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--- isp:contrast [2019/05/08 16:57] – Igor Yefmov
+++ isp:contrast [2023/09/10 21:51] – [In RGB color space] Igor Yefmov
@@ Line 4: / Line 4: @@
 Decreasing the contrast brings the shadows up and highlights down to make them closer to one another.
-Essentially when the contrast is increased, the pixel's ''L'' value gets "pushed away" from the center of the values' scale, and when the contrast is decreased the ''L'' is being "pushed towards" the center.
+Essentially when the contrast is increased, the pixel's \(L\) value gets "pushed away" from the center of the values' scale, and when the contrast is decreased the \(L\) is being "pushed towards" the center.
-For our purposes we are going to adjust the pixel's ''L'' component in the HSL color space, leaving the other two components intact as the Saturation shouldn't be affected by the Contrast and the Hue (at least for now) is also maintaining its invariance during this operation.
+===== In RGB color space =====
-Assuming the ''L'' component to be in range ''[0..100]''% the contrast adjustment looks like this:
+Each component's contrast adjustment is based on its value and the adjustment itself is calculated as
+\[
+component = (component' - 0.5) * k^2 + 0.5 \\
+component' \in [0..1.0] \\
+contrast \in ]0..2.0]%%
+\]
+==== Integer arithmetic ====
+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:
+\[
+\frac{
+\left(\begin{bmatrix}
+R  \\
+G  \\
+B
+\end{bmatrix}
+-
+\begin{bmatrix}
+\\
+\\
+
+\end{bmatrix} \right)
+\begin{bmatrix}
+& 0 & 0 \\
+& 536358 & 0 \\
+& 0 & 5466
+\end{bmatrix} \times contrast^2
+}{1024*1024*1024*1024} +
+\begin{bmatrix}
+\\
+\\
+
+\end{bmatrix}
+\]
+===== In HSL color space =====
+For our purposes we are going to adjust the pixel's \(L\) component in the HSL color space, leaving the other two components intact as the Saturation shouldn't be affected by the Contrast and the Hue (at least for now) is also maintaining its invariance during this operation.
+Assuming the \(L\) component to be in range \([0..100]%%\) the contrast adjustment looks like this:
+\[
+luma = (luma - 50) * contrast^2 + 50 \\
+luma \in [0..100] \\
+contrast \in ]0..2.0]%%
+\]
 <code c++>// pseudo-code
 void contrast(/*array of pixels*/image, double _cst){
-  for(const & pixel: image){
+  for(auto & pixel: image){
-    pixel.luma = std::clamp((pixel.luma - 50) * _sct + 50, 0, 100);
+    pixel.luma = std::clamp((pixel.luma - 50) * _sct * _sct + 50, 0, 100);
   }
 }</code>

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