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canvas dynamic ball overlapping effect

高洛峰
高洛峰Original
2017-02-09 17:16:491221browse

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In the javascript sports series, various sports are introduced in detail, including wall collision sports. However, if you use canvas to implement it, it is another way of thinking. This article will introduce in detail the overlapping effect of canvas dynamic balls

static balls

First, generate 50 static balls with random radius and random position

<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 = &#39;rgb(&#39; + tempR + &#39;,&#39; + tempG + &#39;,&#39; + tempB + &#39;)&#39;;            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>

Random movement

Next, these 50 balls make random movements, and the movement status of the balls needs to be updated in conjunction with the timer. At this time, the above code needs to be rewritten

<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 = &#39;rgb(&#39; + tempR + &#39;,&#39; + tempG + &#39;,&#39; + tempB + &#39;)&#39;;            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(&#39;2d&#39;);        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>

Block detection

Next, add the ball collision detection Function, when the ball hits the wall, it changes to the opposite direction

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;
    }
}


当前浏览器不支持canvas，请更换浏览器后再试
<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 &lt;= ele.r){
        ele.x = ele.r;
        ele.stepX = -ele.stepX;
    }    //右侧
    if(ele.x &gt;= W - ele.r){
        ele.x = W - ele.r;
        ele.stepX = -ele.stepX;
    }    //上侧
    if(ele.y &lt;= ele.r){
        ele.y = ele.r;
        ele.stepY = -ele.stepY;
    }    //下侧
    if(ele.y &gt;= 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>

Overlap effect

The composite attribute globalCompositeOperation of canvas indicates how the graphics drawn later are combined with the graphics drawn first. The attribute value is a string, and the possible values ​​are as follows: 

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

Add the overlapping effect of the ball to 'xor', which is the final effect display 


当前浏览器不支持canvas，请更换浏览器后再试
<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 &lt;= ele.r){
        ele.x = ele.r;
        ele.stepX = -ele.stepX;
    }    //右侧
    if(ele.x &gt;= W - ele.r){
        ele.x = W - ele.r;
        ele.stepX = -ele.stepX;
    }    //上侧
    if(ele.y &lt;= ele.r){
        ele.y = ele.r;
        ele.stepY = -ele.stepY;
    }    //下侧
    if(ele.y &gt;= 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 = &#39;xor&#39;;
            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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