使用鼠标绘制 HTML5画布

小开

如果你在实施过程中遇到困难，请告诉我。它使用 processing.js，并具有改变颜色和使绘制点越来越小的特性。

<html>
<head>
<!--script librarires-->
<script type="text/javascript" src="processing.js"></script>
<script type="text/javascript" src="init.js"></script>


<!--styles -->
<style type="text/css" src="stylesheet.css">
</style>
</head>
<body>
<!--toolbox -->
<div id="draggable toolbox"></div>
<script type="application/processing">
// new script
int prevx, prevy;
int newx, newy;
boolean cliked;
color c1 = #000000;
int largeur=2;
int ps = 20;
int px = 50;
int py = 50;


void setup() {
size(500,500);
frameRate(25);
background(50);


prevx = mouseX;
prevy = mouseY;
cliked = false;
}


void draw() {
//couleur
noStroke(0);
fill(#FFFFFF);//blanc
rect(px, py, ps, ps);
fill(#000000);
rect(px, py+(ps), ps, ps);
fill(#FF0000);
rect(px, py+(ps*2), ps, ps);
fill(#00FF00);
rect(px, py+(ps*3), ps, ps);
fill(#FFFF00);
rect(px, py+(ps*4), ps, ps);
fill(#0000FF);
rect(px, py+(ps*5), ps, ps);
//largeur
fill(#FFFFFF);
rect(px, py+(ps*7), ps, ps);
fill(#FFFFFF);
rect(px, py+(ps*8), ps, ps);
stroke(#000000);
line(px+2, py+(ps*7)+(ps/2), px+(ps-2), py+(ps*7)+(ps/2));
line(px+(ps/2), py+(ps*7)+1, px+(ps/2), py+(ps*8)-1);
line(px+2, py+(ps*8)+(ps/2), px+(ps-2), py+(ps*8)+(ps/2));


if(cliked==false){
prevx = mouseX;
prevy = mouseY;
}
if(mousePressed) {
cliked = true;
newx = mouseX;
newy = mouseY;
strokeWeight(largeur);
stroke(c1);
line(prevx, prevy, newx, newy);
prevx = newx;
prevy = newy;
}else{
cliked= false;
}
}
void mouseClicked() {
if (mouseX>=px && mouseX<=(px+ps)){
//couleur
if (mouseY>=py && mouseY<=py+(ps*6)){
c1 = get(mouseX, mouseY);
}
//largeur
if (mouseY>=py+(ps*7) && mouseY<=py+(ps*8)){
largeur += 2;
}
if (mouseY>=py+(ps*8) && mouseY<=py+(ps*9)){
if (largeur>2){
largeur -= 2;
}
}
}
}
</script><canvas></canvas>
</body>
</html>

小开

下面是用画布创建绘图应用程序的最简单方法:

将 mousedown、 mousemove和 mouseup事件侦听器附加到画布 DOM
在 mousedown上，获取鼠标坐标，并使用 moveTo()方法定位绘图光标，使用 beginPath()方法开始新的绘图路径。
在 mousemove上，用 lineTo()连续添加一个新的点到路径上，用 stroke()给最后一段染色。
在 mouseup上，设置一个标志来禁用绘图。

从那里，您可以添加各种其他功能，比如让用户能够选择线条粗细、颜色、笔刷笔触，甚至图层。

小开

最佳答案

这是一个工作样本。

 <html>
<script type="text/javascript">
var canvas, ctx, flag = false,
prevX = 0,
currX = 0,
prevY = 0,
currY = 0,
dot_flag = false;


var x = "black",
y = 2;
    

function init() {
canvas = document.getElementById('can');
ctx = canvas.getContext("2d");
w = canvas.width;
h = canvas.height;
    

canvas.addEventListener("mousemove", function (e) {
findxy('move', e)
}, false);
canvas.addEventListener("mousedown", function (e) {
findxy('down', e)
}, false);
canvas.addEventListener("mouseup", function (e) {
findxy('up', e)
}, false);
canvas.addEventListener("mouseout", function (e) {
findxy('out', e)
}, false);
}
    

function color(obj) {
switch (obj.id) {
case "green":
x = "green";
break;
case "blue":
x = "blue";
break;
case "red":
x = "red";
break;
case "yellow":
x = "yellow";
break;
case "orange":
x = "orange";
break;
case "black":
x = "black";
break;
case "white":
x = "white";
break;
}
if (x == "white") y = 14;
else y = 2;
    

