canvas怎樣做出黑色背景的紅色煙花

<canvas id="canvas"></canvas>

body { background: #000; margin: 0;
}canvas { cursor: crosshair; display: block;}

// when animating on canvas, it is best to use requestAnimationFrame instead of setTimeout or setInterval// not supported in all browsers though and sometimes needs a prefix, so we need a shimwindow.requestAnimFrame = (function () { return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function (callback) { window.setTimeout(callback, 1000 / 60);
 };
})();// now we will setup our basic variables for the demovar canvas = document.getElementById('canvas'),
 ctx = canvas.getContext('2d'), // full screen dimensions
 cw = window.innerWidth,
 ch = window.innerHeight, // firework collection
 fireworks = [], // particle collection
 particles = [], // starting hue
 hue = 120, // when launching fireworks with a click, too many get launched at once without a limiter, one launch per 5 loop ticks
 limiterTotal = 5,
 limiterTick = 0, // this will time the auto launches of fireworks, one launch per 80 loop ticks
 timerTotal = 80,
 timerTick = 0,
 mousedown = false, // mouse x coordinate,
 mx, // mouse y coordinate
 my;// set canvas dimensionscanvas.width = cw;
canvas.height = ch;// now we are going to setup our function placeholders for the entire demo// get a random number within a rangefunction random(min, max) { return Math.random() * (max - min) + min;
}// calculate the distance between two pointsfunction calculateDistance(p1x, p1y, p2x, p2y) { var xDistance = p1x - p2x,
 yDistance = p1y - p2y; return Math.sqrt(Math.pow(xDistance, 2) + Math.pow(yDistance, 2));
}// create fireworkfunction Firework(sx, sy, tx, ty) { // actual coordinates
 this.x = sx; this.y = sy; // starting coordinates
 this.sx = sx; this.sy = sy; // target coordinates
 this.tx = tx; this.ty = ty; // distance from starting point to target
 this.distanceToTarget = calculateDistance(sx, sy, tx, ty); this.distanceTraveled = 0; // track the past coordinates of each firework to create a trail effect, increase the coordinate count to create more prominent trails
 this.coordinates = []; this.coordinateCount = 3; // populate initial coordinate collection with the current coordinates
 while (this.coordinateCount--) { this.coordinates.push([this.x, this.y]);
 } this.angle = Math.atan2(ty - sy, tx - sx); this.speed = 2; this.acceleration = 1.05; this.brightness = random(50, 70); // circle target indicator radius
 this.targetRadius = 1;
}// update fireworkFirework.prototype.update = function (index) { // remove last item in coordinates array
 this.coordinates.pop(); // add current coordinates to the start of the array
 this.coordinates.unshift([this.x, this.y]); // cycle the circle target indicator radius
 if (this.targetRadius < 8) { this.targetRadius += 0.3;
 } else { this.targetRadius = 1;
 } // speed up the firework
 this.speed *= this.acceleration; // get the current velocities based on angle and speed
 var vx = Math.cos(this.angle) * this.speed,
 vy = Math.sin(this.angle) * this.speed; // how far will the firework have traveled with velocities applied?
 this.distanceTraveled = calculateDistance(this.sx, this.sy, this.x + vx, this.y + vy); // if the distance traveled, including velocities, is greater than the initial distance to the target, then the target has been reached
 if (this.distanceTraveled >= this.distanceToTarget) {
 createParticles(this.tx, this.ty); // remove the firework, use the index passed into the update function to determine which to remove
 fireworks.splice(index, 1);
 } else { // target not reached, keep traveling
 this.x += vx; this.y += vy;
 }
}// draw fireworkFirework.prototype.draw = function () {
 ctx.beginPath(); // move to the last tracked coordinate in the set, then draw a line to the current x and y
 ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][ 1
 ]);
 ctx.lineTo(this.x, this.y);
 ctx.strokeStyle = 'hsl(' + hue + ', 100%, ' + this.brightness + '%)';
 ctx.stroke();
 ctx.beginPath(); // draw the target for this firework with a pulsing circle
 ctx.arc(this.tx, this.ty, this.targetRadius, 0, Math.PI * 2);
 ctx.stroke();
}// create particlefunction Particle(x, y) { this.x = x; this.y = y; // track the past coordinates of each particle to create a trail effect, increase the coordinate count to create more prominent trails
 this.coordinates = []; this.coordinateCount = 5; while (this.coordinateCount--) { this.coordinates.push([this.x, this.y]);
