我一直在使用画布制作草图应用程序。我似乎无法使用橡皮擦来处理我的代码。我已经尝试了Stack Overflow的大量答案，其中大多数都不起作用。
例如：
globalcompositeoperation alpha设为0 clearRect删除。这很有效，但浏览器速度很慢。所以我希望有人可以帮我解决这个问题。
这是a demo，这是我的代码：
<html>
<head>
<script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>
<style type="text/css">
/*#canvas{background: #F6F683}*/
.canvasBackground{ position: absolute; top: 8px; left: 8px; width: 1000px; height: 2400px;z-index: -10;}
#main { position: fixed; top: 5px; left: 1020px; width: 280px; height: 250px;}
.icon{ cursor: pointer; cursor: hand; }
</style>
<script type="text/javascript" src="../js/jquery-1.9.1.js"></script>
<script type="text/javascript">
var lWidthE = 15; //Line width eraser
var lWidthM = 2; //Line width marker
$(document).ready(function() {
$('.canvasBackground').css("background-image", "url(http://s22.postimg.org/i83b7ztch/notepad_page.png)");
});
var canvas;
var ctx;
var started = false;
var lastx = 0;
var lasty = 0;
var memCanvas;
var memCtx;
var pointerCanvas;
var pCtx;
var points = [];
function init() {
// Bind canvas to listeners
canvas = document.getElementById('canvas');
canvas.addEventListener('mousedown', mouseDown, false);
document.addEventListener('mousemove', mouseMove, false);
document.addEventListener('mouseup', mouseUp, false);
ctx = canvas.getContext('2d');
// create an in-memory canvas
memCanvas = document.createElement('canvas');
memCanvas.width = 1000;
memCanvas.height = 2400;
memCtx = memCanvas.getContext('2d');
ctx.lineJoin = 'round';
ctx.lineCap = 'round';
}
function ctx_stuff() {
if (v) {
ctx.lineWidth = lWidthE;
ctx.globalCompositeOperation = "source-over";
ctx.strokeStyle = "rgba(246,246,131,0)";
}
else {
ctx.lineWidth = lWidthM;
ctx.globalCompositeOperation = "source-over";
ctx.strokeStyle = "rgba(0,0,0,1)";
}
}
function mouseDown(e) {
var m = getMouse(e, canvas);
points.push({
x: m.x,
y: m.y
});
started = true;
ctx.clearRect(0, 0, 1000, 2400);
// put back the saved content
ctx.drawImage(memCanvas, 0, 0);
memCtx.clearRect(0, 0, 1000, 2400);
memCtx.drawImage(canvas, 0, 0);
drawPoints(ctx, points);
}
function mouseMove(e) {
if (started) {//to doodle
ctx.clearRect(0, 0, 1000, 2400);
// // put back the saved content
ctx.drawImage(memCanvas, 0, 0);
var m = getMouse(e, canvas);
points.push({
x: m.x,
y: m.y
});
drawPoints(ctx, points);
} else {//to show where start point of doodle
var m = getMouse(e, canvas);
points.push({
x: m.x,
y: m.y
});
ctx.clearRect(0, 0, 1000, 2400);
// put back the saved content
ctx.drawImage(memCanvas, 0, 0);
drawPoints(ctx, points);
points = [];
}
}
function mouseUp(e) {
if (started) {
started = false;
// When the pen is done, save the resulting context
// to the in-memory canvas
memCtx.clearRect(0, 0, 1000, 2400);
memCtx.drawImage(canvas, 0, 0);
ctx.drawImage(memCanvas, 0, 0);
points = [];
}
}
// clear both canvases!
