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--- isp:hue_rotation [2023/09/06 10:21] – [In RGB color space] Igor Yefmov
+++ isp:hue_rotation [2023/09/12 12:25] (current) – [In RGB color space] Igor Yefmov
@@ Line 4: / Line 4: @@
 ===== In RGB color space =====
-To rotate the color vector by an angle \(\Theta\) the following matrix multiplication is done:
+To rotate the color vector by an angle \(\Theta\) is to rotate the RGB cube around the \((0,0,0)-(1,1,)\) diagonal axis, for which the [[https://en.wikipedia.org/wiki/Rotation_matrix#Rotation_matrix_from_axis_and_angle|rotation matrix]] looks like the following:
 \[
+cos_1 = cos(\Theta) \\
+cos_2 = \frac{1 - cos_1}{3} \\
+sin_1 = sin(\Theta)\sqrt{1/3} \\
 \begin{bmatrix}
-R \\
+cos_1 + cos_2 & cos_2 - sin_1 & cos_2 + sin_1 \\
-G \\
+cos_2 + sin_1 & cos_1 + cos_2 & cos_2 - sin_1 \\
-B
+cos_2 - sin_1 & cos_2 + sin_1 & cos_1 + cos_2
-\end{bmatrix}
-\times
-\begin{bmatrix}
-cox(\Theta) & sin(\Theta) & 0 \\
--sin(\Theta) & cos(\Theta) & 0 \\
- & 0 & 1
 \end{bmatrix}
 \]
@@ Line 22: / Line 20: @@
 In our codebase the Hue adjustment is a 14-bit signed integer in range \([-8192..8191]\) that corresponds to \([-180°..+180°)\).
-M.B. this does **not** take into account the fact that different chroma components contribute differently to the luma of the pixel. A more correct implementation must first separate luma and chroma, perform rotation on chroma, and then convert back to RGB triplet FIXME
+M.B. this does **not** take into account the fact that different chroma components contribute differently to the luma of the pixel. A more correct implementation must first separate luma and chroma, perform rotation on chroma, and then convert back to RGB triplet, but that is way too expensive to perform in-line during video transmission.
 ===== In HSL color space =====

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