}
    

function draw() {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = x;
ctx.lineWidth = y;
ctx.stroke();
ctx.closePath();
}
    

function erase() {
var m = confirm("Want to clear");
if (m) {
ctx.clearRect(0, 0, w, h);
document.getElementById("canvasimg").style.display = "none";
}
}
    

function save() {
document.getElementById("canvasimg").style.border = "2px solid";
var dataURL = canvas.toDataURL();
document.getElementById("canvasimg").src = dataURL;
document.getElementById("canvasimg").style.display = "inline";
}
    

function findxy(res, e) {
if (res == 'down') {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.offsetLeft;
currY = e.clientY - canvas.offsetTop;
    

flag = true;
dot_flag = true;
if (dot_flag) {
ctx.beginPath();
ctx.fillStyle = x;
ctx.fillRect(currX, currY, 2, 2);
ctx.closePath();
dot_flag = false;
}
}
if (res == 'up' || res == "out") {
flag = false;
}
if (res == 'move') {
if (flag) {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.offsetLeft;
currY = e.clientY - canvas.offsetTop;
draw();
}
}
}
</script>
<body onload="init()">
<canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;"></canvas>
<div style="position:absolute;top:12%;left:43%;">Choose Color</div>
<div style="position:absolute;top:15%;left:45%;width:10px;height:10px;background:green;" id="green" onclick="color(this)"></div>
<div style="position:absolute;top:15%;left:46%;width:10px;height:10px;background:blue;" id="blue" onclick="color(this)"></div>
<div style="position:absolute;top:15%;left:47%;width:10px;height:10px;background:red;" id="red" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:45%;width:10px;height:10px;background:yellow;" id="yellow" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:46%;width:10px;height:10px;background:orange;" id="orange" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:47%;width:10px;height:10px;background:black;" id="black" onclick="color(this)"></div>
<div style="position:absolute;top:20%;left:43%;">Eraser</div>
<div style="position:absolute;top:22%;left:45%;width:15px;height:15px;background:white;border:2px solid;" id="white" onclick="color(this)"></div>
<img id="canvasimg" style="position:absolute;top:10%;left:52%;" style="display:none;">
<input type="button" value="save" id="btn" size="30" onclick="save()" style="position:absolute;top:55%;left:10%;">
<input type="button" value="clear" id="clr" size="23" onclick="erase()" style="position:absolute;top:55%;left:15%;">
</body>
</html>

小开

我正在寻找使用这种方法的签名以及，我在 http://codetheory.in/上发现了一个样本。

我已经将下面的代码添加到 Jsfiddle

网址:

<div id="sketch">
<canvas id="paint"></canvas>
</div>

Javascript:

 (function() {
var canvas = document.querySelector('#paint');
var ctx = canvas.getContext('2d');


var sketch = document.querySelector('#sketch');
var sketch_style = getComputedStyle(sketch);
canvas.width = parseInt(sketch_style.getPropertyValue('width'));
canvas.height = parseInt(sketch_style.getPropertyValue('height'));


var mouse = {x: 0, y: 0};
var last_mouse = {x: 0, y: 0};


/* Mouse Capturing Work */
canvas.addEventListener('mousemove', function(e) {
last_mouse.x = mouse.x;
last_mouse.y = mouse.y;


mouse.x = e.pageX - this.offsetLeft;
mouse.y = e.pageY - this.offsetTop;
}, false);




/* Drawing on Paint App */
ctx.lineWidth = 5;
ctx.lineJoin = 'round';
ctx.lineCap = 'round';
ctx.strokeStyle = 'blue';


canvas.addEventListener('mousedown', function(e) {
canvas.addEventListener('mousemove', onPaint, false);
}, false);


canvas.addEventListener('mouseup', function() {
canvas.removeEventListener('mousemove', onPaint, false);
}, false);


var onPaint = function() {
ctx.beginPath();
ctx.moveTo(last_mouse.x, last_mouse.y);
ctx.lineTo(mouse.x, mouse.y);
ctx.closePath();
ctx.stroke();
};


}());

小开

Alco 看看这个:
例如:
Https://github.com/williammalone/simple-html5-drawing-app

文件:
Http://www.williammalone.com/articles/create-html5-canvas-javascript-drawing-app/

本文件包括以下代码:-

HTML:

