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isp:contrast [2023/09/06 09:38] – [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
  
 \[ \[
-lum = 0.2126 * R + 0.7152 * G + 0.0722 * B +component = (component' - 0.5) * k^2 + 0.5 \\
-\] +
- +
- +
- +
-With that in mind each component's contrast adjustment is based on its luma's weight \(k\) and the adjustment itself is calculated as +
- +
-\[ +
-component = (component' - 0.5) * (contrast * k)^2 + 0.5 \\+
 component' \in [0..1.0] \\ component' \in [0..1.0] \\
 contrast \in ]0..2.0]%% contrast \in ]0..2.0]%%
 \] \]
  
-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 - 0.5 \\ +
-G - 0.5 \\ +
-B - 0.5 +
-\end{bmatrix} +
- \times +
-\begin{bmatrix} +
-0.2126^2 & 0 & 0 \\ +
-0 & 0.7152^2 & 0 \\ +
-0 & 0 & 0.0722^2 +
-\end{bmatrix} \times contrast^2 +
-+ +
-\begin{bmatrix} +
-0.5 \\ +
-0.5 \\ +
-0.5 +
-\end{bmatrix} \\ +
-R, G, B \in [0..1.0] \\ +
-contrast \in [0..2.0] +
-\]  +
- +
-Of course, much like in [[isp::brightness]] case, we'd like to not use floating point arithmetic, so for the 12-bit pixel components the values are scaled by 4095. Taking into account that the \(contrast\) is also scaled to \([0..2048]\), the math looks like the following:+
  
 \[ \[
Line 63: Line 32:
 2047 \\ 2047 \\
 2047 2047
-\end{bmatrix} \right) +\end{bmatrix} \right) \times contrast^2 
-\begin{bmatrix} +}{1024*1024} +
-47394 & 0 & 0 \\ +
-0 & 536358 & 0 \\ +
-0 & 0 & 5466 +
-\end{bmatrix} \times contrast^2 +
-}{1024*1024*1024*1024} ++
 \begin{bmatrix} \begin{bmatrix}
 2047 \\ 2047 \\
 2047 \\ 2047 \\
 2047 2047
-\end{bmatrix} \\ +\end{bmatrix}
- +
-R, G, B \in [0..4095] \\ +
-contrast \in [0..2048] +
 \]  \] 
- 
  
 ===== In HSL color space ===== ===== In HSL color space =====

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