 } // set a random angle in all possible directions, in radians
 this.angle = random(0, Math.PI * 2); this.speed = random(1, 10); // friction will slow the particle down
 this.friction = 0.95; // gravity will be applied and pull the particle down
 this.gravity = 1; // set the hue to a random number +-20 of the overall hue variable
 this.hue = random(hue - 20, hue + 20); this.brightness = random(50, 80); this.alpha = 1; // set how fast the particle fades out
 this.decay = random(0.015, 0.03);
}// update particleParticle.prototype.update = function (index) { // remove last item in coordinates array
 this.coordinates.pop(); // add current coordinates to the start of the array
 this.coordinates.unshift([this.x, this.y]); // slow down the particle
 this.speed *= this.friction; // apply velocity
 this.x += Math.cos(this.angle) * this.speed; this.y += Math.sin(this.angle) * this.speed + this.gravity; // fade out the particle
 this.alpha -= this.decay; // remove the particle once the alpha is low enough, based on the passed in index
 if (this.alpha <= this.decay) {
 particles.splice(index, 1);
 }
}// draw particleParticle.prototype.draw = function () {
 ctx.beginPath(); // move to the last tracked coordinates in the set, then draw a line to the current x and y
 ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][ 1
 ]);
 ctx.lineTo(this.x, this.y);
 ctx.strokeStyle = 'hsla(' + this.hue + ', 100%, ' + this.brightness + '%, ' + this.alpha + ')';
 ctx.stroke();
}// create particle group/explosionfunction createParticles(x, y) { // increase the particle count for a bigger explosion, beware of the canvas performance hit with the increased particles though
 var particleCount = 30; while (particleCount--) {
 particles.push(new Particle(x, y));
 }
}// main demo loopfunction loop() { // this function will run endlessly with requestAnimationFrame
 requestAnimFrame(loop); // increase the hue to get different colored fireworks over time
 hue += 0.5; // normally, clearRect() would be used to clear the canvas
 // we want to create a trailing effect though
 // setting the composite operation to destination-out will allow us to clear the canvas at a specific opacity, rather than wiping it entirely
 ctx.globalCompositeOperation = 'destination-out'; // decrease the alpha property to create more prominent trails
 ctx.fillStyle = 'rgba(0, 0, 0, 0.5)';
 ctx.fillRect(0, 0, cw, ch); // change the composite operation back to our main mode
 // lighter creates bright highlight points as the fireworks and particles overlap each other
 ctx.globalCompositeOperation = 'lighter'; var text = "HAPPY NEW YEAR !";
 ctx.font = "50px sans-serif"; var textData = ctx.measureText(text);
 ctx.fillStyle = "rgba(" + parseInt(random(0, 255)) + "," + parseInt(random(0, 255)) + "," + parseInt(random(0, 255)) + ",0.3)";
 ctx.fillText(text, cw / 2 - textData.width / 2, ch / 2); // loop over each firework, draw it, update it
 var i = fireworks.length; while (i--) {
 fireworks[i].draw();
 fireworks[i].update(i);
 } // loop over each particle, draw it, update it
 var i = particles.length; while (i--) {
 particles[i].draw();
 particles[i].update(i);
 } // launch fireworks automatically to random coordinates, when the mouse isn't down
 if (timerTick >= timerTotal) { if (!mousedown) { // start the firework at the bottom middle of the screen, then set the random target coordinates, the random y coordinates will be set within the range of the top half of the screen
 for (var h = 0; h < 50; h++) {
 fireworks.push(new Firework(cw / 2, ch / 2, random(0, cw), random(0, ch)));
 }
 timerTick = 0;
 }
 } else {
 timerTick++;
 } // limit the rate at which fireworks get launched when mouse is down
 if (limiterTick >= limiterTotal) { if (mousedown) { // start the firework at the bottom middle of the screen, then set the current mouse coordinates as the target
 fireworks.push(new Firework(cw / 2, ch / 2, mx, my));
 limiterTick = 0;
 }
 } else {
 limiterTick++;
 }
}// mouse event bindings// update the mouse coordinates on mousemovecanvas.addEventListener('mousemove', function (e) {
 mx = e.pageX - canvas.offsetLeft;
 my = e.pageY - canvas.offsetTop;
});// toggle mousedown state and prevent canvas from being selectedcanvas.addEventListener('mousedown', function (e) {
 e.preventDefault();
 mousedown = true;
});
canvas.addEventListener('mouseup', function (e) {
 e.preventDefault();
 mousedown = false;
});// once the window loads, we are ready for some fireworks!window.onload = loop;