javascript - Canvas 淡出粒子

Question

我是 Canvas 初学者，如果这是一个微不足道的问题，我很抱歉。怎样才能让作品中的烟花爆炸后淡出呢？

https://jsfiddle.net/ccwhryvv/

var SCREEN_WIDTH = window.innerWidth,
SCREEN_HEIGHT = window.innerHeight,
mousePos = {
    x: 400,
    y: 300
},

// create canvas
canvas = document.createElement('canvas'),
context = canvas.getContext('2d'),
particles = [],
rockets = [],
MAX_PARTICLES = 400,
colorCode = 0;

// init
$(document).ready(function() {
    $('#content')[0].appendChild(canvas);
    canvas.width = SCREEN_WIDTH;
    canvas.height = SCREEN_HEIGHT;
    setInterval(launch, 800);
    setInterval(loop, 1000 / 50);
});


// update mouse position
$(document).mousemove(function(e) {
    e.preventDefault();
    mousePos = {
        x: e.clientX,
        y: e.clientY
    };
});

// launch more rockets!!!
$(document).mousedown(function(e) {
    for (var i = 0; i < 5; i++) {
        launchFrom(Math.random() * SCREEN_WIDTH * 2 / 3 + SCREEN_WIDTH / 6);
    }
});

function launch() {
    launchFrom(SCREEN_WIDTH / 2);
}

function launchFrom(x) {
    if (rockets.length < 10) {
        var rocket = new Rocket(x);
        rocket.explosionColor = Math.floor(Math.random() * 360 / 10) * 10;
        rocket.vel.y = Math.random() * -3 - 4;
        rocket.vel.x = Math.random() * 6 - 3;
        rocket.size = 8;
        rocket.shrink = 0.999;
        rocket.gravity = 0.01;
        rockets.push(rocket);
    }
}

function loop() {
    // update screen size
    if (SCREEN_WIDTH != window.innerWidth) {
        canvas.width = SCREEN_WIDTH = window.innerWidth;
    }
    if (SCREEN_HEIGHT != window.innerHeight) {
        canvas.height = SCREEN_HEIGHT = window.innerHeight;
    }

    // clear canvas
    context.fillStyle = "rgba(0, 0, 0, 0.0)";
    context.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

    var existingRockets = [];

    for (var i = 0; i < rockets.length; i++) {
        // update and render
        rockets[i].update();
        rockets[i].render(context);

        // calculate distance with Pythagoras
        var distance = Math.sqrt(Math.pow(mousePos.x - rockets[i].pos.x, 2) + Math.pow(mousePos.y - rockets[i].pos.y, 2));

        // random chance of 1% if rockets is above the middle
        var randomChance = rockets[i].pos.y < (SCREEN_HEIGHT * 2 / 3) ? (Math.random() * 100 <= 1) : false;

/* Explosion rules
            - 80% of screen
            - going down
            - close to the mouse
            - 1% chance of random explosion
        */
        if (rockets[i].pos.y < SCREEN_HEIGHT / 5 || rockets[i].vel.y >= 0 || distance < 50 || randomChance) {
            rockets[i].explode();
        } else {
            existingRockets.push(rockets[i]);
        }
    }

    rockets = existingRockets;

    var existingParticles = [];

    for (var i = 0; i < particles.length; i++) {
        particles[i].update();

        // render and save particles that can be rendered
        if (particles[i].exists()) {
            particles[i].render(context);
            existingParticles.push(particles[i]);
        }
    }

    // update array with existing particles - old particles should be garbage collected
    particles = existingParticles;

    while (particles.length > MAX_PARTICLES) {
        particles.shift();
    }
}

function Particle(pos) {
    this.pos = {
        x: pos ? pos.x : 0,
        y: pos ? pos.y : 0
    };
    this.vel = {
        x: 0,
        y: 0
    };
    this.shrink = .97;
    this.size = 2;

    this.resistance = 1;
    this.gravity = 0;

    this.flick = false;

    this.alpha = 1;
    this.fade = 0;
    this.color = 0;
}

Particle.prototype.update = function() {
    // apply resistance
    this.vel.x *= this.resistance;
    this.vel.y *= this.resistance;

    // gravity down
    this.vel.y += this.gravity;

    // update position based on speed
    this.pos.x += this.vel.x;
    this.pos.y += this.vel.y;

    // shrink
    this.size *= this.shrink;

    // fade out
    this.alpha -= this.fade;
};

