canvas動態小球重疊效果，canvas小球重疊

canvas動態小球重疊效果，canvas小球重疊
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

 

效果展示

靜態小球

　　首先，產生隨機半徑、隨機位置的50個靜態小球

<button id="btn">按鈕</button><canvas id="canvas" width="500" height="300" style="border:1px solid black">當前瀏覽器不支援canvas，請更換瀏覽器後再試</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>    

隨機運動

　　接著，這50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，需要對上面代碼進行改寫

<button id="btn">更新</button><canvas id="canvas" width="500" height="300" style="border:1px solid black">當前瀏覽器不支援canvas，請更換瀏覽器後再試</canvas><script>btn.onclick = function(){history.go();}var canvas = document.getElementById('canvas');//儲存畫布寬高var H=300,W=500;//儲存小球個數var NUM = 50;//儲存小球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(){    //重設畫布高度，達到清空畫布的效果    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(){    //更新小球運動狀態    updateBalls();    //渲染小球    renderBalls();},50);</script>

碰壁檢測

　　下面，增加小球的碰壁檢測功能，當小球碰壁時，變為相反方向

function bumpTest(ele){    //左側    if(ele.x <= ele.r){        ele.x = ele.r;        ele.stepX = -ele.stepX;    }    //右側    if(ele.x >= W - ele.r){        ele.x = W - ele.r;        ele.stepX = -ele.stepX;    }    //上側    if(ele.y <= ele.r){        ele.y = ele.r;        ele.stepY = -ele.stepY;    }    //下側    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">btn.onclick = function(){history.go();}var canvas = document.getElementById('canvas');//儲存畫布寬高var H=300,W=500;//儲存小球個數var NUM = 30;//儲存小球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){    //左側    if(ele.x <= ele.r){        ele.x = ele.r;        ele.stepX = -ele.stepX;    }    //右側    if(ele.x >= W - ele.r){        ele.x = W - ele.r;        ele.stepX = -ele.stepX;    }    //上側    if(ele.y <= ele.r){        ele.y = ele.r;        ele.stepY = -ele.stepY;    }    //下側    if(ele.y >= H - ele.r){        ele.y = H - ele.r;        ele.stepY = -ele.stepY;    }}function renderBalls(){    //重設畫布高度，達到清空畫布的效果    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(){    //更新小球運動狀態    updateBalls();    //渲染小球    renderBalls();},50);</script>    

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：

source-over(預設)：後繪製的圖形位於先繪製的圖形上方source-in:後繪製的圖形與先繪製的圖形重疊的部分可見，兩者其他部分完全透明source-out:後繪製的圖形與先繪製的圖形不重疊的部分可見，先繪製的圖形完全透明source-atop:後繪製的圖形與先繪製的圖形重疊的部分可見，先繪製的圖形不受影響destination-over:後繪製的圖形位於先繪製的圖形下方，只有之前透明像素下的部分才可見destination-in:後繪製的圖形位於先繪製的圖形下方，兩者不重疊的部分完全透明destination-out:後繪製的圖形擦除與先繪製的圖形重疊的部分destination-atop:後繪製的圖形位於先繪製的圖形下方，在兩者不重疊的地方，先繪製的圖形會變透明lighter:後繪製的圖形與先繪製的圖形重疊部分的值相加，使該部分變亮copy:後繪製的圖形完全替代與之重疊的先繪製圖形 xor:後繪製的圖形與先繪製的圖形重疊的部分執行"異或"操作

　　增加小球的重疊效果為'xor'，即為最終的效果展示 

<button id="btn">變換</button><canvas id="canvas" width="500" height="300">btn.onclick = function(){history.go();}var canvas = document.getElementById('canvas');//儲存畫布寬高var H=300,W=500;//儲存小球個數var NUM = 30;//儲存小球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){    //左側    if(ele.x <= ele.r){        ele.x = ele.r;        ele.stepX = -ele.stepX;    }    //右側    if(ele.x >= W - ele.r){        ele.x = W - ele.r;        ele.stepX = -ele.stepX;    }    //上側    if(ele.y <= ele.r){        ele.y = ele.r;        ele.stepY = -ele.stepY;    }    //下側    if(ele.y >= H - ele.r){        ele.y = H - ele.r;        ele.stepY = -ele.stepY;    }}function renderBalls(){    //重設畫布高度，達到清空畫布的效果    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(){    //更新小球運動狀態    updateBalls();    //渲染小球    renderBalls();},50);</script>    

源碼查看