我有这个javascript程序在屏幕上绘制了一百个圆圈，它们可以在画布上自行弹跳。目前，我在它们上方绘制一个空矩形以清除它们的下一代。但是有更好的方法可以在HTML页面上擦除和刷新Canvas。
代码：
function drawScreen () {
context.fillStyle = '#EEEEEE';
context.fillRect(0, 0, theCanvas.width, theCanvas.height);
//Box
context.strokeStyle = '#000000';
context.strokeRect(1, 1, theCanvas.width-2, theCanvas.height-2);
update();
testWalls();
collide();
render();
}
如果您想尝试以下所有代码：
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>CH5EX7: Balls With Simple Interactions</title>
<script type="text/javascript">
window.addEventListener('load', eventWindowLoaded, false);
function eventWindowLoaded() {
canvasApp();
}
function canvasApp() {
var numBalls = 200 ;
var maxSize = 15;
var minSize = 5;
var maxSpeed = maxSize+5;
var balls = new Array();
var tempBall;
var tempX;
var tempY;
var tempSpeed;
var tempAngle;
var tempRadius;
var tempRadians;
var tempvelocityx;
var tempvelocityy;
function drawScreen () {
context.fillStyle = '#EEEEEE';
context.fillRect(0, 0, theCanvas.width, theCanvas.height);
//Box
context.strokeStyle = '#000000';
context.strokeRect(1, 1, theCanvas.width-2, theCanvas.height-2);
update();
testWalls();
collide();
render();
}
function update() {
var ball;
for (var i =0; i <balls.length; i++) {
ball = balls[i];
ball.nextx = (ball.x += ball.velocityx);
ball.nexty = (ball.y += ball.velocityy);
}
}
function testWalls() {
var ball;
var testBall;
for (var i =0; i <balls.length; i++) {
ball = balls[i];
if (ball.nextx+ball.radius > theCanvas.width) {
ball.velocityx = ball.velocityx*-1;
ball.nextx = theCanvas.width - ball.radius;
} else if (ball.nextx-ball.radius < 0 ) {
ball.velocityx = ball.velocityx*-1;
ball.nextx = ball.radius;
} else if (ball.nexty+ball.radius > theCanvas.height ) {
ball.velocityy = ball.velocityy*-1;
ball.nexty = theCanvas.height - ball.radius;
} else if(ball.nexty-ball.radius < 0) {
ball.velocityy = ball.velocityy*-1;
ball.nexty = ball.radius;
}
}
}
function render() {
var ball;
context.fillStyle = "#000000";
for (var i =0; i <balls.length; i++) {
ball = balls[i];
ball.x = ball.nextx;
ball.y = ball.nexty;
context.beginPath();
context.arc(ball.x,ball.y,ball.radius,0,Math.PI*2,true);
context.closePath();
context.fill();
}
}
function collide() {
var ball;
var testBall;
for (var i =0; i <balls.length; i++) {
ball = balls[i];
for (var j = i+1; j < balls.length; j++) {
testBall = balls[j];
if (hitTestCircle(ball,testBall)) {
collideBalls(ball,testBall);
}
}
}
}
function hitTestCircle(ball1,ball2) {
var retval = false;
var dx = ball1.nextx - ball2.nextx;
var dy = ball1.nexty - ball2.nexty;
var distance = (dx * dx + dy * dy);
if (distance <= (ball1.radius + ball2.radius) * (ball1.radius + ball2.radius) ) {
retval = true;
}
return retval;
}
function collideBalls(ball1,ball2) {
var dx = ball1.nextx - ball2.nextx;
var dy = ball1.nexty - ball2.nexty;
var collisionAngle = Math.atan2(dy, dx);
var speed1 = Math.sqrt(ball1.velocityx * ball1.velocityx + ball1.velocityy * ball1.velocityy);
var speed2 = Math.sqrt(ball2.velocityx * ball2.velocityx + ball2.velocityy * ball2.velocityy);
var direction1 = Math.atan2(ball1.velocityy, ball1.velocityx);
var direction2 = Math.atan2(ball2.velocityy, ball2.velocityx);
var velocityx_1 = speed1 * Math.cos(direction1 - collisionAngle);
var velocityy_1 = speed1 * Math.sin(direction1 - collisionAngle);
var velocityx_2 = speed2 * Math.cos(direction2 - collisionAngle);
var velocityy_2 = speed2 * Math.sin(direction2 - collisionAngle);
var final_velocityx_1 = ((ball1.mass - ball2.mass) * velocityx_1 + (ball2.mass + ball2.mass) * velocityx_2)/(ball1.mass + ball2.mass);
