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--- isp:contrast [2023/09/05 05:16] – [In RGB color space] Igor Yefmov
+++ isp:contrast [2023/09/10 21:53] (current) – [Integer arithmetic] Igor Yefmov
@@ Line 7: / Line 7: @@
 ===== In RGB color space =====
-As with the [[isp::brightness]] adjusting contrast in RGB color space requires us to pay attention to proportionally adjust the individual color components of the pixel to preserve their relative ratios.
-Traditionally the luminance is calculated as
+Each component's contrast adjustment is based on its value and the adjustment itself is calculated as
 \[
-lum = 0.2126 * R + 0.7152 * G + 0.0722 * B
+component = (component' - 0.5) * k^2 + 0.5 \\
+component' \in [0..1.0] \\
+contrast \in ]0..2.0]%%
 \]
+==== Integer arithmetic ====
-With that in mind each component's contrast adjustment is based on its luma's weight \(k\) and the adjustment itself is calculated as
+Of course, much like in [[isp::brightness]] case, we'd like to not use floating point arithmetic, so for the 12-bit pixel components and taking into account that the \(contrast\) is in range \([0..2047]\), the math looks like the following:
 \[
-component = (component - 50*k) * contrast^2 + 50*k \\
+\frac{
-luma \in [0..100] \\
+\left(\begin{bmatrix}
-contrast \in ]0..2.0]%%
+R  \\
-\]
+G  \\
+B
-which looks like this:
-\[
-\begin{bmatrix}
-R - 50 * 0.2126 \\
-G - 50 * 0.7152 \\
-B - 50 * 0.0722
-\end{bmatrix} \times contrast^2
-+
-\begin{bmatrix}
-* 0.2126 \\
-* 0.7152 \\
-* 0.0722
 \end{bmatrix}
-\]
+-
-which easily simplifies to
-\[
 \begin{bmatrix}
-R - 10.63 \\
+ \\
-G - 35.76 \\
+ \\
-B - 3.61
-\end{bmatrix} \times contrast^2
+\end{bmatrix} \right) \times contrast^2
-+
+}{1024*1024} +
 \begin{bmatrix}
-.63 \\
+ \\
-.76 \\
+ \\
-.61
 \end{bmatrix}
 \]
 ===== In HSL color space =====

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