畫線準備
準備一個canvas
<canvas id="canvasId" width="1000" height="800"></canvas>
使用pointer事件監聽，落筆，拖拽，收筆。
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);
}
主要的邏輯在Handwritinglff 上，儲存了當前繪製中的線條的所有點集合，所有繪製過的線條集合pointLines 。
class Handwritinglff {
constructor(canvas) {
this.canvas = canvas;
this.ctx = canvas.getContext("2d")
this.line = new Line();
this.pointLines = new Array();//Line陣列
this.k = 0.5;
this.begin = null;
this.middle = null;
this.end = null;
this.lineWidth = 10;
this.isDown = false;
}
down事件的時候初始化當前繪製線條line；
move事件的時候將點加入到當前線條line，並開始繪製
up的時候將點加入繪製線條，並繪製完整一條線。
需要注意的點：
加入點的時候，距離太近的點不需要重複新增；
怎麼形成筆鋒效果呢
很簡單！就是在一條線段的最後幾個點的lineWidth不斷減小，我們這裡選用了最後6個點，如果只選用六個階梯變化，效果是很難看的，會看到一節節明顯的線條變細的過程，如下圖：
所以我們有個關鍵的補點過程，我們會再每6個畫素之間補一個點，根據線條粗細變化的範圍和最後計算出來的點數，就可以知道每兩點連線lineWidth的粗細。
這裡的補點過程我們用到了在貝塞爾曲線上補點的演算法。具體不明白的可以留言哈
bezierCalculate(poss, precision) {
//維度，座標軸數（二維座標，三維座標...）
let dimersion = 2;
//貝塞爾曲線控制點數（階數）
let number = poss.length;
//控制點數不小於 2 ，至少為二維座標系
if (number < 2 || dimersion < 2)
return null;
let result = new Array();
//計算楊輝三角
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];
}
}
//計算座標點
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;
}
部分程式碼如下；
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)";
//繪製不包含this.line的線條
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();//將之前的線條不變的path繪製完
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();//為了開啟新的路徑 否則每次stroke 都會把之前的路徑在描一遍
// 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);//移動到之前的點
this.ctx.lineTo(points[i].x, points[i].y);
this.ctx.stroke();//將之前的線條不變的path繪製完
} 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) {//上一個是控制點 當前點已經被繪製
} else
this.ctx.lineTo(points[i].x, points[i].y);
}
}
})
//繪製this.line線條
let points;
if (isUp)
points = this.line.points;
else
points = this.line.points.clone();
//當前繪製的線條最後幾個補點 貝塞爾方式增加點
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) {//使用貝塞爾計算補點
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 變成一個適合splice的陣列（包含splice前2個引數的陣列）
insertPoints.unshift(index, 1);
Array.prototype.splice.apply(points, insertPoints);
//補完點後
endPoint = startPoint;
startPoint = null;
}
if (count >= 6)
break;
i--;
}
//確定最後線寬變化的點數
let changeWidthCount = count + insertCount;
if (isUp)
this.line.changeWidthCount = changeWidthCount;
//製造橢圓頭
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;
}
//最後的一些點線寬變細
if (i > points.length - changeWidthCount) {
if (!isChangeW) {
this.ctx.stroke();//將之前的線條不變的path繪製完
isChangeW = true;
if (i > 1 && points[i - 1].isControl)
continue;
}
//計算線寬
let w = (lastW - minLineW) / changeWidthCount * (points.length - i) + minLineW;
points[i - 1].lineWidth = w;
this.ctx.beginPath();//為了開啟新的路徑 否則每次stroke 都會把之前的路徑在描一遍
// 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);//移動到之前的點
this.ctx.lineTo(points[i].x, points[i].y);
this.ctx.stroke();//將之前的線條不變的path繪製完
} 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) {//上一個是控制點 當前點已經被繪製
} else
this.ctx.lineTo(points[i].x, points[i].y);
}
}
}
draw
最終效果
動手試試：拖拽寫字即可
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