HTML 5画布覆盖在网页上？

Question

我们的目标是让烟花爆竹出现在一个现有网页的顶部，这样你就可以看到现有的网页，以及烟花爆竹在上面爆炸。我成功地让他们越过了页面的顶端，然而，现在他们并没有褪色。我在网页顶部留下了白色的积木。

我有个小玩意：

http://jsfiddle.net/2EQ2w/1/

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() {
    document.body.insertBefore(canvas, document.body.firstChild);
    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(mousePos.x);
}

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, 0.001)";
    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%, 0)");
    gradient.addColorStop(1, "hsla(" + this.color + ", 100%, 50%, 0)");

    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 ,255)");
    gradient.addColorStop(0.1, "rgba(0, 0, 0, 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();
};

是在这个基地上建造的：

http://jsfiddle.net/dtrooper/AceJJ/

有人知道我怎样才能让这些烟火褪色吗？或者让粒子在几毫秒内没有移动后消失？

Answer 1

你绝对可以用褪色的痕迹来实现这一点：

http://jsfiddle.net/LgjG8/

只需设置第二个屏幕外画布就可以减少全球阿尔法。

// create 2nd canvas
var canvas2 = document.createElement('canvas'),
    context2 = canvas2.getContext('2d');

canvas2.width = canvas.width;
canvas2.height = canvas.height;

// reduce alpha of second canvas
context2.globalAlpha = 0.8;

然后，不要简单地擦拭画布的每一个帧，复制第一个屏幕上的帆布到第二个。由于全局alpha值降低，这将产生可见画布的褪色副本。然后，在复制褪色版本之前，先擦拭第一个画布。最后，只需按常规更新画布即可。这会产生一条线索。

// produce faded copy of current canvas
context2.clearRect(0, 0, canvas2.width, canvas2.height);
context2.drawImage(canvas, 0, 0);

// redraw faded copy on original canvas
context.clearRect(0, 0, canvas.width, canvas.height);
context.drawImage(canvas2, 0, 0);

我没有仔细看过你的代码，所以你可能需要稍微玩一下，但你明白了。

Answer 2

因为您希望内容可见，所以可以尝试更改生成跟踪的方式并使用clearRect。相反，要让c.fill()进行跟踪，您可以通过粒子来完成，这样您就可以查看它们。

在Rocket.prototype.render中，您可以这样做：

//c.fill();

var particle = new Particle(this.pos);
particle.shrink = Math.random() * 0.05 + 0.93;
particle.size = 10;

particles.push(particle);

然后这些小径就可以看到了。

示例

在经过编辑的答案之前(不像预期的那样工作)：

在loop()函数中，您有一个非常小的alpha，这样烟花就不会消失。

试着改变：

context.fillStyle = "rgba(0, 0, 0, 0.001)";

至

context.fillStyle = "rgba(0, 0, 0, 0.05)";

希望能帮上忙！

Answer 3

烟花使用低不透明度的fillRect()清除(淡出)旧烟花。因此，画布后面什么也不会显示出来。

但是，您可以使用clearRect()，这样画布就没有坚实的背景。这样做的问题是，烟花不会留下很好的痕迹，因为没有低的不透明度填补他们淡出。

不是最佳的，但至少烟花在前面的其他网页内容。我希望有一个clearStyle，你可以设置为低不透明度，但遗憾的是，没有。

// clear canvas
//context.fillStyle = "rgba(0, 0, 0, 0.05)";
//context.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
context.clearRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

工作实例