画一个箭头 HummusJS

Question

如何在hummusJS 中绘制箭头？ 我正在使用鹰嘴豆泥在 pdf 中绘图。我需要在pdf中画一个箭头。我能够划清界限。但是我怎样才能画一个箭头呢？ 我尝试了以下

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

      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'})

但这不是在正确的位置绘制箭头 这是图片

Answer 1

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http://localhost:3000/ 生成的。 它可以绘制水平和垂直箭头，并在开始或结束处放置箭头。 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);

Answer 2

基于此问题的答案：How to calculate the coordinates of a arrowhead based on the arrow? 特别是被接受的：https://stackoverflow.com/a/10316601/1713942

您可以使用矢量算术计算所需的坐标：

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 确定长度和半宽度。该函数返回两个点：箭头的两端。

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'});