<canvas id="canvas" width="490" height="220"></canvas>

约翰逊:

context = document.getElementById('canvas').getContext("2d");


$('#canvas').mousedown(function(e){
var mouseX = e.pageX - this.offsetLeft;
var mouseY = e.pageY - this.offsetTop;


paint = true;
addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop);
redraw();
});


$('#canvas').mouseup(function(e){
paint = false;
});


$('#canvas').mouseleave(function(e){
paint = false;
});


var clickX = new Array();
var clickY = new Array();
var clickDrag = new Array();
var paint;


function addClick(x, y, dragging)
{
clickX.push(x);
clickY.push(y);
clickDrag.push(dragging);
}


//Also redraw
function redraw(){
context.clearRect(0, 0, context.canvas.width, context.canvas.height); // Clears the canvas


context.strokeStyle = "#df4b26";
context.lineJoin = "round";
context.lineWidth = 5;


for(var i=0; i < clickX.length; i++) {
context.beginPath();
if(clickDrag[i] && i){
context.moveTo(clickX[i-1], clickY[i-1]);
}else{
context.moveTo(clickX[i]-1, clickY[i]);
}
context.lineTo(clickX[i], clickY[i]);
context.closePath();
context.stroke();
}
}

还有一个很棒的例子
 Http://perfectionkills.com/exploring-canvas-drawing-techniques/

小开

我认为，这里的其他例子太复杂了。这个更简单，只有 JS..。

// create canvas element and append it to document body
var canvas = document.createElement('canvas');
document.body.appendChild(canvas);


// some hotfixes... ( ≖_≖)
document.body.style.margin = 0;
canvas.style.position = 'fixed';


// get canvas 2D context and set him correct size
var ctx = canvas.getContext('2d');
resize();


// last known position
var pos = { x: 0, y: 0 };


window.addEventListener('resize', resize);
document.addEventListener('mousemove', draw);
document.addEventListener('mousedown', setPosition);
document.addEventListener('mouseenter', setPosition);


// new position from mouse event
function setPosition(e) {
pos.x = e.clientX;
pos.y = e.clientY;
}


// resize canvas
function resize() {
ctx.canvas.width = window.innerWidth;
ctx.canvas.height = window.innerHeight;
}


function draw(e) {
// mouse left button must be pressed
if (e.buttons !== 1) return;


ctx.beginPath(); // begin


ctx.lineWidth = 5;
ctx.lineCap = 'round';
ctx.strokeStyle = '#c0392b';


ctx.moveTo(pos.x, pos.y); // from
setPosition(e);
ctx.lineTo(pos.x, pos.y); // to


ctx.stroke(); // draw it!
}

小开

这是我非常简单的工作画布绘制和擦除。

Https://jsfiddle.net/richardcwc/d2gxjdva/

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;
var tooltype = 'draw';


//Mousedown
$(canvas).on('mousedown', function(e) {
last_mousex = mousex = parseInt(e.clientX-canvasx);
last_mousey = mousey = parseInt(e.clientY-canvasy);
mousedown = true;
});


//Mouseup
$(canvas).on('mouseup', function(e) {
mousedown = false;
});


//Mousemove
$(canvas).on('mousemove', function(e) {
mousex = parseInt(e.clientX-canvasx);
mousey = parseInt(e.clientY-canvasy);
if(mousedown) {
ctx.beginPath();
if(tooltype=='draw') {
ctx.globalCompositeOperation = 'source-over';
ctx.strokeStyle = 'black';
ctx.lineWidth = 3;
} else {
ctx.globalCompositeOperation = 'destination-out';
ctx.lineWidth = 10;
}
ctx.moveTo(last_mousex,last_mousey);
ctx.lineTo(mousex,mousey);
ctx.lineJoin = ctx.lineCap = 'round';
ctx.stroke();
}
last_mousex = mousex;
last_mousey = mousey;
//Output
$('#output').html('current: '+mousex+', '+mousey+'<br/>last: '+last_mousex+', '+last_mousey+'<br/>mousedown: '+mousedown);
});


//Use draw|erase
use_tool = function(tool) {
tooltype = tool; //update
}

canvas {
cursor: crosshair;
border: 1px solid #000000;
}

<canvas id="canvas" width="800" height="500"></canvas>
<input type="button" value="draw" onclick="use_tool('draw');" />
<input type="button" value="erase" onclick="use_tool('erase');" />
<div id="output"></div>