var final_velocityx_2 = ((ball1.mass + ball1.mass) * velocityx_1 + (ball2.mass - ball1.mass) * velocityx_2)/(ball1.mass + ball2.mass);
var final_velocityy_1 = velocityy_1;
var final_velocityy_2 = velocityy_2;
ball1.velocityx = Math.cos(collisionAngle) * final_velocityx_1 + Math.cos(collisionAngle + Math.PI/2) * final_velocityy_1;
ball1.velocityy = Math.sin(collisionAngle) * final_velocityx_1 + Math.sin(collisionAngle + Math.PI/2) * final_velocityy_1;
ball2.velocityx = Math.cos(collisionAngle) * final_velocityx_2 + Math.cos(collisionAngle + Math.PI/2) * final_velocityy_2;
ball2.velocityy = Math.sin(collisionAngle) * final_velocityx_2 + Math.sin(collisionAngle + Math.PI/2) * final_velocityy_2;
ball1.nextx = (ball1.nextx += ball1.velocityx);
ball1.nexty = (ball1.nexty += ball1.velocityy);
ball2.nextx = (ball2.nextx += ball2.velocityx);
ball2.nexty = (ball2.nexty += ball2.velocityy);
}
theCanvas = document.getElementById('canvasOne');
context = theCanvas.getContext('2d');
for (var i = 0; i < numBalls; i++) {
tempRadius = 5;
var placeOK = false;
while (!placeOK) {
tempX = tempRadius*3 + (Math.floor(Math.random()*theCanvas.width)-tempRadius*3);
tempY = tempRadius*3 + (Math.floor(Math.random()*theCanvas.height)-tempRadius*3);
tempSpeed = 4;
tempAngle = Math.floor(Math.random()*360);
tempRadians = tempAngle * Math.PI/ 180;
tempvelocityx = Math.cos(tempRadians) * tempSpeed;
tempvelocityy = Math.sin(tempRadians) * tempSpeed;
tempBall = {x:tempX,y:tempY, nextX: tempX, nextY: tempY, radius:tempRadius, speed:tempSpeed, angle:tempAngle, velocityx:tempvelocityx, velocityy:tempvelocityy, mass:tempRadius};
placeOK = canStartHere(tempBall);
}
balls.push(tempBall);
}
function canStartHere(ball) {
var retval = true;
for (var i =0; i <balls.length; i++) {
if (hitTestCircle(ball, balls[i])) {
retval = false;
}
}
return retval;
}
function gameLoop() {
window.setTimeout(gameLoop, 20);
drawScreen()
}
gameLoop();
}
</script>
</head>
<body>
<div style="position: absolute; top: 50px; left: 50px;">
<canvas id="canvasOne" width="500" height="500">
Your browser does not support the HTML 5 Canvas.
</canvas>
</div>
</body>
</html>
我已经在这里放置了最新的工作代码，以防有人将此帖作为参考。画布明显不同，效果非常好。
代码：
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Moving In Circle</title>
<script type="text/javascript">
window.addEventListener('load', eventWindowLoaded, false);
function eventWindowLoaded() {
canvasApp();
}
function canvasApp() {
var radius = 100;
var circle = {centerX:250, centerY:250, radius:125, angle:0}
var ball = {x:0, y:0,speed:.1};
theCanvas = document.getElementById('canvasOne');
context = theCanvas.getContext('2d');
function erraseCanvas() {
context.clearRect(0,0,theCanvas.width,theCanvas.height);
}
function drawScreen () {
erraseCanvas();
ball.x = circle.centerX + Math.cos(circle.angle) * circle.radius;
ball.y = circle.centerY + Math.sin(circle.angle) * circle.radius;
circle.angle += ball.speed;
context.fillStyle = "#000000";
context.beginPath();
context.arc(ball.x,ball.y,15,0,Math.PI*2,true);
context.closePath();
context.fill();
}
function gameLoop() {
window.setTimeout(gameLoop, 20);
drawScreen()
}
gameLoop();
}
</script>
</head>
<body>
<div style="position: absolute; top: 50px; left: 50px;">
<canvas id="canvasOne" width="500" height="500">
Your browser does not support the HTML 5 Canvas.
</canvas>
</div>
</body>
</html>
答案 0 :(得分：3)
清除画布
context.clearRect ( x , y , w , h );
x：x坐标，以像素为单位 y：Y坐标，以像素为单位 w：宽度，以像素为单位 h：高度，以像素为单位
答案 1 :(得分：3)
调整画布大小将导致画布被清除。
myCanvasElement.width+=0; // causes the canvas to resize (to the same size)
请注意，这也将清除您的转换