请用c++详细描述把rgb24转换为yuv420

RGB24到YUV420是一种常见的颜色空间转换，特别是在视频编码和图像处理中。RGB (Red Green Blue) 是一种加色混合模式，而 YUV (Luma - Chrominance) 则是一种减色混合模式，特别适合于传输和显示。YUV420表示每个像素由一个亮度分量(Y) 和两个 chrominance (Cb and Cr) 分量组成，其中Cb和Cr分量每四个像素共享。

以下是使用C++描述将RGB24（通常是一个字节的红、绿、蓝三个分量）转换为YUV420的一种简单方法：

#include <cstdint>

// RGB24 struct representing a pixel in ARGB format (alpha is often ignored)
struct RGB24 {
    uint8_t b; // blue
    uint8_t g; // green
    uint8_t r; // red
};

// YUV420 pixel structure, assuming 8-bit components
struct YUV420 {
    uint8_t y;   // luma
    uint8_t uc[2]; // Cb/Cr packed as two bytes (since they're often downsampled by a factor of 2)
};

// Convert a single RGB24 pixel to YUV420
YUV420 rgbToYuv(RGB24 rgb) {
    const int16_t R = static_cast<int16_t>(rgb.r);
    const int16_t G = static_cast<int16_t>(rgb.g);
    const int16_t B = static_cast<int16_t>(rgb.b);

    // Luma calculation
    int16_t Y = 17 * R + 17 * G + 24 * B / 32;

    // Chrominance calculation (approximate)
    int16_t Cr = (R - G) + 128;
    int16_t Cb = (B - G) / 2 + 128;

    // Pack Cb and Cr into a single byte array for YUV420 format
    YUV420 yuv;
    yuv.y = static_cast<uint8_t>(Y >> 3); // Shift and truncate to 8 bits
    yuv.uc[0] = static_cast<uint8_t>((Cr >> 3) + 16); // Downsample and pack Cb
    yuv.uc[1] = static_cast<uint8_t>((Cb >> 2) + 16); // Downsample and pack Cr

    return yuv;
}