小开

如果你有你的画布背景图像，你将不得不作出一些调整，使其工作正常，因为白色擦除技巧将隐藏背景。

这是带密码的大意。

<html>
<script type="text/javascript">
var canvas, canvasimg, backgroundImage, finalImg;
var mouseClicked = false;
var prevX = 0;
var currX = 0;
var prevY = 0;
var currY = 0;
var fillStyle = "black";
var globalCompositeOperation = "source-over";
var lineWidth = 2;


function init() {
var imageSrc = '/abstract-geometric-pattern_23-2147508597.jpg'
backgroundImage = new Image();
backgroundImage.src = imageSrc;
canvas = document.getElementById('can');
finalImg = document.getElementById('finalImg');
canvasimg = document.getElementById('canvasimg');
canvas.style.backgroundImage = "url('" + imageSrc + "')";
canvas.addEventListener("mousemove", handleMouseEvent);
canvas.addEventListener("mousedown", handleMouseEvent);
canvas.addEventListener("mouseup", handleMouseEvent);
canvas.addEventListener("mouseout", handleMouseEvent);
}


function getColor(btn) {
globalCompositeOperation = 'source-over';
lineWidth = 2;
switch (btn.getAttribute('data-color')) {
case "green":
fillStyle = "green";
break;
case "blue":
fillStyle = "blue";
break;
case "red":
fillStyle = "red";
break;
case "yellow":
fillStyle = "yellow";
break;
case "orange":
fillStyle = "orange";
break;
case "black":
fillStyle = "black";
break;
case "eraser":
globalCompositeOperation = 'destination-out';
fillStyle = "rgba(0,0,0,1)";
lineWidth = 14;
break;
}


}


function draw(dot) {
var ctx = canvas.getContext("2d");
ctx.beginPath();
ctx.globalCompositeOperation = globalCompositeOperation;
if(dot){
ctx.fillStyle = fillStyle;
ctx.fillRect(currX, currY, 2, 2);
} else {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = fillStyle;
ctx.lineWidth = lineWidth;
ctx.stroke();
}
ctx.closePath();
}


function erase() {
if (confirm("Want to clear")) {
var ctx = canvas.getContext("2d");
ctx.clearRect(0, 0, canvas.width, canvas.height);
document.getElementById("canvasimg").style.display = "none";
}
}


function save() {
canvas.style.border = "2px solid";
canvasimg.width = canvas.width;
canvasimg.height = canvas.height;
var ctx2 = canvasimg.getContext("2d");
// comment next line to save the draw only
ctx2.drawImage(backgroundImage, 0, 0);
ctx2.drawImage(canvas, 0, 0);
finalImg.src = canvasimg.toDataURL();
finalImg.style.display = "inline";
}


function handleMouseEvent(e) {
if (e.type === 'mousedown') {
prevX = currX;
prevY = currY;
currX = e.offsetX;
currY = e.offsetY;
mouseClicked = true;
draw(true);
}
if (e.type === 'mouseup' || e.type === "mouseout") {
mouseClicked = false;
}
if (e.type === 'mousemove') {
if (mouseClicked) {
prevX = currX;
prevY = currY;
currX = e.offsetX;
currY = e.offsetY;
draw();
}
}
}
</script>
<body onload="init()">
<canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;">
</canvas>
<div style="position:absolute;top:12%;left:43%;">Choose Color</div>
<div style="position:absolute;top:15%;left:45%;width:10px;height:10px;background:green;" data-color="green" onclick="getColor(this)"></div>
<div style="position:absolute;top:15%;left:46%;width:10px;height:10px;background:blue;" data-color="blue" onclick="getColor(this)"></div>
<div style="position:absolute;top:15%;left:47%;width:10px;height:10px;background:red;" data-color="red" onclick="getColor(this)"></div>
<div style="position:absolute;top:17%;left:45%;width:10px;height:10px;background:yellow;" data-color="yellow" onclick="getColor(this)"></div>
<div style="position:absolute;top:17%;left:46%;width:10px;height:10px;background:orange;" data-color="orange" onclick="getColor(this)"></div>
<div style="position:absolute;top:17%;left:47%;width:10px;height:10px;background:black;" data-color="black" onclick="getColor(this)"></div>
<div style="position:absolute;top:20%;left:43%;">Eraser</div>
<div style="position:absolute;top:22%;left:45%;width:15px;height:15px;background:white;border:2px solid;" data-color="eraser" onclick="getColor(this)"></div>
<canvas id="canvasimg" style="display:none;" ></canvas>
<img id="finalImg" style="position:absolute;top:10%;left:52%;display:none;" >
<input type="button" value="save" id="btn" size="30" onclick="save()" style="position:absolute;top:55%;left:10%;">
<input type="button" value="clear" id="clr" size="23" onclick="erase()" style="position:absolute;top:55%;left:15%;">
</body>
</html>

