使用 canvas 绘制图片，然后下载、上传图片操作指南

最近在做项目时，遇到这样一个需求：设计提供几张切图。前端将切图和后端返回的文本信息拼接在一块，绘制为一张图片。然后会下载或上传。实现起来其实很简单。现在，让我来详细分享一下大概流程。

一、绘图基础：使用 canvas 进行绘制

1. 初始步骤：创建画布

首先，我们需要创建一个画布（canvas）来容纳我们的绘图。您可以使用 JavaScript 的 createCanvas() 方法创建 canvas 元素。例如：

const canvas = document.createElement('canvas');
canvas.width = 500;
canvas.height = 500;

2. 绘制图像

创建好画布后，就可以开始绘制图像了。您可以使用 canvas 的 getContext() 方法获取画布的绘图上下文，然后使用该上下文来绘制图像。例如，要绘制一个矩形，您可以使用以下代码：

const ctx = canvas.getContext('2d');
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, 100, 100);

3. 保存图像

绘制完图像后，您需要将其保存下来。您可以使用 canvas 的 toDataURL() 方法将图像转换为一个 Data URL。例如：

const dataURL = canvas.toDataURL('image/png');

二、数据处理：下载和上传图像

1. 下载图像

要下载图像，您可以使用 HTML5 的 download 属性。例如：

<a href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADIAAAAyCAYAAAAeP4ixAAAACXBIWXMAAAsTAAALEwEAmpwYAAAKT2lDQ1BQaG90b3Nob3AgSUNDIHByb2ZpbGUAAHjanVNnVFPpFj33foXD8Q7UxAOPAjv6Soyee7H624e8B43++44LlBSjmvH8Rx86PlL3+NuTIxN+63tSetPC+ERUa7RBG9ounFhbAgapSMRXWOKremaE2WUzIselle5KOg5fGUZ+D0JXAu 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