前端创造的图片粒子动画效果：HTML5 Canvas 技术详解
我们将深入探讨如何通过 HTML5 的 Canvas 功能，将上传的图片转换成引人入胜的粒子动画效果。这种效果将图片分解成小粒子，并在用户与它们交互时产生动态变化。我们将分步骤详细解析代码，让你能够理解每一行代码的作用，并自己实现这一效果。
环境准备
首先，你需要一个简单的 HTML 元素和一些样式设置：
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Particle Image Animation from Uploaded Image</title> <style> body { display: flex; justify-content: center; align-items: center; height: 100vh; margin: 0; background-color: #f0f0f0; overflow: hidden; } canvas, input { display: block; margin: auto; } </style> </head> <body> <input type="file" id="upload" accept="image/*"> <canvas id="canvas" hidden></canvas> </body> </html>
这段 HTML 设置了一个文件输入控件供用户上传图片，以及一个 Canvas 元素用于渲染动画效果。样式使页面内容居中显示，并将背景设置为浅灰色。
JavaScript 部分
JavaScript 脚本是这个效果的核心，下面我们逐一解析每个部分的功能。
1. 初始化和载入图片：
const canvas = document.getElementById('canvas'); const ctx = canvas.getContext('2d'); let particles = []; const numOfParticles = 5000; const uploadInput = document.getElementById('upload'); uploadInput.addEventListener('change', function(event) { const file = event.target.files[0]; if (file && file.type.startsWith('image')) { const reader = new FileReader(); reader.onload = function(e) { const maxSize = 500; // 最大尺寸 let width = img.width; let height = img.height; let scale = Math.min(maxSize / width, maxSize / height); if (scale < 1) { width *= scale; height *= scale; } canvas.width = width; canvas.height = height; ctx.drawImage(img, 0, 0, width, height); canvas.hidden = false; const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); createParticles(imageData); animate(); }; }; reader.readAsDataURL(file); } });
在这部分代码中，我们首先获取 Canvas 元素并配置基本画布（context）。监听文件输入控件的变化事件，当用户选择一个图片文件时，使用 FileReader 对象读取文件内容，将其转换为 Base64 编码的 URL，然后载入 <img> 元素。图片载入完毕后，把它绘制到 Canvas 上，然后提取图片的像素数据。
2. 创建粒子：
function createParticles(imageData) { particles = []; const { width, height } = imageData; for (let i = 0; i < numOfParticles; i++) { const x = Math.random() * width; const y = Math.random() * height; const color = imageData.data[(~~y * width + ~~x) * 4]; particles.push(new Particle(x, y, color)); } }
这个函数根据图片的像素数据随机生成指定数量的粒子。每个粒子具有位置（x，y）和基于图片某一点的颜色。粒子的初始位置是随机分布的。
3. 定义粒子对象：
function Particle(x, y, color) { this.x = x; this.originalX = x; this.y = y; this.originalY = y; this.color = `rgba(${color},${color},${color}, 0.5)`; this.draw = function() { ctx.fillStyle = this.color; ctx.fillRect(this.x, this.y, 2, 2); }; this.update = function() { let dx = this.originalX - this.x; let dy = this.originalY - this.y; this.x += dx * 0.1; this.y += dy * 0.1; this.draw(); }; }
粒子对象具有 draw 和 update 方法。draw 方法用来在 Canvas 上绘制粒子，update 方法则负责更新粒子的位置，使它们逐渐回到原始位置。
4. 动画循环和鼠标交互：
function animate() { ctx.clearRect(0, 0, canvas.width, canvas.height); particles.forEach(particle => particle.update()); requestAnimationFrame(animate); }
animate 函数清空画布并更新所有粒子的位置，然后通过 requestAnimationFrame 递归调用自身以形成动画循环。
完整代码
复制这段代码到一个.html文件，可以直接在浏览器允许该demo，实际操作一番。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Particle Image Animation from Uploaded Image</title> <style> body { display: flex; justify-content: center; align-items: center; height: 100vh; margin: 0; background-color: #f0f0f0; overflow: hidden; } canvas, input { display: block; margin: auto; } </style> </head> <body> <input type="file" id="upload" accept="image/*"> <canvas id="canvas" hidden></canvas> <script> const canvas = document.getElementById('canvas'); const ctx = canvas.getContext('2d'); let particles = []; const numOfParticles = 5000; const uploadInput = document.getElementById('upload'); uploadInput.addEventListener('change', function (event) { const file = event.target.files[0]; if (file && file.type.startsWith('image')) { const reader = new FileReader(); reader.onload = function (e) { const img = new Image(); img.src = e.target.result; img.onload = function () { const maxSize = 500; // 最大尺寸 let width = img.width; let height = img.height; let scale = Math.min(maxSize / width, maxSize / height); if (scale < 1) { width *= scale; height *= scale; } canvas.width = width; canvas.height = height; ctx.drawImage(img, 0, 0, width, height); canvas.hidden = false; const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); createParticles(imageData); animate(); }; }; reader.readAsDataURL(file); } }); function createParticles(imageData) { particles = []; const { width, height } = imageData; for (let i = 0; i < numOfParticles; i++) { const x = Math.random() * width; const y = Math.random() * height; const color = imageData.data[(~~y * width + ~~x) * 4]; particles.push(new Particle(x, y, color)); } } function Particle(x, y, color) { this.x = x; this.originalX = x; this.y = y; this.originalY = y; this.color = `rgba(${color},${color},${color}, 0.5)`; this.draw = function () { ctx.fillStyle = this.color; ctx.fillRect(this.x, this.y, 2, 2); }; this.update = function () { let dx = this.originalX - this.x; let dy = this.originalY - this.y; this.x += dx * 0.1; this.y += dy * 0.1; this.draw(); }; } function animate() { ctx.clearRect(0, 0, canvas.width, canvas.height); particles.forEach(particle => particle.update()); requestAnimationFrame(animate); } canvas.addEventListener('mousemove', function (e) { const rect = canvas.getBoundingClientRect(); const mouseX = e.clientX - rect.left; const mouseY = e.clientY - rect.top; particles.forEach(particle => { const dx = mouseX - particle.x; const dy = mouseY - particle.y; const dist = Math.sqrt(dx * dx + dy * dy); if (dist < 50) { const angle = Math.atan2(dy, dx); particle.x -= Math.cos(angle); particle.y -= Math.sin(angle); } }); }); </script> </body> </html>
