HTML5 Canvas火焰效果 像火球发射一样
我们可以在这里查看火焰球的demo演示
当然，我们要来分析一下源代码，主要是一些js代码。
首先很简单地在页面上放一个canvas标签，并且给它点简单的样式：
<canvas></canvas>
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canvas{
position: absolute;
height: 100%;
width: 100%;
left: 0;
top: 0;
cursor: crosshair;
}
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接下来就来分析一下js代码。我们来逐步分解js。
由于这个是二维动画，所以我们利用canvas的getcontext方法来返回一个对象，这个对象包含我们对二维动画操作的api，代码如下：
canvas = document.queryselector('canvas');
ctx = canvas.getcontext('2d');
下面我们来定义粒子：
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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
};
};
})();
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然后我们来定义火球：
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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
};
};
})();
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这里life表示火球的生命周期，下面我们可以看到，life值会随着火球发射力度的改变而改变。
接下来是定义鼠标拖动弹弓，准备发射火球：
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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;
}
};
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可以看到，当鼠标按键弹起时，新建一个火球，并初始化life值。
下面是火球运动时的动画执行代码，包括碰到浏览器边缘时的反射效果：
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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);
});