前面的话
在javascript运动系列中，详细介绍了各种运动，其中就包括碰壁运动。但是，如果用canvas去实现，却是另一种思路。本文将详细介绍canvas动态小球重叠效果
静态小球
首先，生成随机半径、随机位置的50个静态小球
<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>随机运动
接着，这50个小球做随机运动，需要配合定时器更新小球的运动状态。这时，需要对上面代码进行改写
<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>碰壁检测
下面，增加小球的碰壁检测功能，当小球碰壁时，变为相反方向
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; }}<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 = 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>重叠效果
canvas的合成属性globalCompositeOperation表示后绘制的图形怎样与先绘制的图形结合，属性值是字符串，可能值如下：source-over(默认)：后绘制的图形位于先绘制的图形上方source-in:后绘制的图形与先绘制的图形重叠的部分可见，两者其他部分完全透明source-out:后绘制的图形与先绘制的图形不重叠的部分可见，先绘制的图形完全透明source-atop:后绘制的图形与先绘制的图形重叠的部分可见，先绘制的图形不受影响destination-over:后绘制的图形位于先绘制的图形下方，只有之前透明像素下的部分才可见destination-in:后绘制的图形位于先绘制的图形下方，两者不重叠的部分完全透明destination-out:后绘制的图形擦除与先绘制的图形重叠的部分destination-atop:后绘制的图形位于先绘制的图形下方，在两者不重叠的地方，先绘制的图形会变透明lighter:后绘制的图形与先绘制的图形重叠部分的值相加，使该部分变亮copy:后绘制的图形完全替代与之重叠的先绘制图形xor:后绘制的图形与先绘制的图形重叠的部分执行"异或"操作
增加小球的重叠效果为'xor'，即为最终的效果展示
<button id="btn">变换</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 = 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() * 21 -10), stepY:Math.floor(Math.random() * 21 -10), 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.globalCompositeOperation = 'xor'; 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>源码查看
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