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isp:contrast [2023/09/05 05:38] – [In RGB color space] Igor Yefmovisp:contrast [2023/09/10 21:53] (current) – [Integer arithmetic] Igor Yefmov
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 ===== In RGB color space ===== ===== 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 (see [[isp:luminance]])+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] \\
- +
- +
- +
-With that in mind each component's contrast adjustment is based on its luma's weight \(k\) and the adjustment itself is calculated as +
- +
-\[ +
-component = (component - 50*k) * contrast^2 + 50*k \\ +
-luma \in [0..100] \\+
 contrast \in ]0..2.0]%% contrast \in ]0..2.0]%%
 \] \]
  
-which looks like this: 
  
-\[ +==== Integer arithmetic ==== 
-\begin{bmatrix} +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:
-R - 50 * 0.2126 \\ +
-G - 50 * 0.7152 \\ +
-B - 50 * 0.0722 +
-\end{bmatrix} \times contrast^2 +
-+ +
-\begin{bmatrix} +
-50 * 0.2126 \\ +
-50 * 0.7152 \\ +
-50 * 0.0722 +
-\end{bmatrix} +
-\]  +
- +
-which easily simplifies to +
- +
-\[ +
-\begin{bmatrix} +
-R - 10.63 \\ +
-G - 35.76 \\ +
-B - 3.61 +
-\end{bmatrix} \times contrast^2 +
-+ +
-\begin{bmatrix} +
-10.63 \\ +
-35.76 \\ +
-3.61 +
-\end{bmatrix} +
-\]  +
- +
-Of course, much like in [[isp::brightness]] case, we'd like to not use floating point arithmetic:+
  
 \[ \[
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 G  \\ G  \\
 B B
-\end{bmatrix} \times 1024+\end{bmatrix}
 - -
 \begin{bmatrix} \begin{bmatrix}
-10885 \\ +2047 \\ 
-36618 \\ +2047 \\ 
-3697 +2047 
-\end{bmatrix} \right) +\end{bmatrix} \right) \times contrast^2 
- \times contrast^2 ++}{1024*1024} +
 \begin{bmatrix} \begin{bmatrix}
-10885 \\ +2047 \\ 
-36618 \\ +2047 \\ 
-3697+2047
 \end{bmatrix} \end{bmatrix}
-}{1024} 
 \]  \] 
- 
  
 ===== In HSL color space ===== ===== In HSL color space =====

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