清除canvas画布内容--点擦除+线擦除

效果结尾处可验收。

画线准备

准备一个canvas

 <canvas id="canvasId" width="1000" height="800"></canvas>

使用pointer事件监听，落笔，拖拽，收笔。

       document.onpointerdown = function (e) {
            if (e.type == "touchstart")
                handwriting.down(e.touches[0].pageX, e.touches[0].pageY);
            else
                handwriting.down(e.x, e.y);
        }
        
        document.onpointermove = function (e) {
            if (e.type == "touchmove")
                handwriting.move(e.touches[0].pageX, e.touches[0].pageY);
            else
                handwriting.move(e.x, e.y);
        }
        
        document.onpointerup = function (e) {
            if (e.type == "touchend")
                handwriting.up(e.touches[0].pageX, e.touches[0].pageY);
            else
                handwriting.up(e.x, e.y);
        }

主要的逻辑在Handwritinglff 上，存储了当前绘制中的线条的所有点集合，所有绘制过的线条集合pointLines 。

class Handwritinglff {

    constructor(canvas) {
        this.canvas = canvas;
        this.ctx = canvas.getContext("2d")
        this.line = new Line();
        this.pointLines = new Array();//Line数组
        this.k = 0.5;
        this.begin = null;
        this.middle = null;
        this.end = null;
this.lineWidth = 10;
        this.isDown = false;
    }

down事件的时候初始化当前绘制线条line；

move事件的时候将点加入到当前线条line，并开始绘制

up的时候将点加入绘制线条，并绘制完整一条线。

需要注意的点：

加入点的时候，距离太近的点不需要重复添加；

怎么形成笔锋效果呢

很简单！就是在一条线段的最后几个点的lineWidth不断减小，我们这里选用了最后6个点，如果只选用六个阶梯变化，效果是很难看的，会看到一节节明显的线条变细的过程，如下图：

所以我们有个关键的补点过程，我们会再每6个像素之间补一个点，根据线条粗细变化的范围和最后计算出来的点数，就可以知道每两点连线lineWidth的粗细。

这里的补点过程我们用到了在贝塞尔曲线上补点的算法。具体不明白的可以留言哈

bezierCalculate(poss, precision) {

            //维度，坐标轴数（二维坐标，三维坐标...）
            let dimersion = 2;

            //贝塞尔曲线控制点数（阶数）
            let number = poss.length;

            //控制点数不小于 2 ，至少为二维坐标系
            if (number < 2 || dimersion < 2)
                return null;

            let result = new Array();

            //计算杨辉三角
            let mi = new Array();
            mi[0] = mi[1] = 1;
            for (let i = 3; i <= number; i++) {

                let t = new Array();
                for (let j = 0; j < i - 1; j++) {
                    t[j] = mi[j];
                }

                mi[0] = mi[i - 1] = 1;
                for (let j = 0; j < i - 2; j++) {
                    mi[j + 1] = t[j] + t[j + 1];
                }
            }

            //计算坐标点
            for (let i = 0; i < precision; i++) {
                let t = i / precision;
                let p = new Point(0, 0);
                result.push(p);
                for (let j = 0; j < dimersion; j++) {
                    let temp = 0.0;
                    for (let k = 0; k < number; k++) {
                        temp += Math.pow(1 - t, number - k - 1) * (j == 0 ? poss[k].x : poss[k].y) * Math.pow(t, k) * mi[k];
                    }
                    j == 0 ? p.x = temp : p.y = temp;
                }
                p.x = this.toDecimal(p.x);
                p.y = this.toDecimal(p.y);
            }

            return result;
        }

  部分代码如下；

    addPoint(p) {
        if (this.line.points.length >= 1) {
            let last_point = this.line.points[this.line.points.length - 1]
            let distance = this.z_distance(p, last_point);
            if (distance < 10) {
                return;
            }
        }

        if (this.line.points.length == 0) {
            this.begin = p;
            p.isControl = true;
            this.pushPoint(p);
        } else {
            this.middle = p;
            let controlPs = this.computeControlPoints(this.k, this.begin, this.middle, null);
            this.pushPoint(controlPs.first);
            this.pushPoint(p);
            p.isControl = true;

            this.begin = this.middle;
        }
    }

addPoint

draw(isUp = false) {
        this.ctx.clearRect(0, 0, this.canvas.width, this.canvas.height);
        this.ctx.strokeStyle = "rgba(255,20,87,1)";