Particle.prototype.render = function(c) {
    if (!this.exists()) {
        return;
    }

    c.save();

    c.globalCompositeOperation = 'lighter';

    var x = this.pos.x,
        y = this.pos.y,
        r = this.size / 2;

    var gradient = c.createRadialGradient(x, y, 0.1, x, y, r);
    gradient.addColorStop(0.1, "rgba(255,255,255," + this.alpha + ")");
    gradient.addColorStop(0.8, "hsla(" + this.color + ", 100%, 50%, " + this.alpha + ")");
    gradient.addColorStop(1, "hsla(" + this.color + ", 100%, 50%, 0.1)");

    c.fillStyle = gradient;

    c.beginPath();
    c.arc(this.pos.x, this.pos.y, this.flick ? Math.random() * this.size : this.size, 0, Math.PI * 2, true);
    c.closePath();
    c.fill();

    c.restore();
};

Particle.prototype.exists = function() {
    return this.alpha >= 0.1 && this.size >= 1;
};

function Rocket(x) {
    Particle.apply(this, [{
        x: x,
        y: SCREEN_HEIGHT}]);

    this.explosionColor = 0;
}

Rocket.prototype = new Particle();
Rocket.prototype.constructor = Rocket;

Rocket.prototype.explode = function() {
    var count = Math.random() * 10 + 80;

    for (var i = 0; i < count; i++) {
        var particle = new Particle(this.pos);
        var angle = Math.random() * Math.PI * 2;

        // emulate 3D effect by using cosine and put more particles in the middle
        var speed = Math.cos(Math.random() * Math.PI / 2) * 15;

        particle.vel.x = Math.cos(angle) * speed;
        particle.vel.y = Math.sin(angle) * speed;

        particle.size = 10;

        particle.gravity = 0.2;
        particle.resistance = 0.92;
        particle.shrink = Math.random() * 0.05 + 0.93;

        particle.flick = true;
        particle.color = this.explosionColor;



        particles.push(particle);
    }
};

Rocket.prototype.render = function(c) {
    if (!this.exists()) {
        return;
    }

    c.save();

    c.globalCompositeOperation = 'lighter';

    var x = this.pos.x,
        y = this.pos.y,
        r = this.size / 2;

    var gradient = c.createRadialGradient(x, y, 0.1, x, y, r);
    gradient.addColorStop(0.1, "rgba(255, 255, 255 ," + this.alpha + ")");
    // gradient.addColorStop(1, "rgba(255, 255, 255, " + this.alpha + ")");
    gradient.addColorStop(1, "rgba(255, 255, 255, 0)");

    c.fillStyle = gradient;

    c.beginPath();
    c.arc(this.pos.x, this.pos.y, this.flick ? Math.random() * this.size / 2 + this.size / 2 : this.size, 0, Math.PI * 2, true);
    c.closePath();
    c.fill();

    c.restore();
};

谢谢!

Answer 1

创建渐变的成本很高——尤其是在动画循环内。

更有效的方法是在应用启动之前预先创建渐变爆炸 Sprite 的 Sprite 表:

• 创建一个内存中 Canvas 作为 Sprite 表，
• 为你的爆炸选择十几种标准颜色。
• 按照爆炸顺序创建渐变 Sprite 。

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var ss=makeSpritesheet(10,15);

ctx.fillStyle='navy';
ctx.fillRect(0,0,cw,ch);
ctx.drawImage(ss,0,0);

function makeSpritesheet(maxRadius,colorCount){
    var c=document.createElement('canvas');
    var ctx=c.getContext('2d');
    var spacing=maxRadius*2.5;
    c.width=spacing*maxRadius;
    c.height=spacing*(colorCount+1);
    for(var colors=0;colors<colorCount;colors++){
        var y=(colors)*spacing+spacing/2;
        var color = parseInt(colors/colorCount*360);
        for(r=2;r<=maxRadius;r++){
            var x=(r-1)*spacing;
            var gradient = ctx.createRadialGradient(x, y, 0, x, y, r);
            gradient.addColorStop(0.2, "white");
            gradient.addColorStop(0.7, 'hsla('+color+', 100%, 50%, 1)');
            gradient.addColorStop(1.0, "rgba(0,0,0,0)");
            ctx.fillStyle = gradient;
            ctx.beginPath();
            ctx.arc(x,y,r,0,Math.PI*2);
            ctx.closePath();
            ctx.fill();
        }
    }
    return(c);
}

body{ background-color:white; }
#canvas{border:1px solid red; }

<canvas id="canvas" width=640 height=512></canvas>

在动画过程中，将 Sprite 从 Sprite 表绘制到 Canvas 上。

在绘制每个 Sprite 之前，通过设置 context.globalAlpha 淡化每个 Sprite 的不透明度。