转：查表法转换YV12到RGB24的例子（附代码）

最近频繁有网友问到关于YUV转RGB的问题，这里我统一回答一下，并给出代码示例，供大家参考（以YV12转RGB24为例）。

YV12字节排列方式如下：

<–width–>
YYYY…YYYY  ^
Y                    Y  |
.                      . height
.                      .   |
YYYY…YYYY  V
<-width/2->
VV…VV      ^
.            .      |
VV…VV      height
UU…UU      |
.            .      |
UU…UU      V

RGB24的字节排列方式如下：
<—width*3—>
BGRBGR…BGRBGR   ^
.                                 .   |
.                                 .  height
.                                 .   |
BGRBGR…BGRBGR   V

详细的像素点布局和字节流排列方式可以参考微软网站。
转换公式我们采用下面的形式：
R = Y + 1.402 ( Cr - 128 ) 
G = Y - 0.34414 ( Cb - 128 )  -  0.71414 ( Cr - 128 ) 
B = Y + 1.772 ( Cb - 128 ) 
这个公式中有大量的浮点运算，严重影响速度，这里要对浮点运算进行优化，变成整形的运算，公式优化如下：
rdif = v + ( ( v  *  103 )  >>  8 );
invgdif = ( ( u  *  88 )  >>  8 )  + ( ( v  *  183 )  >>  8 );
bdif = u +(  ( u * 198 )  >>  8 );

r = YUVdata[YPOS] + rdif;
g = YUVdata[YPOS] - invgdif;
b = YUVdata[YPOS] + bdif;

上面的公式已经比浮点运算提高了很多速度，但是我们可以发现，还可以继续做优化。
y u v的取值都在0~255之间，所以rdif invgdif bdif 取值也都在一定的范围，我们可以预先把这些值计算出来，放到内存中，转换的时候直接查表来获取，这样可以省去了大量的整形的加法，乘法和移位操作。查表出来的rdif invgdif bdif 值再进行r g b公式的计算。当然这里我们还可以通过二次查表来计算rgb的值。这里以一次查表法为例来描述yv12到rgb24的转换，速度应该在300fps以上，至于二次查表法，请读者自己来实现。