        //绘制不包含this.line的线条
        this.pointLines.forEach((line, index) => {
            let points = line.points;
            this.ctx.beginPath();
            this.ctx.ellipse(points[0].x - 1.5, points[0].y, 6, 3, Math.PI / 4, 0, Math.PI * 2);
            this.ctx.fill();
            this.ctx.beginPath();
            this.ctx.moveTo(points[0].x, points[0].y);
            let lastW = line.lineWidth;
            this.ctx.lineWidth = line.lineWidth;
            this.ctx.lineJoin = "round";
            this.ctx.lineCap = "round";
            let minLineW = line.lineWidth / 4;
            let isChangeW = false;

            let changeWidthCount = line.changeWidthCount;
            for (let i = 1; i <= points.length; i++) {
                if (i == points.length) {
                    this.ctx.stroke();
                    break;
                }
                if (i > points.length - changeWidthCount) {
                    if (!isChangeW) {
                        this.ctx.stroke();//将之前的线条不变的path绘制完
                        isChangeW = true;
                        if (i > 1 && points[i - 1].isControl)
                            continue;
                    }
                    let w = (lastW - minLineW) / changeWidthCount * (points.length - i) + minLineW;
                    points[i - 1].lineWidth = w;
                    this.ctx.beginPath();//为了开启新的路径 否则每次stroke 都会把之前的路径在描一遍
                    // this.ctx.strokeStyle = "rgba("+Math.random()*255+","+Math.random()*255+","+Math.random()*255+",1)";
                    this.ctx.lineWidth = w;
                    this.ctx.moveTo(points[i - 1].x, points[i - 1].y);//移动到之前的点
                    this.ctx.lineTo(points[i].x, points[i].y);
                    this.ctx.stroke();//将之前的线条不变的path绘制完
                } else {
                    if (points[i].isControl && points[i + 1]) {
                        this.ctx.quadraticCurveTo(points[i].x, points[i].y, points[i + 1].x, points[i + 1].y);
                    } else if (i >= 1 && points[i - 1].isControl) {//上一个是控制点 当前点已经被绘制
                    } else
                        this.ctx.lineTo(points[i].x, points[i].y);
                }
            }
        })

        //绘制this.line线条
        let points;
        if (isUp)
            points = this.line.points;
        else
            points = this.line.points.clone();
        //当前绘制的线条最后几个补点 贝塞尔方式增加点
        let count = 0;
        let insertCount = 0;
        let i = points.length - 1;
        let endPoint = points[i];
        let controlPoint;
        let startPoint;
        while (i >= 0) {
            if (points[i].isControl == true) {
                controlPoint = points[i];
                count++;
            } else {
                startPoint = points[i];
            }
            if (startPoint && controlPoint && endPoint) {//使用贝塞尔计算补点
                let dis = this.z_distance(startPoint, controlPoint) + this.z_distance(controlPoint, endPoint);
                let insertPoints = this.BezierCalculate([startPoint, controlPoint, endPoint], Math.floor(dis / 6) + 1);
                insertPoints.splice(0, 1);
                insertCount += insertPoints.length;
                var index = i;//插入位置
                // 把arr2 变成一个适合splice的数组（包含splice前2个参数的数组） 
                insertPoints.unshift(index, 1);
                Array.prototype.splice.apply(points, insertPoints);

                //补完点后
                endPoint = startPoint;
                startPoint = null;
            }
            if (count >= 6)
                break;
            i--;
        }
        //确定最后线宽变化的点数
        let changeWidthCount = count + insertCount;
        if (isUp)
            this.line.changeWidthCount = changeWidthCount;

        //制造椭圆头
        this.ctx.fillStyle = "rgba(255,20,87,1)"
        this.ctx.beginPath();
        this.ctx.ellipse(points[0].x - 1.5, points[0].y, 6, 3, Math.PI / 4, 0, Math.PI * 2);
        this.ctx.fill();

        this.ctx.beginPath();
        this.ctx.moveTo(points[0].x, points[0].y);
        let lastW = this.line.lineWidth;
        this.ctx.lineWidth = this.line.lineWidth;
        this.ctx.lineJoin = "round";
        this.ctx.lineCap = "round";
        let minLineW = this.line.lineWidth / 4;
        let isChangeW = false;
        for (let i = 1; i <= points.length; i++) {
            if (i == points.length) {
                this.ctx.stroke();
                break;
            }
            //最后的一些点线宽变细
            if (i > points.length - changeWidthCount) {
                if (!isChangeW) {
                    this.ctx.stroke();//将之前的线条不变的path绘制完
                    isChangeW = true;
                    if (i > 1 && points[i - 1].isControl)
                        continue;
                }

                //计算线宽
                let w = (lastW - minLineW) / changeWidthCount * (points.length - i) + minLineW;
                points[i - 1].lineWidth = w;
                this.ctx.beginPath();//为了开启新的路径 否则每次stroke 都会把之前的路径在描一遍
                // this.ctx.strokeStyle = "rgba(" + Math.random() * 255 + "," + Math.random() * 255 + "," + Math.random() * 255 + ",0.5)";
                this.ctx.lineWidth = w;
                this.ctx.moveTo(points[i - 1].x, points[i - 1].y);//移动到之前的点
                this.ctx.lineTo(points[i].x, points[i].y);
                this.ctx.stroke();//将之前的线条不变的path绘制完
            } else {
                if (points[i].isControl && points[i + 1]) {
                    this.ctx.quadraticCurveTo(points[i].x, points[i].y, points[i + 1].x, points[i + 1].y);
                } else if (i >= 1 && points[i - 1].isControl) {//上一个是控制点 当前点已经被绘制
                } else
                    this.ctx.lineTo(points[i].x, points[i].y);
            }
        }
    }

draw

 最终效果

动手试试：拖拽写字即可 

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清除canvas画布内容--点擦除+线擦除