RGB和YCbCr颜色空间的转换及优化算法

RGB和YCbCr颜色空间转换和优化

转载于颜色空间系列3: RGB和YCbCr颜色空间的转换及优化算法

在常用的几种颜色空间中，YCbCr颜色空间在学术论文中出现的频率是相当高的，常用于肤色检测等等。其和RGB空间之间的相互转换公式在网上也有多种，我们这里取http://en.wikipedia.org/wiki/YCbCr 描述的JPG转换时使用的计算公式：

　　JPEG conversion

　　JFIF usage of JPEG allows Y′CbCr where Y′, C_B and C_R have the full 8-bit range of 0-255:

　　And back:

上述公式的主要优点是转换后的各分量的范围也在0到255之间，因此用 byte类型的变量即可容纳新的颜色空间。

为了可以指定位移的大小，我们采用常量的定义方式是计算各放大系数。

        const float YCbCrYRF = 0.299F;              // RGB转YCbCr的系数(浮点类型）const float YCbCrYGF = 0.587F;const float YCbCrYBF = 0.114F;const float YCbCrCbRF = -0.168736F;        const float YCbCrCbGF = -0.331264F;const float YCbCrCbBF = 0.500000F;const float YCbCrCrRF = 0.500000F;const float YCbCrCrGF = -0.418688F;const float YCbCrCrBF = -0.081312F;const float RGBRYF = 1.00000F;            // YCbCr转RGB的系数(浮点类型）const float RGBRCbF = 0.0000F;const float RGBRCrF = 1.40200F;const float RGBGYF = 1.00000F;          const float RGBGCbF = -0.34414F;const float RGBGCrF = -0.71414F;const float RGBBYF = 1.00000F;          const float RGBBCbF = 1.77200F;const float RGBBCrF = 0.00000F; const int Shift = 20;const int HalfShiftValue = 1 << (Shift - 1);const int YCbCrYRI = (int)(YCbCrYRF * (1 << Shift) + 0.5);         // RGB转YCbCr的系数(整数类型）const int YCbCrYGI = (int)(YCbCrYGF * (1 << Shift) + 0.5);const int YCbCrYBI = (int)(YCbCrYBF * (1 << Shift) + 0.5);const int YCbCrCbRI = (int)(YCbCrCbRF * (1 << Shift) + 0.5);const int YCbCrCbGI = (int)(YCbCrCbGF * (1 << Shift) + 0.5);const int YCbCrCbBI = (int)(YCbCrCbBF * (1 << Shift) + 0.5);const int YCbCrCrRI = (int)(YCbCrCrRF * (1 << Shift) + 0.5);const int YCbCrCrGI = (int)(YCbCrCrGF * (1 << Shift) + 0.5);const int YCbCrCrBI = (int)(YCbCrCrBF * (1 << Shift) + 0.5);const int RGBRYI = (int)(RGBRYF * (1 << Shift) + 0.5);              // YCbCr转RGB的系数(整数类型）const int RGBRCbI = (int)(RGBRCbF * (1 << Shift) + 0.5);const int RGBRCrI = (int)(RGBRCrF * (1 << Shift) + 0.5);const int RGBGYI = (int)(RGBGYF * (1 << Shift) + 0.5);const int RGBGCbI = (int)(RGBGCbF * (1 << Shift) + 0.5);const int RGBGCrI = (int)(RGBGCrF * (1 << Shift) + 0.5);const int RGBBYI = (int)(RGBBYF * (1 << Shift) + 0.5);const int RGBBCbI = (int)(RGBBCbF * (1 << Shift) + 0.5);const int RGBBCrI = (int)(RGBBCrF * (1 << Shift) + 0.5);

RGB2YCbCr

public static void ToYCbCr(byte* From, byte* To, int Length = 1){if (Length < 1) return;byte* End = From + Length * 3;int Red, Green, Blue;// int Y, Cb, Cr;while (From != End){Blue = *From; Green = *(From + 1); Red = *(From + 2);// 无需判断是否存在溢出，因为测试过整个RGB空间的所有颜色值，无颜色存在溢出*To = (byte)((YCbCrYRI * Red + YCbCrYGI * Green + YCbCrYBI * Blue + HalfShiftValue) >> Shift);     *(To + 1) = (byte)( 128 + ( (YCbCrCbRI * Red + YCbCrCbGI * Green + YCbCrCbBI * Blue + HalfShiftValue) >> Shift));*(To + 2) = (byte) (128+( (YCbCrCrRI * Red + YCbCrCrGI * Green + YCbCrCrBI * Blue + HalfShiftValue) >> Shift));// *To = (byte)Y;          // 不要把直接计算的代码放在这里，会降低速度，//*(To + 1) = (byte)Cb;//*(To + 2) = (byte)Cr;From += 3;To += 3;}}

被注释掉的代码时原始的，因为这种比较简单的代码，直接对表达式进行强制类型转换比用中间变量要少几条汇编码，并且中间变量越少，在编译后越有可能让CPU用寄存器来缓存一些变量，而不是用内存。

比如我们比较下注释部分和上述代码的反编译码：

注释掉的部分的反编译码：

Y = (YCbCrYRI * Red + YCbCrYGI * Green + YCbCrYBI * Blue + HalfShiftValue) >> Shift;
0000003a  imul        eax,eax,4C8B4h imul        edx,ebx,9645Ah add         eax,edx imul        edx,edi,1D2F2h
0000004e  lea         eax,[eax+edx+00080000h] sar         eax,14h mov         dword ptr [ebp-18h],eax *To = (byte)Y;
0000005b  mov         byte ptr [esi],al

上述代码的反编译码：

*To = (byte) ( (YCbCrYRI * Red + YCbCrYGI * Green + YCbCrYBI * Blue + HalfShiftValue) >> Shift);
0000003a  imul        eax,ebx,4C8B4h imul        edx,edi,9645Ah add         eax,edx imul        edx,dword ptr [ebp-14h],1D2F2h
0000004f  lea         eax,[eax+edx+00080000h] sar         eax,14h mov         byte ptr [esi],al

当然，如果循环中的代码复杂一些，这个就不一定了。

YCbCr转为RGB空间的代码：

public static void ToRGB(byte* From, byte* To, int Length = 1){if (Length < 1) return;byte* End = From + Length * 3;int Red, Green, Blue;int Y, Cb, Cr;while (From != End){Y = *From; Cb = *(From + 1)-128; Cr = *(From + 2)-128;Red = Y + ((RGBRCrI * Cr + HalfShiftValue) >> Shift);Green = Y + ((RGBGCbI * Cb + RGBGCrI * Cr+ HalfShiftValue) >> Shift);Blue = Y + ((RGBBCbI * Cb + HalfShiftValue) >> Shift);if (Red > 255) Red = 255; else if (Red < 0) Red = 0;if (Green > 255) Green = 255; else if (Green < 0) Green = 0;    // 编译后应该比三目运算符的效率高if (Blue > 255) Blue = 255; else if (Blue < 0) Blue = 0;*To = (byte)Blue;                                               // 由于不是一一对应的，需要判断是否越界*(To + 1) = (byte)Green;*(To + 2) = (byte)Red;From += 3;To += 3;}}