const int Table_fv1[256]={ -180, -179, -177, -176, -174, -173, -172, -170, -169, -167, -166, -165, -163, -162, -160, -159, -158, -156, -155, -153, -152, -151, -149, -148, -146, -145, -144, -142, -141, -139, -138, -137, -135, -134, -132, -131, -130, -128, -127, -125, -124, -123, -121, -120, -118, -117, -115, -114, -113, -111, -110, -108, -107, -106, -104, -103, -101, -100, -99, -97, -96, -94, -93, -92, -90, -89, -87, -86, -85, -83, -82, -80, -79, -78, -76, -75, -73, -72, -71, -69, -68, -66, -65, -64, -62, -61, -59, -58, -57, -55, -54, -52, -51, -50, -48, -47, -45, -44, -43, -41, -40, -38, -37, -36, -34, -33, -31, -30, -29, -27, -26, -24, -23, -22, -20, -19, -17, -16, -15, -13, -12, -10, -9, -8, -6, -5, -3, -2, 0, 1, 2, 4, 5, 7, 8, 9, 11, 12, 14, 15, 16, 18, 19, 21, 22, 23, 25, 26, 28, 29, 30, 32, 33, 35, 36, 37, 39, 40, 42, 43, 44, 46, 47, 49, 50, 51, 53, 54, 56, 57, 58, 60, 61, 63, 64, 65, 67, 68, 70, 71, 72, 74, 75, 77, 78, 79, 81, 82, 84, 85, 86, 88, 89, 91, 92, 93, 95, 96, 98, 99, 100, 102, 103, 105, 106, 107, 109, 110, 112, 113, 114, 116, 117, 119, 120, 122, 123, 124, 126, 127, 129, 130, 131, 133, 134, 136, 137, 138, 140, 141, 143, 144, 145, 147, 148, 150, 151, 152, 154, 155, 157, 158, 159, 161, 162, 164, 165, 166, 168, 169, 171, 172, 173, 175, 176, 178 };
const int Table_fv2[256]={ -92, -91, -91, -90, -89, -88, -88, -87, -86, -86, -85, -84, -83, -83, -82, -81, -81, -80, -79, -78, -78, -77, -76, -76, -75, -74, -73, -73, -72, -71, -71, -70, -69, -68, -68, -67, -66, -66, -65, -64, -63, -63, -62, -61, -61, -60, -59, -58, -58, -57, -56, -56, -55, -54, -53, -53, -52, -51, -51, -50, -49, -48, -48, -47, -46, -46, -45, -44, -43, -43, -42, -41, -41, -40, -39, -38, -38, -37, -36, -36, -35, -34, -33, -33, -32, -31, -31, -30, -29, -28, -28, -27, -26, -26, -25, -24, -23, -23, -22, -21, -21, -20, -19, -18, -18, -17, -16, -16, -15, -14, -13, -13, -12, -11, -11, -10, -9, -8, -8, -7, -6, -6, -5, -4, -3, -3, -2, -1, 0, 0, 1, 2, 2, 3, 4, 5, 5, 6, 7, 7, 8, 9, 10, 10, 11, 12, 12, 13, 14, 15, 15, 16, 17, 17, 18, 19, 20, 20, 21, 22, 22, 23, 24, 25, 25, 26, 27, 27, 28, 29, 30, 30, 31, 32, 32, 33, 34, 35, 35, 36, 37, 37, 38, 39, 40, 40, 41, 42, 42, 43, 44, 45, 45, 46, 47, 47, 48, 49, 50, 50, 51, 52, 52, 53, 54, 55, 55, 56, 57, 57, 58, 59, 60, 60, 61, 62, 62, 63, 64, 65, 65, 66, 67, 67, 68, 69, 70, 70, 71, 72, 72, 73, 74, 75, 75, 76, 77, 77, 78, 79, 80, 80, 81, 82, 82, 83, 84, 85, 85, 86, 87, 87, 88, 89, 90, 90 };
const int Table_fu1[256]={ -44, -44, -44, -43, -43, -43, -42, -42, -42, -41, -41, -41, -40, -40, -40, -39, -39, -39, -38, -38, -38, -37, -37, -37, -36, -36, -36, -35, -35, -35, -34, -34, -33, -33, -33, -32, -32, -32, -31, -31, -31, -30, -30, -30, -29, -29, -29, -28, -28, -28, -27, -27, -27, -26, -26, -26, -25, -25, -25, -24, -24, -24, -23, -23, -22, -22, -22, -21, -21, -21, -20, -20, -20, -19, -19, -19, -18, -18, -18, -17, -17, -17, -16, -16, -16, -15, -15, -15, -14, -14, -14, -13, -13, -13, -12, -12, -11, -11, -11, -10, -10, -10, -9, -9, -9, -8, -8, -8, -7, -7, -7, -6, -6, -6, -5, -5, -5, -4, -4, -4, -3, -3, -3, -2, -2, -2, -1, -1, 0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 14, 14, 14, 15, 15, 15, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 20, 20, 20, 21, 21, 22, 22, 22, 23, 23, 23, 24, 24, 24, 25, 25, 25, 26, 26, 26, 27, 27, 27, 28, 28, 28, 29, 29, 29, 30, 30, 30, 31, 31, 31, 32, 32, 33, 33, 33, 34, 34, 34, 35, 35, 35, 36, 36, 36, 37, 37, 37, 38, 38, 38, 39, 39, 39, 40, 40, 40, 41, 41, 41, 42, 42, 42, 43, 43 };
const int Table_fu2[256]={ -227, -226, -224, -222, -220, -219, -217, -215, -213, -212, -210, -208, -206, -204, -203, -201, -199, -197, -196, -194, -192, -190, -188, -187, -185, -183, -181, -180, -178, -176, -174, -173, -171, -169, -167, -165, -164, -162, -160, -158, -157, -155, -153, -151, -149, -148, -146, -144, -142, -141, -139, -137, -135, -134, -132, -130, -128, -126, -125, -123, -121, -119, -118, -116, -114, -112, -110, -109, -107, -105, -103, -102, -100, -98, -96, -94, -93, -91, -89, -87, -86, -84, -82, -80, -79, -77, -75, -73, -71, -70, -68, -66, -64, -63, -61, -59, -57, -55, -54, -52, -50, -48, -47, -45, -43, -41, -40, -38, -36, -34, -32, -31, -29, -27, -25, -24, -22, -20, -18, -16, -15, -13, -11, -9, -8, -6, -4, -2, 0, 1, 3, 5, 7, 8, 10, 12, 14, 15, 17, 19, 21, 23, 24, 26, 28, 30, 31, 33, 35, 37, 39, 40, 42, 44, 46, 47, 49, 51, 53, 54, 56, 58, 60, 62, 63, 65, 67, 69, 70, 72, 74, 76, 78, 79, 81, 83, 85, 86, 88, 90, 92, 93, 95, 97, 99, 101, 102, 104, 106, 108, 109, 111, 113, 115, 117, 118, 120, 122, 124, 125, 127, 129, 131, 133, 134, 136, 138, 140, 141, 143, 145, 147, 148, 150, 152, 154, 156, 157, 159, 161, 163, 164, 166, 168, 170, 172, 173, 175, 177, 179, 180, 182, 184, 186, 187, 189, 191, 193, 195, 196, 198, 200, 202, 203, 205, 207, 209, 211, 212, 214, 216, 218, 219, 221, 223, 225 };

