 = "#ccc"; //Color of other auxiliary lines
drawLine(dot, [dot[0] 9*r,dot[1]], {color: color2}); // Extend the horizontal line where the center of the circle is located
drawLine(dot, [dot[0],dot[1]-4*r], {color: color2}); // Draw the vertical line where the center of the circle is located
drawPoint({
         pw:2,ph:2,color:'DarkRed',point: [dot[0] 9*r,dot[1],'x']
});
drawPoint({
         pw:2,ph:2,color:'DarkRed',point: [dot[0],dot[1]-4*r,'y']
});
drawLine(point, [point[0],dot[1]], {color: color2}); // Draw a vertical line from point to x-axis
DrawLine(point, dot, {color: color2}); // Connect point and dot
drawLine([point[0]-2*r, point[1]], [point[0] 2*r, point[1]], {color: color2}); //Draw the horizontal line where point is located
//Draw auxiliary lines-end
//point.push('point');
drawPoint({
         pw:2,ph:2,color:'DarkRed',point: point
});
//dot.push('centre');
var r_square = Math.pow(r,2); // square of r
var point_v = point[1]-dot[1]; //The square of the distance from point to the x-axis
var point_h = point[0]-dot[0]; //The square of the distance from point to y-axis
var c_square = Math.pow(point_v,2) Math.pow(point_h,2); //The square of the distance from point to the center of the circle
var c = Math.sqrt(c_square); //Distance from point to center of circle
var sinA = Math.abs(point_v)/c; //sinA
var cosA = Math.abs(point_h)/c; //cosA
var b_square = c_square-r_square; //The square of the distance from point to tangent point
var b = Math.sqrt(b_square); //Distance from point to tangent point
var sinB = b/c; //sinB
var cosB = r/c; //cosB
//Using the center of the circle as the coordinate point, determine the quadrant where the point is located
var quadrant = 1; //Default value
var pm_h = point_h == 0? point_h: point_h/Math.abs(point_h); //Horizontal direction
var pm_v = point_v == 0? point_v: point_v/Math.abs(point_v); //Vertical direction
var hv = pm_h*pm_v; //Multiply, when -1, the point is in the first and third quadrants, when 1, the point is in the second and fourth quadrants, and when 0, the point is on the axis
switch(hv){
case 1:
If((pm_h pm_v)==-2){
                  quadrant = 2; //The second quadrant
               }else{
                 quadrant = 4; //The fourth quadrant
            }
             break;
case -1:
If((pm_h-pm_v)==-2){
                  quadrant = 3; //The third quadrant
            }
             break;
case 0:
If((pm_h pm_v)==-1){ //When the point is on the negative half-axis of the x-axis or the positive half-axis of the y-axis, it is concluded that the point is in the second quadrant
               quadrant = 2;
            }
If((pm_h pm_v)==1){ //When the point is on the positive half-axis of the x-axis or the negative half-axis of the y-axis, it is concluded that the point is in the fourth quadrant
               quadrant = 4;
            }
             break;
           default:
}
var sinC = 0;
    var conC = 0;
    was sinD = 0;
    var conD = 0;
    switch(quadrant){
        case 1:
            sinC = cosB*cosA sinB*sinA; //sinC = sin(90 (B-A)) = cos(B-A) = cosB*cosA sinB*sinA
            withC = -(withoutB*cosA-cosB*withoutA); //cosC = cos(90 (B-A)) = -sin(B-A) = -(sinB*cosA-cosB*sinA)
            sinD = -(cosA*cosB-sinA*sinB); //sinD = sin(270-(A B))
            withD = -(withoutA*cosB cosA*withoutB); //conD = cos(270-(A B))
            break;
        case 2:
            sinC = -(cosB*cosA-sinB*sinA); //sinC = sin(-90 (A B))
            conC = sinA*cosB cosA*sinB; //conC = cos(-90 (A B))
            sinD = cosA*cosB sinA*sinB; //sinD = sin(90 (A-B))
            withD = -(withoutA*cosB-cosA*withoutB); //conD = cos(90 (A-B))
            break;
        case 3:
            sinC = -(cosA*cosB sinA*sinB); //sinC = sin(-90 (A-B))
            withC = -(withoutA*cosB-cosA*withoutB); //conC = cos(-90 (A-B))
            sinD = (cosA*cosB-sinA*sinB); 
            conD = sinA*cosB cosA*sinB;
            break;
        case 4:
            sinC = cosA*cosB-sinA*sinB;
            withC = -(withoutA*cosB cosA*withoutB)
            sinD = -(cosA*cosB sinA*sinB); //sinD = sin(270 (A-B))
            withD = (withoutA*cosB-cosA*withoutB); //conD = cos(270 (A-B))
            break;
        default:
    }
    var tangentPointA = [point[0] b*conC, point[1] b*sinC]; //Suit A位置
    drawLine(point, tangentPointA, {color: color}); //画出切线
    drawLine(dot, tangentPointA, {color: color2}); //I'm looking forward to seeing you
    //drawArc(point, 17, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.and(sinC )*180/Math.PI, 0);
    var tangentPointB = [point[0] b*conD, point[1] b*sinD]; //SuitcaseB
    drawLine(point, tangentPointB, {color: color}); //画出切线
    drawLine(dot, tangentPointB, {color: color2}); //I'm looking forward to seeing you
    //drawArc(point, 27, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.and(sinD )*180/Math.PI, 0);
    drawPoint({ //描切点
        pw:3,ph:3,color:'DarkRed',point: tangentPointA
    });
    drawPoint({ //描切点
        pw:3,ph:3,color:'DarkRed',point: tangentPointB
    });
    //画辅助弧
    //(quadrant ==1 ||quadrant==4?360:0)
    drawArc(point, b, 60, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.and(sinC )*180/Math.PI-5);
}