前面的話
在javascript運動系列中，詳細介紹了各種運動，其中就包括碰壁運動。但是，如果用canvas去實現，卻是另一種思路。本文將詳細介紹canvas動態小球重疊效果
靜態小球
首先，生成隨機半徑、隨機位置的50個靜態小球
<button id="btn">按钮</button><canvas id="canvas" width="500" height="300" style="border:1px solid black">当前浏览器不支持canvas，请更换浏览器后再试</canvas><script>var canvas = document.getElementById('canvas');var H=300,W=500;
btn.onclick = function(){
getBalls();
}
getBalls();function getBalls(){
canvas.height = H; if(canvas.getContext){ var cxt = canvas.getContext('2d'); for(var i = 0; i < 50; i++){ var tempR = Math.floor(Math.random()*255); var tempG = Math.floor(Math.random()*255); var tempB = Math.floor(Math.random()*255);
cxt.fillStyle = 'rgb(' + tempR + ',' + tempG + ',' + tempB + ')'; var tempW = Math.floor(Math.random()*W); var tempH = Math.floor(Math.random()*H); var tempR = Math.floor(Math.random()*50);
cxt.beginPath();
cxt.arc(tempW,tempH,tempR,0,Math.PI*2);
cxt.fill();
}
}
}</script>登入後複製
接個小球做隨機運動，需要配合定時器更新小球的運動狀態。這時，需要對上面程式碼進行改寫
<button id="btn">更新</button><canvas id="canvas" width="500" height="300" style="border:1px solid black">当前浏览器不支持canvas，请更换浏览器后再试</canvas><script>btn.onclick = function(){history.go();}var canvas = document.getElementById('canvas');//存储画布宽高var H=300,W=500;//存储小球个数var NUM = 50;//存储小球var balls = [];function getBalls(){ if(canvas.getContext){ var cxt = canvas.getContext('2d'); for(var i = 0; i < NUM; i++){ var tempR = Math.floor(Math.random()*255); var tempG = Math.floor(Math.random()*255); var tempB = Math.floor(Math.random()*255); var tempColor = 'rgb(' + tempR + ',' + tempG + ',' + tempB + ')'; var tempX = Math.floor(Math.random()*W); var tempY = Math.floor(Math.random()*H); var tempR = Math.floor(Math.random()*30+20); var tempBall = {
x:tempX,
y:tempY,
r:tempR,
stepX:Math.floor(Math.random() * 4 -2),
stepY:Math.floor(Math.random() * 4 -2),
color:tempColor,
disX:Math.floor(Math.random() * 3 -1),
disY:Math.floor(Math.random() * 3 -1)
};
balls.push(tempBall);
}
}
}function updateBalls(){ for(var i = 0; i < balls.length; i++){
balls[i].stepY += balls[i].disY;
balls[i].stepX += balls[i].disX;
balls[i].x += balls[i].stepX;
balls[i].y += balls[i].stepY;
}
}function renderBalls(){ //重置画布高度，达到清空画布的效果 canvas.height = H;
if(canvas.getContext){ var cxt = canvas.getContext('2d'); for(var i = 0; i < balls.length; i++){
cxt.beginPath();
cxt.arc(balls[i].x,balls[i].y,balls[i].r,0,2*Math.PI);
cxt.fillStyle = balls[i].color;
cxt.closePath();
cxt.fill();
}
}
}
getBalls();
clearInterval(oTimer);var oTimer = setInterval(function(){ //更新小球运动状态 updateBalls(); //渲染小球 renderBalls();
},50);</script>登入後複製
碰壁檢測
下面，增加小球的碰壁偵測功能，當小球面
function bumpTest(ele){ //左侧
if(ele.x <= ele.r){
ele.x = ele.r;
ele.stepX = -ele.stepX;
} //右侧
if(ele.x >= W - ele.r){
ele.x = W - ele.r;
ele.stepX = -ele.stepX;
} //上侧
if(ele.y <= ele.r){
ele.y = ele.r;
ele.stepY = -ele.stepY;
} //下侧
if(ele.y >= H - ele.r){
ele.y = H - ele.r;
ele.stepY = -ele.stepY;
}
}登入後複製
重疊效果
canvas的合成屬性globalCompositeOperation表示後繪製的圖形怎樣與先繪製的圖形結合，屬性值是字串，可能值如下：
㜎的重疊效果為'xor'，即為最終的效果展示
<script>btn.onclick = function(){history.go();}var canvas = document.getElementById('canvas');//存储画布宽高var H=300,W=500;//存储小球个数var NUM = 30;//存储小球var balls = [];function getBalls(){ if(canvas.getContext){ var cxt = canvas.getContext('2d'); for(var i = 0; i < NUM; i++){ var tempR = Math.floor(Math.random()*255); var tempG = Math.floor(Math.random()*255); var tempB = Math.floor(Math.random()*255); var tempColor = 'rgb(' + tempR + ',' + tempG + ',' + tempB + ')'; var tempR = Math.floor(Math.random()*30+20); var tempX = Math.floor(Math.random()*(W-tempR) + tempR); var tempY = Math.floor(Math.random()*(H-tempR) + tempR);
var tempBall = {
x:tempX,
y:tempY,
r:tempR,
stepX:Math.floor(Math.random() * 13 -6),
stepY:Math.floor(Math.random() * 13 -6),
color:tempColor
};
balls.push(tempBall);
}
}
}function updateBalls(){ for(var i = 0; i < balls.length; i++){
balls[i].x += balls[i].stepX;
balls[i].y += balls[i].stepY;
bumpTest(balls[i]);
}
}function bumpTest(ele){ //左侧
if(ele.x <= ele.r){
ele.x = ele.r;
ele.stepX = -ele.stepX;
} //右侧
if(ele.x >= W - ele.r){
ele.x = W - ele.r;
ele.stepX = -ele.stepX;
} //上侧
if(ele.y <= ele.r){
ele.y = ele.r;
ele.stepY = -ele.stepY;
} //下侧
if(ele.y >= H - ele.r){
ele.y = H - ele.r;
ele.stepY = -ele.stepY;
}
}function renderBalls(){ //重置画布高度，达到清空画布的效果
canvas.height = H;
if(canvas.getContext){ var cxt = canvas.getContext('2d'); for(var i = 0; i < balls.length; i++){
cxt.beginPath();
cxt.arc(balls[i].x,balls[i].y,balls[i].r,0,2*Math.PI);
cxt.fillStyle = balls[i].color;
cxt.closePath();
cxt.fill();
}
}
}
getBalls();
clearInterval(oTimer);var oTimer = setInterval(function(){ //更新小球运动状态 updateBalls(); //渲染小球 renderBalls();
},50);</script>登入後複製
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