function clear123() {
ctx.clearRect(0, 0, 1000, 2400);
memCtx.clearRect(0, 0, 1000, 2400);
cleanUpArray();
}
var small_x = 0, small_y = 0, big_x = 0, big_y = 0;
function drawPoints(ctx, points) {
ctx_stuff();
// draw a basic circle instead
if (points.length < 6) {
var b = points[0];
if (v) {
ctx.beginPath(), ctx.arc(b.x, b.y, ctx.lineWidth / 2, 0, Math.PI * 2, !0), ctx.closePath(), ctx.fillStyle = "rgba(246,246,131,0)", ctx.fill();
// ctx.clearRect(b.x - (lWidthE / 2), b.y - (lWidthE / 2), (lWidthE), (lWidthE));
} else {
ctx.beginPath(), ctx.arc(b.x, b.y, ctx.lineWidth / 2, 0, Math.PI * 2, !0), ctx.closePath(), ctx.fillStyle = "rgba(0,0,0,1)", ctx.fill();
}
return;
}
ctx.beginPath(), ctx.moveTo(points[0].x, points[0].y);
// draw a bunch of quadratics, using the average of two points as the control point
for (var i = 1; i < points.length - 2; i++) {
var c = (points[i].x + points[i + 1].x) / 2,
d = (points[i].y + points[i + 1].y) / 2;
// if (v) {
// ctx.clearRect(points[i].x - (lWidthE / 2), points[i].y - (lWidthE / 2), lWidthE, lWidthE);
// }
// else {
ctx.quadraticCurveTo(points[i].x, points[i].y, c, d);
// }
}
// if (!v) {
ctx.quadraticCurveTo(points[i].x, points[i].y, points[i + 1].x, points[i + 1].y), ctx.stroke();
// }
}
// Creates an object with x and y defined,
// set to the mouse position relative to the state's canvas
// If you wanna be super-correct this can be tricky,
// we have to worry about padding and borders
// takes an event and a reference to the canvas
function getMouse(e, canvas) {
var element = canvas, offsetX = 0, offsetY = 0, mx, my;
// Compute the total offset. It's possible to cache this if you want
if (element.offsetParent !== undefined) {
do {
offsetX += element.offsetLeft;
offsetY += element.offsetTop;
} while ((element == element.offsetParent));
}
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
ex = mx;
ey = my;
var tr;
if (v) {
tr = lWidthE + 250;
} else {
tr = lWidthM + 250;
}
if (mx < small_x || small_x === 0) {
small_x = mx - tr;
if (small_x < 0) {
small_x = 0;
}
}
if (mx > big_x || big_x === 0) {
big_x = mx + tr;
if (big_x > 1000) {
big_x = 1000;
}
}
if (my < small_y || small_y === 0) {
small_y = my - tr;
if (small_y < 0) {
small_y = 0;
}
}
if (my > big_y || big_y === 0) {
big_y = my + tr;
if (big_y > 2400) {
big_y = 2400;
}
}
// We return a simple javascript object with x and y defined
return {x: mx, y: my};
}
var v = false;
function erase() {
if (v) {
v = false;
}
else {
v = true;
}
}
</script>
</head>
<body onload="init();">
<div class="canvasBackground"></div>
<div class="canvasBackground"></div>
<canvas id='canvas' width='1000' height='2400'></canvas>
<div id="main">
<p>
<label for="amountM">Marker size:</label>
<input type="text" id="amountM" style="border: 0; color: #f6931f; font-weight: bold;"/>
</p>
<div title="Slide the bar to change size of marker" id="slider-range-minM"></div>
<p>
<label for="amountE">Eraser size:</label>
<input type="text" id="amountE" style="border: 0; color: #f6931f; font-weight: bold;"/>
</p>
<div title="Slide the bar to change size of eraser" id="slider-range-minE"></div>
<br/>
<button title="Clear the canvas area." onclick='clear123();'>Clear</button>
<input type="button" id="btnErase" title="Click to change between eraser and marker." onclick='erase();' value="Eraser/Marker"/>
<img class="icon" src="icons/undo-icon.png" alt="Undo." title="Undo" onclick="javascript:undo();">
<img class="icon" src="icons/redo-icon.png" alt="Redo." title="Redo" onclick="javascript:redo();">
</div>
</body>
</html>
答案 0 :(得分：10)
以下是使用globalCompositeOperation =“destination-out”创建橡皮擦的说明
此合成将“擦除”橡皮擦所覆盖的任何先前像素。
提示：将橡皮擦画成圆圈有助于您不留下草图线条的“剪切”。
这是一个mousemove函数，它说明了草绘线和擦除。
我看到你以不同的方式处理草图（录制笔画），但这将说明如何将目的地合成用作橡皮擦。
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
if(isMouseDown){
ctx.beginPath();
if(mode=="pen"){
ctx.globalCompositeOperation="source-over";
ctx.moveTo(lastX,lastY);
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
}else{
ctx.globalCompositeOperation="destination-out";
ctx.arc(lastX,lastY,5,0,Math.PI*2,false);
ctx.fill();
}
lastX=mouseX;
lastY=mouseY;
}
}
这是代码和小提琴：http://jsfiddle.net/m1erickson/uSMxU/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<!--[if lt IE 9]><script type="text/javascript" src="../excanvas.js"></script><![endif]-->
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var lastX;
var lastY;
var strokeColor="red";
var strokeWidth=2;
var mouseX;
var mouseY;
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var isMouseDown=false;
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
lastX=mouseX;
lastY=mouseY;
isMouseDown=true;
}
function handleMouseUp(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mouseup stuff here
isMouseDown=false;
}
function handleMouseOut(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mouseOut stuff here
isMouseDown=false;
}
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
if(isMouseDown){
ctx.beginPath();
if(mode=="pen"){
ctx.globalCompositeOperation="source-over";
ctx.moveTo(lastX,lastY);
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
}else{
ctx.globalCompositeOperation="destination-out";
ctx.arc(lastX,lastY,5,0,Math.PI*2,false);
ctx.fill();
}
lastX=mouseX;
lastY=mouseY;
}
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
var mode="pen";
$("#pen").click(function(){ mode="pen"; });
$("#eraser").click(function(){ mode="eraser"; });
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas></br>
<button id="pen">Pen</button>
<button id="eraser">Eraser</button>
</body>
</html>