小开

我必须为这个主题提供一个简单的例子，所以我将在这里分享:

Http://jsfiddle.net/haelle/v6tfp2e1

class SignTool {
constructor() {
this.initVars()
this.initEvents()
}


initVars() {
this.canvas = $('#canvas')[0]
this.ctx = this.canvas.getContext("2d")
this.isMouseClicked = false
this.isMouseInCanvas = false
this.prevX = 0
this.currX = 0
this.prevY = 0
this.currY = 0
}


initEvents() {
$('#canvas').on("mousemove", (e) => this.onMouseMove(e))
$('#canvas').on("mousedown", (e) => this.onMouseDown(e))
$('#canvas').on("mouseup", () => this.onMouseUp())
$('#canvas').on("mouseout", () => this.onMouseOut())
$('#canvas').on("mouseenter", (e) => this.onMouseEnter(e))
}
  

onMouseDown(e) {
this.isMouseClicked = true
this.updateCurrentPosition(e)
}
  

onMouseUp() {
this.isMouseClicked = false
}
  

onMouseEnter(e) {
this.isMouseInCanvas = true
this.updateCurrentPosition(e)
}
  

onMouseOut() {
this.isMouseInCanvas = false
}


onMouseMove(e) {
if (this.isMouseClicked && this.isMouseInCanvas) {
this.updateCurrentPosition(e)
this.draw()
}
}
  

updateCurrentPosition(e) {
this.prevX = this.currX
this.prevY = this.currY
this.currX = e.clientX - this.canvas.offsetLeft
this.currY = e.clientY - this.canvas.offsetTop
}
  

draw() {
this.ctx.beginPath()
this.ctx.moveTo(this.prevX, this.prevY)
this.ctx.lineTo(this.currX, this.currY)
this.ctx.strokeStyle = "black"
this.ctx.lineWidth = 2
this.ctx.stroke()
this.ctx.closePath()
}
}


var canvas = new SignTool()

canvas {
position: absolute;
border: 2px solid;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<canvas id="canvas" width="500" height="300"></canvas>

小开

这个问题已经过去很多年了。

对于任何寻找一个简单的绘图画布(例如，从用户/客户那里获得签名)的人，这里我发布了一个当前接受的答案的更简化的 jquery 版本

$(document).ready(function() {
var flag, dot_flag = false,
prevX, prevY, currX, currY = 0,
color = 'black', thickness = 2;
var $canvas = $('#canvas');
var ctx = $canvas[0].getContext('2d');


$canvas.on('mousemove mousedown mouseup mouseout', function(e) {
prevX = currX;
prevY = currY;
currX = e.clientX - $canvas.offset().left;
currY = e.clientY - $canvas.offset().top;
if (e.type == 'mousedown') {
flag = true;
}
if (e.type == 'mouseup' || e.type == 'mouseout') {
flag = false;
}
if (e.type == 'mousemove') {
if (flag) {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = color;
ctx.lineWidth = thickness;
ctx.stroke();
ctx.closePath();
}
}
});


$('.canvas-clear').on('click', function(e) {
c_width = $canvas.width();
c_height = $canvas.height();
ctx.clearRect(0, 0, c_width, c_height);
$('#canvasimg').hide();
});
});

<html>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.0/jquery.min.js"></script>
<body>
<canvas id="canvas" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;"></canvas>
<input type="button" value="Clear" class="canvas-clear" />
</body>
</html>

小开

一个超级简短的版本，给你，在普通的 JavaScript 中没有 position:absolute。主要思想是将画布的上下文移动到正确的坐标并绘制一条直线。取消注释 click处理程序，并在下面注释 mousedown和 mousemove处理程序，以便了解它是如何工作的。

<!DOCTYPE html>
<html>
<body>


<p style="margin: 50px">Just some padding in y direction</p>


<canvas id="myCanvas" width="300" height="300" style="background: #000; margin-left: 100px;">Your browser does not support the HTML5 canvas tag.</canvas>


