生成三层影像金字塔，要求用C++语言编写

以下是用C语言实现生成三层影像金字塔的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define WIDTH 640
#define HEIGHT 480

void buildPyramid(unsigned char *srcImg, unsigned char *dstImg1, unsigned char *dstImg2, unsigned char *dstImg3)
{
    int i, j, k;
    double tmp;

    // 第一层
    for(i = 0; i < HEIGHT; i += 2)
    {
        for(j = 0; j < WIDTH; j += 2)
        {
            k = (i * WIDTH + j) * 3;
            dstImg1[(i / 2) * (WIDTH / 2) * 3 + (j / 2) * 3] = srcImg[k];
            dstImg1[(i / 2) * (WIDTH / 2) * 3 + (j / 2) * 3 + 1] = srcImg[k + 1];
            dstImg1[(i / 2) * (WIDTH / 2) * 3 + (j / 2) * 3 + 2] = srcImg[k + 2];
        }
    }

    // 第二层
    for(i = 0; i < HEIGHT / 2; i += 2)
    {
        for(j = 0; j < WIDTH / 2; j += 2)
        {
            k = (i * (WIDTH / 2) + j) * 3;
            tmp = (dstImg1[i * (WIDTH / 2) * 3 + j * 3] + dstImg1[(i + 1) * (WIDTH / 2) * 3 + j * 3] + dstImg1[i * (WIDTH / 2) * 3 + (j + 1) * 3] + dstImg1[(i + 1) * (WIDTH / 2) * 3 + (j + 1) * 3]) / 4.0;
            dstImg2[i * (WIDTH / 2) * 3 + j * 3] = (unsigned char)tmp;
            dstImg2[i * (WIDTH / 2) * 3 + j * 3 + 1] = (unsigned char)tmp;
            dstImg2[i * (WIDTH / 2) * 3 + j * 3 + 2] = (unsigned char)tmp;
        }
    }

    // 第三层
    for(i = 0; i < HEIGHT / 4; i += 2)
    {
        for(j = 0; j < WIDTH / 4; j += 2)
        {
            k = (i * (WIDTH / 4) + j) * 3;
            tmp = (dstImg2[i * (WIDTH / 4) * 3 + j * 3] + dstImg2[(i + 1) * (WIDTH / 4) * 3 + j * 3] + dstImg2[i * (WIDTH / 4) * 3 + (j + 1) * 3] + dstImg2[(i + 1) * (WIDTH / 4) * 3 + (j + 1) * 3]) / 4.0;
            dstImg3[i * (WIDTH / 4) * 3 + j * 3] = (unsigned char)tmp;
            dstImg3[i * (WIDTH / 4) * 3 + j * 3 + 1] = (unsigned char)tmp;
            dstImg3[i * (WIDTH / 4) * 3 + j * 3 + 2] = (unsigned char)tmp;
        }
    }
}

int main()
{
    unsigned char *srcImg = (unsigned char *)malloc(WIDTH * HEIGHT * 3);
    unsigned char *dstImg1 = (unsigned char *)malloc((WIDTH / 2) * (HEIGHT / 2) * 3);
    unsigned char *dstImg2 = (unsigned char *)malloc((WIDTH / 4) * (HEIGHT / 4) * 3);
    unsigned char *dstImg3 = (unsigned char *)malloc((WIDTH / 8) * (HEIGHT / 8) * 3);
    // TODO: 读入源图像数据到srcImg中

    buildPyramid(srcImg, dstImg1, dstImg2, dstImg3);

    // TODO: 将三层金字塔图像数据写入文件

    free(srcImg);
    free(dstImg1);
    free(dstImg2);
    free(dstImg3);

    return 0;
}

其中，`buildPyramid`函数用于生成三层影像金字塔，`srcImg`为源图像数据，`dstImg1`、`dstImg2`、`dstImg3`分别为三层金字塔的图像数据。在函数中，首先生成第一层金字塔，然后基于第一层生成第二层，最后基于第二层生成第三层。具体实现过程可以参考代码中的注释。