Javascript 画一个箭头

如何在hummusJS中绘制箭头？ 我正在使用hummus以pdf格式绘制。我需要画一个pdf格式的箭头。我能划清界限。但是我怎样才能画箭头呢？ 我尝试了以下方法

if(x2 > y2)
            {
                a1 = x2-5;
                b1 = y2-5;
                a2 = x2-5;
                b2 = y2+5;
                a3 = x2+5;
                b3 = y2;
            }
            else
            {
                a1 = x2-5;
                b1 = y2+5;
                a2 = x2+5;
                b2 = y2+5;
                a3 = x2;
                b3 = y2-5;                
            }

 cxt.drawPath(a1,b1,a2,b2,a3,b3,{type: 'fill',
            color: '#000000'})

我也试过这样做

      var d =5;
            a1 = x2-d*Math.sin(45);
            b1 = y2-d*Math.cos(45);
            a2 = x2+d*Math.sin(45);
            b2 = y2+d*Math.cos(45);
cxt.drawPath(x2,y2,a1,b1,{type: 'fill',
                color: '#000000'})
cxt.drawPath(x2,y2,a2,b2,{type: 'fill',
                color: '#000000'})

但这并不是在正确的位置绘制箭头

---

这是一个使用函数

drawArrow

在PDF上绘制箭头的示例。PDF是通过浏览器访问生成的。 它可以绘制水平和垂直箭头，并将箭头置于起点或终点。 您可以排练输入变量

startPoint

，

endPoint

，

endArrowHead

var express = require('express');
var app = express();


const DELTA = 5; //used to draw triangle arrowhead points

app.get('/', function(req, res){

var startPoint = {x:100, y:100}
var endPoint = {x:150, y:100}
var endArrowHead = true;

res.writeHead(200, {'Content-Type': 'application/pdf'});

var hummus = require('hummus');

var pdfWriter = hummus.createWriter(new hummus.PDFStreamForResponse(res));
var page = pdfWriter.createPage(0,0,595,842);
var cxt = pdfWriter.startPageContentContext(page);

drawArrow(cxt, startPoint, endPoint, endArrowHead);

startPoint = {x:200, y:100}
endPoint = {x:250, y:100}
endArrowHead = false;

drawArrow(cxt, startPoint, endPoint, endArrowHead);


startPoint = {x:300, y:100}
endPoint = {x:300, y:150}
endArrowHead = true;

drawArrow(cxt, startPoint, endPoint, endArrowHead);

startPoint = {x:400, y:200}
endPoint = {x:400, y:250}
endArrowHead = false;

drawArrow(cxt, startPoint, endPoint, endArrowHead);

pdfWriter.writePage(page);
pdfWriter.end();

res.end();

});

function drawArrow(cxt, startPoint, endPoint, endArrowHead) {
    cxt.drawPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y);

    if (endPoint.x > startPoint.x) { //horizontal line
        if (endArrowHead) {// right arrowhead
            cxt.drawPath(endPoint.x, endPoint.y + DELTA, endPoint.x, endPoint.y - DELTA, endPoint.x + DELTA, endPoint.y, {
                type: 'fill',
                color: '#000000'
            });
        } else {// left arrowhead
            cxt.drawPath(startPoint.x, startPoint.y + DELTA, startPoint.x, startPoint.y - DELTA, startPoint.x - DELTA, startPoint.y, {
                type: 'fill',
                color: '#000000'
            });
        }

    } else {//vertical line
        if (endArrowHead) { //up arrowhead
            cxt.drawPath(endPoint.x + DELTA, endPoint.y, endPoint.x - DELTA, endPoint.y, endPoint.x, endPoint.y + DELTA, {
                type: 'fill',
                color: '#000000'
            });
        } else { //down arrowhead
            cxt.drawPath(startPoint.x + DELTA, startPoint.y, startPoint.x - DELTA, startPoint.y, startPoint.x, startPoint.y - DELTA, {
                type: 'fill',
                color: '#000000'
            });
        }
    }
}

app.listen(3000);

---

基于此问题的答案： 特别是公认的：

可以使用矢量算法计算所需的坐标：

function arrowhead(A, B) {
    var h = 10 * Math.sqrt(3);
    var w = 10;
    var d = {x: B.x - A.x, y: B.y - A.y};
    var length = Math.sqrt(d.x*d.x + d.y*d.y);
    var U = {x: d.x/length, y: d.y/length};
    var V = {x: -U.y, y: U.x};
    return [
        {x: B.x - U.x*h + V.x*w, y: B.y - U.y*h + V.y*w},
        {x: B.x - U.x*h - V.x*w, y: B.y - U.y*h - V.y*w}
    ];
}

h

和

w

确定长度和半宽。该函数返回两个点：箭头的端点

用法示例：

var startPoint = {x:100, y:100};
var endPoint = {x:200, y:200};

var arrowPoint = arrowhead(startPoint, endPoint);

// draw line
cxt.drawPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y);

// draw arrow head
cxt.drawPath(endPoint.x, endPoint.y, arrowPoint[0].x, arrowPoint[0].y, arrowPoint[1].x, arrowPoint[1].y, endPoint.x, endPoint.y, {type: 'fill', color: '#000000'});

---

你能分享完整的代码吗？如何绘制箭头（水平/垂直和箭头方向左/右）？箭头由用户绘制。水平/垂直方向将是左或右。我试过这个。我得到正确的箭头只为右侧。我在问题中附上了图片。对于其他方面，箭头是不正确的。我更新了我的帖子，并附加了一个输出图像和更多的例子。即使是左侧和垂直箭头，一切看起来都很好。。。