<script>
const c = document.getElementById("myCanvas");
// c.addEventListener("click", penTool); // fires after mouse left btn is released
c.addEventListener("mousedown", setLastCoords); // fires before mouse left btn is released
c.addEventListener("mousemove", freeForm);




const ctx = c.getContext("2d");


function setLastCoords(e) {
const {x, y} = c.getBoundingClientRect();
lastX = e.clientX - x;
lastY = e.clientY - y;
}


function freeForm(e) {
if (e.buttons !== 1) return; // left button is not pushed yet
penTool(e);
}


function penTool(e) {
const {x, y} = c.getBoundingClientRect();
const newX = e.clientX - x;
const newY = e.clientY - y;


ctx.beginPath();
ctx.lineWidth = 5;
ctx.moveTo(lastX, lastY);
ctx.lineTo(newX, newY);
ctx.strokeStyle = 'white';
ctx.stroke();
ctx.closePath();


lastX = newX;
lastY = newY;
}


let lastX = 0;
let lastY = 0;
</script>


</body>
</html>

小开

我使用了1083202所做的，但是删除了所有的控件，并实现了 KWILLIAMS 建议的更改，使其对滚动不敏感。我还把画布做得很大，基本上覆盖了整个页面2000x1600px，除了空白处。我没有删除所有的绘图工具和按钮，并使用“蓝色”作为唯一的颜色。

我将 JS 代码放在一个名为 myJS.JS 的单独文件中，并将其放在一个名为“ JS”localy 的文件夹中:

然后我在笔记本电脑上用手写笔在触控板上写字，这样比用手指或鼠标更好一些。

我使用它的文件是一种非正式的，为内部使用，但它是很好的，能够把一些笔画在它之前，使一个 pdf 文件。

    var canvas, ctx, flag = false,
prevX = 0,
currX = 0,
prevY = 0,
currY = 0,
dot_flag = false;


var x = "blue",
y = 3;
    

function init() {
canvas = document.getElementById('can');
ctx = canvas.getContext("2d");
w = canvas.width;
h = canvas.height;
    

canvas.addEventListener("mousemove", function (e) {
findxy('move', e)
}, false);
canvas.addEventListener("mousedown", function (e) {
findxy('down', e)
}, false);
canvas.addEventListener("mouseup", function (e) {
findxy('up', e)
}, false);
canvas.addEventListener("mouseout", function (e) {
findxy('out', e)
}, false);
}
 

function draw() {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = x;
ctx.lineWidth = y;
ctx.stroke();
ctx.closePath();
}
   

function findxy(res, e) {
if (res == 'down') {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.getBoundingClientRect().left;
currY = e.clientY - canvas.getBoundingClientRect().top;
    

flag = true;
dot_flag = true;
if (dot_flag) {
ctx.beginPath();
ctx.fillStyle = x;
ctx.fillRect(currX, currY, 2, 2);
ctx.closePath();
dot_flag = false;
}
}
if (res == 'up' || res == "out") {
flag = false;
}
if (res == 'move') {
if (flag) {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.getBoundingClientRect().left;
currY = e.clientY - canvas.getBoundingClientRect().top;
draw();
}
}
}

<html>






<body onload="init()">
<p>Below you can draw:</p>
<canvas id="can" width="750" height="1050" style="position:absolute;">Below you can write:</canvas>
</body>

小开

我选择了第一个答案，并根据我的需要改变。现在

也适用于移动设备。
反应更灵敏,
卷轴没问题

 <html>
<head>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.2/dist/css/bootstrap.min.css" integrity="sha384-xOolHFLEh07PJGoPkLv1IbcEPTNtaed2xpHsD9ESMhqIYd0nLMwNLD69Npy4HI+N" crossorigin="anonymous">
<style>
.selected::before {


content: 'x';
height: 30px;
width: 30px;
left: 18px;
top: -8px;
font-size: 3rem;
position: absolute;
}


