jpg_function

代码说明：

  // RGB -> YCbCr // Y = Coef. for red*Red+Coef. for green*Green+Coef. for blue*Blue // Cb = (Blue-Y)/(2-2*Coef. for blue) // Cr = (Red-Y)/(2-2*Coef. for red) // YCbCr -> RGB // Red = Cr*(2-2*Coef. for red)+Y // Green = (Y-Coef. for blue*Blue-Coef. for red*Red)/Coef. for green // Blue = Cb*(2-2*Coef. for blue)+Y (// RGB-> YCbCr // Y = Coef. for red*Red+Coef. for green*Green+Coef. for blue*Blue // Cb = (Blue-Y)/(2-2*Coef. for blue) // Cr = (Red-Y)/(2-2*Coef. for red) // YCbCr-> RGB // Red = Cr*(2-2*Coef. for red)+Y // Green = (Y-Coef. for blue*Blue-Coef. for red*Red)/Coef. for green // Blue = Cb*(2-2*Coef. for blue)+Y )