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>
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以上就是canvas实现动态小球重叠的效果代码的详细内容。