Hue rotation

“Hue rotation” is a global (every pixel) shift of the image's color spectrum by a given radial offset (considering the color wheel).

To rotate the color vector by an angle \(\Theta\) the following matrix multiplication is done:

\[ \begin{bmatrix} R \\ G \\ B \end{bmatrix} \times \begin{bmatrix} cox(\Theta) & sin(\Theta) & 0 \\ -sin(\Theta) & cos(\Theta) & 0 \\ 0 & 0 & 1 \end{bmatrix} \]

In our codebase the Hue adjustment is a 14-bit signed integer in range \([-16383..16384]\) that corresponds to \((-180°..+180°]\).

Really, no magic here, once you process the image in HSL color space. The operation is as trivial as adding or subtracting a specified value from the pixel's \(H\) component:

// pseudo-code
void hue_rotation(/*array of pixels*/image, double _hue){
  for(const & pixel: image){
    pixel.hue = std::clamp(pixel.hue + _hue, 0., 360.); // hue is in range [0..360]°
  }
}