.select-colour{
height: 30px;
width: 30px;
}


</style>
</head>
<script type="text/javascript">
var canvas, ctx, flag = false,
prevX = 0,
currX = 0,
prevY = 0,
currY = 0,
dot_flag = false;


var x = "black",
y = 2;
    

function init() {
canvas = document.getElementById('can');
ctx = canvas.getContext("2d");
w = canvas.width;
h = canvas.height;
    

canvas.addEventListener("mousemove", function (e) {
findxy('move', e, '')
}, false);
canvas.addEventListener("mousedown", function (e) {
findxy('down', e, '')
}, false);
canvas.addEventListener("mouseup", function (e) {
findxy('up', e, '')
}, false);
canvas.addEventListener("mouseout", function (e) {
findxy('out', e, '')
}, false);


canvas.addEventListener("touchmove", function (e) {
findxy('move', e, 'touch')
}, false);
canvas.addEventListener("touchstart", function (e) {
findxy('down', e, 'touch')
}, false);
canvas.addEventListener("touchend", function (e) {
findxy('up', e, 'touch')
}, false);


}
    

function color(obj) {
x = obj.id;
if (x == "white") y = 14;
else y = 2;
var prevSelected = document.getElementsByClassName("selected");
// If it exists, remove it.
if(prevSelected.length > 0) {
prevSelected[0].classList.remove("selected");
}
                 

document.getElementById(obj.id).classList.add("selected");
}
    

function draw() {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = x;
ctx.lineWidth = y;
ctx.stroke();
ctx.closePath();
}
    

function erase() {
var m = confirm("Want to clear");
if (m) {
ctx.clearRect(0, 0, w, h);
document.getElementById("canvasimg").style.display = "none";
}
}
    

function save() {
document.getElementById("canvasimg").style.border = "2px solid";
var dataURL = canvas.toDataURL();
document.getElementById("canvasimg").src = dataURL;
document.getElementById("canvasimg").style.display = "inline";
}


function download() {
var link = document.createElement('a');
var dataURL = canvas.toDataURL();
link.href = dataURL;
link.download = 'mydrawing.png';
document.body.appendChild(link);
link.click();
document.body.removeChild(link);
}
    

function findxy(res, e, source) {
if(source){
var clientX = e.changedTouches[0].clientX;
var clientY = e.changedTouches[0].clientY;
}else{
var clientX = e.clientX;
var clientY = e.clientY;
}


if (res == 'down') {
prevX = currX;
prevY = currY;
currX = clientX - canvas.getBoundingClientRect().left;
currY = clientY - canvas.getBoundingClientRect().top;
    

flag = true;
dot_flag = true;
if (dot_flag) {
ctx.beginPath();
ctx.fillStyle = x;
ctx.fillRect(currX, currY, 2, 2);
ctx.closePath();
dot_flag = false;
}
}


if (res == 'up' || res == "out") {
flag = false;
}


if (res == 'move') {
if (flag) {
prevX = currX;
prevY = currY;
currX = clientX - canvas.getBoundingClientRect().left;
currY = clientY - canvas.getBoundingClientRect().top;
draw();
}
}
}
</script>
<body onload="init()">
<div style="">
<div style="float: left;">
<canvas id="can" width="600" height="500" style=" border:5px solid;"></canvas>
</div>
<div style="margin-left:20px; float:left;">
<div class="row">
<div class="col-12">Choose Color</div>
<div class="col-2 pt-3"><div class="select-colour" style="background:green;" id="green" onclick="color(this)"></div></div>
<div class="col-2 pt-3"><div class="select-colour" style="background:blue;" id="blue" onclick="color(this)"></div></div>
<div class="col-2 pt-3"><div class="select-colour" style="background:red;" id="red" onclick="color(this)"></div></div>
<div class="col-2 pt-3"><div class="select-colour" style="background:yellow;" id="yellow" onclick="color(this)"></div></div>
<div class="col-2 pt-3"><div class="select-colour" style="background:orange;" id="orange" onclick="color(this)"></div></div>
<div class="col-2 pt-3"><div class="select-colour" style="background:black;" id="black" onclick="color(this)"></div></div>
<div class="col-12 pt-3 pb-3" style="">Eraser</div>
<div class="col-2 pt-3"><div class="select-colour" style="background:white;border:2px solid;" id="white" onclick="color(this)"></div></div>
</div>


<div class="row">
<div class="col-6 col-lg-12 mt-3">
<input type="button" value="Download" id="download" size="23" onclick="download()" class="btn btn-primary">
</div>
<div class="col-6 col-lg-12 mt-3">
<input type="button" value="clear" id="clr" size="23" onclick="erase()" class="btn btn-light">
</div>
</div>
</div>


</div>
</body>
</html>