Canvas是HTML5中非常重要且有用的東西，我們可以在Canvas上繪製任意的元素，就像你製作Flash一樣。今天我們就在Canvas上來製作一款火焰發射的效果。就像古代的火球砲一樣，而且可以在瀏覽器邊緣反彈，感覺會比較屌。來看看效果圖：
我們可以在這裡查看火焰球的DEMO示範
當然，我們要來分析一下原始程式碼，主要是一些JS程式碼。
首先很簡單地在頁面上放一個canvas標籤，並且給它點簡單的樣式：
<canvas></canvas>
canvas{
position: absolute;
height: 100%;
width: 100%;
left: 0;
top: 0;
cursor: crosshair;
}接下來就來分析一下JS程式碼。我們來逐步分解JS。
由於這個是二維動畫，所以我們利用canvas的getContext方法來傳回一個物件，這個物件包含我們對二維動畫操作的 API，程式碼如下：
canvas = document.querySelector('canvas'); ctx = canvas.getContext('2d');
下面我們來定義粒子：
particles = {};
newParticle = (function(){ var nextIndex = 0; return function(x,y,r,o,c,xv,yv,rv,ov){
particles[++nextIndex] = {
index: nextIndex,
x: x,
y: y,
r: r,
o: o,
c: c,
xv: xv,
yv: yv,
rv: rv,
ov: ov
};
};
})();然後我們來定義火球：
fireballs = {};
newFireball = (function(){ var nextIndex = 0; return function(x,y,xv,yv,life){
fireballs[++nextIndex] = {
index: nextIndex,
x: x,
y: y,
xv: xv,
yv: yv,
life: life
};
};
})();這裡life表示火球的生命週期，下面我們可以看到，life值會隨著火球發射力度的改變而改變。
接下來是定義滑鼠拖曳彈弓，準備發射火球：
mouse = {x:0,y:0,d:0};
onmousemove = function(e){
mouse.x = e.clientX-o.x;
mouse.y = e.clientY-o.y; var dx = mouse.x - pos1.x,
dy = mouse.y - pos1.y;
mouse.d = Math.sqrt(dx*dx+dy*dy);
};
charging = false;
pos1 = {x:0,y:0};
showInstructions = true;
onmousedown = function(e){
pos1.x = mouse.x;
pos1.y = mouse.y;
charging = true;
showInstructions = false;
};
onmouseup = function(){ if(charging){
newFireball(
mouse.x,
mouse.y,
(pos1.x-mouse.x)*0.03,
(pos1.y-mouse.y)*0.03, 600
);
charging = false;
}
};可以看到，當滑鼠按鍵彈起時，新建一個火球，並初始化life值。
以下是火球運動時的動畫執行程式碼，包含碰到瀏覽器邊緣時的反射效果：
time = 0;
requestAnimationFrame(loop = function(){
ctx.setTransform(1,0,0,1,0,0);
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = 1;
ctx.fillStyle = bgColor;
ctx.fillRect(0,0,width,height);
ctx.translate(o.x,o.y);
if(charging){ var c = Math.floor(30+mouse.d/2);
ctx.strokeStyle = 'rgba('+c+','+c+','+c+',1)';
ctx.lineWidth = 4;
ctx.beginPath();
ctx.moveTo(pos1.x,pos1.y);
ctx.lineTo(mouse.x,mouse.y);
ctx.lineCap = 'round';
ctx.stroke();
}
if(showInstructions){
pos1.x = -70;
pos1.y = -35;
if(time<10){ var x = -70,
y = -35,
r = 30-time*2,
a = time/10;
}else if(time<80){ var x = (time-10)*2-70,
y = (time-10)-35,
r = 10,
a = 1;
}else if(time<90){ var x = 70,
y = 35,
r = 10+(time-80)*2,
a = 1-(time-80)/10;
}else if(time<140){ var x = 70,
y = 35,
r = 30,
a = 0;
} var dx = pos1.x-x,
dy = pos1.y-y,
d = Math.sqrt(dx*dx+dy*dy); if(time<80&&time>10){
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = 1; var c = Math.floor(30+d/2);
ctx.strokeStyle = 'rgba('+c+','+c+','+c+',1)';
ctx.lineWidth = 4;
ctx.beginPath();
ctx.moveTo(pos1.x,pos1.y);
ctx.lineTo(x,y);
ctx.lineCap = 'round';
ctx.stroke();
} if(time<140){
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = a;
ctx.beginPath();
ctx.arc(x,y,r,0,Math.PI*2);
ctx.lineWidth = 2;
ctx.strokeStyle = '#aaa';
ctx.stroke();
} if(time==80){
newFireball(
x,
y,
dx*0.03,
dy*0.03, 240
);
}
time = (time+1)%180;
}
ctx.globalCompositeOperation = 'lighter';
for(var i in particles){
var p = particles[i];
ctx.beginPath();
ctx.arc(p.x,p.y,p.r,0,Math.PI*2);
ctx.globalAlpha = p.o;
ctx.fillStyle = p.c;
ctx.fill();
}
for(var i in particles){
var p = particles[i];
p.x += p.xv;
p.y += p.yv;
p.r += p.rv;
p.o += p.ov;
if(p.r<0)delete particles[p.index];
if(p.o<0)delete particles[p.index];
}
for(var i in fireballs){
f = fireballs[i];
var numParticles = Math.sqrt(f.xv*f.xv+f.yv*f.yv)/5;
if(numParticles<1)numParticles=1;
var numParticlesInt = Math.ceil(numParticles),
numParticlesDif = numParticles/numParticlesInt;
for(var j=0;j<numParticlesInt;j++){
newParticle(
f.x-f.xv*j/numParticlesInt,
f.y-f.yv*j/numParticlesInt,
7,
numParticlesDif,
particleColor,
Math.random()*0.6-0.3,
Math.random()*0.6-0.3,
-0.3,
-0.05*numParticlesDif
);
}
f.x += f.xv;
f.y += f.yv;
f.yv += gravity;
var boundary;
if(f.y<(boundary = edge.top+7)){
f.y = boundary;
f.yv *= -1;
}else if(f.y>(boundary = edge.bottom-7)){
f.y = boundary;
f.yv *= -1;
} if(f.x>(boundary = edge.right-7)){
f.x = boundary;
f.xv *= -1;
}else if(f.x<(boundary = edge.left+7)){
f.x = boundary;
f.xv *= -1;
} if(--f.life<0)delete fireballs[f.index];
}
requestAnimationFrame(loop);
});以上是HTML5 Canvas實現火焰效果像火球發射一樣的範例程式碼的詳細內容。更多資訊請關注PHP中文網其他相關文章！
