颜色空间BGR转YCbCr

　颜色空间系列代码下载链接：http://files.cnblogs.com/Imageshop/ImageInfo.rar （同文章同步更新）

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。

要避免浮点运算带来的速度瓶颈，这里同样可以用颜色空间系列1: RGB和CIEXYZ颜色空间的转换及相关优化文章中同样的优化技巧。

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;

    </span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBRYF = <span style="color: #800080;">1.00000F</span>;            <span style="color: #008000;">//</span><span style="color: #008000;"> YCbCr转RGB的系数(浮点类型）</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBRCbF = <span style="color: #800080;">0.0000F</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBRCrF = <span style="color: #800080;">1.40200F</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBGYF = <span style="color: #800080;">1.00000F</span><span style="color: #000000;">;          </span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBGCbF = -<span style="color: #800080;">0.34414F</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBGCrF = -<span style="color: #800080;">0.71414F</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBBYF = <span style="color: #800080;">1.00000F</span><span style="color: #000000;">;          </span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBBCbF = <span style="color: #800080;">1.77200F</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">float</span> RGBBCrF = <span style="color: #800080;">0.00000F</span><span style="color: #000000;">; </span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> Shift = <span style="color: #800080;">20</span><span style="color: #000000;">;</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> HalfShiftValue = <span style="color: #800080;">1</span> &lt;&lt; (Shift - <span style="color: #800080;">1</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrYRI = (<span style="color: #0000ff;">int</span>)(YCbCrYRF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span>);         <span style="color: #008000;">//</span><span style="color: #008000;"> RGB转YCbCr的系数(整数类型）</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrYGI = (<span style="color: #0000ff;">int</span>)(YCbCrYGF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrYBI = (<span style="color: #0000ff;">int</span>)(YCbCrYBF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCbRI = (<span style="color: #0000ff;">int</span>)(YCbCrCbRF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCbGI = (<span style="color: #0000ff;">int</span>)(YCbCrCbGF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCbBI = (<span style="color: #0000ff;">int</span>)(YCbCrCbBF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCrRI = (<span style="color: #0000ff;">int</span>)(YCbCrCrRF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCrGI = (<span style="color: #0000ff;">int</span>)(YCbCrCrGF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> YCbCrCrBI = (<span style="color: #0000ff;">int</span>)(YCbCrCrBF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBRYI = (<span style="color: #0000ff;">int</span>)(RGBRYF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span>);              <span style="color: #008000;">//</span><span style="color: #008000;"> YCbCr转RGB的系数(整数类型）</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBRCbI = (<span style="color: #0000ff;">int</span>)(RGBRCbF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBRCrI = (<span style="color: #0000ff;">int</span>)(RGBRCrF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBGYI = (<span style="color: #0000ff;">int</span>)(RGBGYF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBGCbI = (<span style="color: #0000ff;">int</span>)(RGBGCbF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBGCrI = (<span style="color: #0000ff;">int</span>)(RGBGCrF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBBYI = (<span style="color: #0000ff;">int</span>)(RGBBYF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBBCbI = (<span style="color: #0000ff;">int</span>)(RGBBCbF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span><span style="color: #000000;">);</span><span style="color: #0000ff;">const</span> <span style="color: #0000ff;">int</span> RGBBCrI = (<span style="color: #0000ff;">int</span>)(RGBBCrF * (<span style="color: #800080;">1</span> &lt;&lt; Shift) + <span style="color: #800080;">0.5</span>); </pre>

RGB转为YCbCr的代码：

        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
00000040  imul        edx,ebx,9645Ah
00000046  add         eax,edx
00000048  imul        edx,edi,1D2F2h
0000004e  lea         eax,[eax+edx+00080000h]
00000055  sar         eax,14h
00000058  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
00000040  imul        edx,edi,9645Ah
00000046  add         eax,edx
00000048  imul        edx,dword ptr [ebp-14h],1D2F2h
0000004f  lea         eax,[eax+edx+00080000h]
00000056  sar         eax,14h
00000059  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;}}

实际中这种逆变换用的不多。

照例给出一些转换效果：

YCbCr综合图：

Y分量：

Cb分量：

Cr分量：

在皮肤识别方面，常用YCbCr空间的数据进行分析，从上面几个图中也可以看出，肤色在CbCr有着一定的集聚性，这个在日后的文章中再说。