音視頻入門（三） - RGB和YUV之間的轉換

一、YUV420p轉RGB24

在嵌入式設備上進行神經網路推理時，經常會涉及到YUV420p到RGB之間的轉換

原理

在之前的文章中簡單描述過YUV420p和RGB24的存盤格式，為了方便理解，這里再次列出其存盤格式，

YUV420p                         RGB24
Y Y Y Y Y Y Y Y               R R R R R R R R
Y Y Y Y Y Y Y Y               G G G G G G G G
Y Y Y Y Y Y Y Y               B B B B B B B B
Y Y Y Y Y Y Y Y               R R R R R R R R
U U U U U U U U               G G G G G G G G
V V V V V V V V               B B B B B B B B

上述資訊中簡單回顧了YUV420p和RGB24的存盤格式，它們之間的區別在于:YUV420p屬于分開存盤（Y存盤完再存盤U最后再存盤V），RGB24屬于連續存盤（一個像素點由8位的R元素、8位的G元素、8位的B元素構成），其中，類似于YUV420p的存盤方式稱為Planar方式，RGB24的存盤方式稱為Packed方式，

知道了內部存盤方式之后，我們還需要知道如何從YUV資料轉換成RGB資料，所以我們還需要知道YUV420p到RGB24的資料轉換的公式，

B = Y + 1.779 * (U-128)
G = Y - 0.3455 * (U-128) - 0.7169 * (V-128)
R = Y + 1.4075 * (V-128)

下面是代碼段
注：這里為了轉換后的RGB資料可能不在0~255這個區間內，所以多設定了一個函式避免越過這個區間，

#include <iostream>
#include <string.h>

using namespace std;
#define rgbFileName "rgbTest.rgb"
#define yuvFileName "lena_256x256_yuv420p.yuv"

/* 防止計算后的資料過大或者過小 */
unsigned char clipValue(unsigned char x, unsigned char minVal, unsigned char maxVal)
{
    if (x > maxVal)
    {
        return maxVal;
    }
    else if (x < minVal)
    {
        return minVal;
    }
    else
    {
        return x;
    }
}

bool yuv420ToRGB24(unsigned char *yuvData, int width, int height, unsigned char *rgbData)
{
    int indexY = 0;
    int indexU = 0;
    int indexV = 0;

    unsigned char rData;
    unsigned char gData;
    unsigned char bData;

    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            indexY = i * width + j;
            indexU = width * height + i / 4 * width + j / 2;
            indexV = width * height * 5 / 4 + i / 4 * width + j / 2;

            rData = yuvData[indexY] + 1.402 * (yuvData[indexV] - 128);                                       //R = Y+1.4075*(V-128)
            gData = yuvData[indexY] - 0.34413 * (yuvData[indexU] - 128) - 0.71414 * (yuvData[indexV] - 128); //G = Y-0.3455*(U-128)-0.7169*(V-128)
            bData = yuvData[indexY] + 1.772 * (yuvData[indexU] - 128);                                       //B = Y +1.779*(U-128)

            *(rgbData++) = clipValue(rData, 0, 255);
            *(rgbData++) = clipValue(gData, 0, 255);
            *(rgbData++) = clipValue(bData, 0, 255);
        }
    }

    return true;
}

int main()
{
    int width = 256;
    int height = 256;
    unsigned char *yuvData = (unsigned char *)malloc(width * height * 3 / 2);
    unsigned char *rgbData = (unsigned char *)malloc(width * height * 3);
    FILE *fp1 = NULL;
    FILE *fp2 = NULL;

    fp1 = fopen(yuvFileName, "r+");
    fread(yuvData, 1, width * height * 3 / 2, fp1);

    yuv420ToRGB24(yuvData, width, height, rgbData);

    fp2 = fopen(rgbFileName, "w+");
    fwrite(rgbData, 1, width * height * 3, fp2);

    fclose(fp1);
    fclose(fp2);
    free(yuvData);
    free(rgbData);

    return 0;
}

二、RGB24轉YUV420p

剛剛我們演示了YUV420p轉RGB24，RGB24轉YUV420p就只不過是一個逆程序而已，同樣的我們貼出公式

Y = 0.299 * R + 0.587 * G + 0.114 * B
U = -0.147 * R - 0.289 * G + 0.463 * B
V = 0.615 * R - 0.515 * G - 0.100 * B

注意： 在YUV420p中，U和V占有的位元組數之和應該是Y的一半，所以U，V在水平和垂直方向的取樣數是Y的一半

#include <iostream>
#include <string.h>

using namespace std;
#define rgbFileName "rgbTest.rgb"
#define yuvFileName "yuvTest.yuv"

/* 防止計算后的資料過大或者過小 */
unsigned char clipValue(unsigned char x, unsigned char minVal, unsigned char maxVal)
{
    if (x > maxVal)
    {
        return maxVal;
    }
    else if (x < minVal)
    {
        return minVal;
    }
    else
    {
        return x;
    }
}

bool RGB24ToYuv420(unsigned char *RgbData, int width, int height, unsigned char *YuvData)
{
    unsigned char *ptrY;
    unsigned char *ptrU;
    unsigned char *ptrV;
    unsigned char *ptrRGB;

    memset(YuvData, 0, width * height * 3 / 2);
    ptrY = YuvData;
    ptrU = YuvData + width * height;
    ptrV = ptrU + (width * height * 1 / 4);

    unsigned char yData;
    unsigned char uData;
    unsigned char vData;

    unsigned char rData;
    unsigned char gData;
    unsigned char bData;

    for (int j = 0; j < height; j++)
    {
        ptrRGB = RgbData + width * j * 3;
        for (int i = 0; i < width; i++)
        {

            rData = *(ptrRGB++);
            gData = *(ptrRGB++);
            bData = *(ptrRGB++);
            /*
              原公式
              Y = 0.299 * R + 0.587 * G + 0.114 * B
              U = -0.147 * R - 0.289 * G + 0.463 * B
              V = 0.615 * R - 0.515 * G - 0.100 * B
              這里轉換成整數運算
            */
            yData = (unsigned char)((66 * rData + 129 * gData + 25 * bData + 128) >> 8) + 16;
            uData = (unsigned char)((-38 * rData - 74 * gData + 112 * bData + 128) >> 8) + 128;
            vData = (unsigned char)((112 * rData - 94 * gData - 18 * bData + 128) >> 8) + 128;
            
            /* Y取全部 */
            *(ptrY++) = clipValue(yData, 0, 255);
            if (j % 2 == 0 && i % 2 == 0)
            {
                /* U取偶數行的偶數列 */
                *(ptrU++) = clipValue(uData, 0, 255);
            }
            else
            {
                if (i % 2 == 0)
                {
                    /* V取奇數行的偶數列 */
                    *(ptrV++) = clipValue(vData, 0, 255);
                }
            }
        }
    }
    return true;
}

int main()
{
    int width = 256;
    int height = 256;
    unsigned char *yuvData = (unsigned char *)malloc(width * height * 3 / 2);
    unsigned char *rgbData = (unsigned char *)malloc(width * height * 3);
    FILE *fp1 = NULL;
    FILE *fp2 = NULL;

    fp2 = fopen(rgbFileName, "r+");
    fread(rgbData, 1, width * height * 3, fp2);
    RGB24ToYuv420(rgbData, width, height, yuvData);

    fp1 = fopen(yuvFileName, "w+");
    fwrite(yuvData, 1, width * height * 3 / 2, fp1);

    fclose(fp1);
    fclose(fp2);
    free(yuvData);
    free(rgbData);

    return 0;
}