#define WIDTH 320
#define HEIGHT 240
#define PIXELSIZE WIDTH*HEIGHT

bool YV12_to_RGB24(unsigned char* pYV12, unsigned char* pRGB24)
{
 if(!pYV12 || !pRGB24)
  return false;
 
 const long nYLen = long(PIXELSIZE);
 const int nHfWidth = (WIDTH>>1);
 
 if(nYLen<1 || nHfWidth<1)
  return false;
 
 // Y data
 unsigned char* yData = pYV12;
 // v data
 unsigned char* vData = &yData[nYLen];
 // u data
 unsigned char* uData = &vData[nYLen>>2];
 
 if(!uData || !vData)
  return false;
 
 int rgb[3];
 int i, j, m, n, x, y, pu, pv, py, rdif, invgdif, bdif;
 m = -WIDTH;
 n = -nHfWidth;

 bool addhalf = true;
 for(y=0; y   m += WIDTH;
  if( addhalf ){
   n+=nHfWidth;
   addhalf = false;
  } else {
   addhalf = true;
  }
  for(x=0; x    i = m + x;
   j = n + (x>>1);

   py = yData[i];

   // search tables to get rdif invgdif and bidif
   rdif = Table_fv1[vData[j]];    // fv1
   invgdif = Table_fu1[uData[j]] + Table_fv2[vData[j]]; // fu1+fv2
   bdif = Table_fu2[uData[j]]; // fu2

 

//此处应该修改为

//====================

//   rgb[2] = py+rdif;    // R
   //rgb[1] = py-invgdif; // G
  // rgb[0] = py+bdif;    // B
//====================

   rgb[0] = py+rdif;    // R
   rgb[1] = py-invgdif; // G
    rgb[2] = py+bdif;    // B 
   j = nYLen - WIDTH - m + x;
   i = (j<<1) + j;
   
   // copy this pixel to rgb data
   for(j=0; j<3; j++)
   {
    if(rgb[j]>=0 && rgb[j]<=255){
     pRGB24[i + j] = rgb[j];
    }
    else{
     pRGB24[i + j] = (rgb[j] < 0)? 0 : 255;
    }
   }
  }
 }
 return true;
}