编写C代码 my_canvas.cpp 实现图像数据的管理与圆的绘制，C代码中包含两个函数，get_img_buf()函数根据传入的参数判断是否需要重新创建缓冲区；draw_circle()函数在指定位置以指定半径填充绘制圆。#ifndef EM_PORT_API#if defined(__EMSCRIPTEN__)#include <emscripten.h>#if defined(__cplusplus)#define EM_PORT_API(rettype) extern "C" rettype EMSCRIPTEN_KEEPALIVE#else#define EM_PORT_API(rettype) rettype EMSCRIPTEN_KEEPALIVE#endif#else#if defined(__cplusplus)#define EM_PORT_API(rettype) extern "C" rettype#else#define EM_PORT_API(rettype) rettype#endif#endif#endif#include <stdint.h>#include <malloc.h>uint8_t *img_buf = NULL;//指向位于保存位图的缓冲区int img_width = 0, img_height = 0;EM_PORT_API(uint8_t*) get_img_buf(int w, int h) {//根据传入的参数判断是否需要重新创建缓冲区if (img_buf == NULL || w != img_width || h != img_height) {if (img_buf) {free(img_buf);}img_buf = (uint8_t*)malloc(w * h * 4);img_width = w;img_height = h;}return img_buf;}EM_PORT_API(void) draw_circle(int cx, int cy, int radii) {//在指定位置以指定半径填充绘制圆int sq = radii * radii;for (int y = 0; y < img_height; y++) {for (int x = 0; x < img_width; x++) {int d = (y - cy) * (y - cy) + (x - cx) * (x - cx);if (d < sq) {img_buf[(y * img_width + x) * 4] = 255;//rimg_buf[(y * img_width + x) * 4 + 1] = 0;//gimg_buf[(y * img_width + x) * 4 + 2] = 0;//bimg_buf[(y * img_width + x) * 4 + 3] = 255;//a}else {img_buf[(y * img_width + x) * 4] = 0;//rimg_buf[(y * img_width + x) * 4 + 1] = 255;//gimg_buf[(y * img_width + x) * 4 + 2] = 255;//bimg_buf[(y * img_width + x) * 4 + 3] = 255;//a}}}} 通过命令emcc my_canvas.cpp –s WASM=1 -o my_canvas.js生成my_canvas.js文件和my_canvas.wasm文件。编写HTML代码（my_canvas.html）实现C程序的网页端显示，html中声明了id为myCanvas的canvas元素，在每帧更新时，从Emscripten运行时Module中取出图像数据，创建ImageData对象imgData，并将imgData通过CanvasRenderingContext2D对象ctx更新到canvas上去。<!doctype html><html> <head> <meta charset="utf-8"> <title>Emscripten:Canvas</title> </head> <body> <canvas id="myCanvas"></canvas> <script> Module = {}; Module.onRuntimeInitialized = function() { var canvas = document.getElementById('myCanvas'); canvas.width = 400; canvas.height = 400; window.requestAnimationFrame(update); } var radii = 0, delta = 1; function update() { var buf_addr = Module._get_img_buf(400, 400);//从module读取数据 Module._draw_circle(200, 200, radii); radii += delta; if (radii > 200 || radii < 0) delta = -delta; var u8o = new Uint8ClampedArray(Module.HEAPU8.subarray(buf_addr, buf_addr + 400 * 400 * 4)); var imgData = new ImageData(u8o, 400, 400); var canvas = document.getElementById('myCanvas'); var ctx = canvas.getContext('2d'); ctx.putImageData(imgData, 0, 0); window.requestAnimationFrame(update); } </script> <script src="my_canvas.js"></script> </body></html> 通过命令emrun –no_browser –port 8080 my_canvas.html进行页面浏览
运行效果如下（动态）：