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isp:brightness [2023/09/10 18:32] – [In RGB color space] Igor Yefmov | isp:brightness [2023/09/10 21:49] (current) – [Calculation reference] Igor Yefmov | ||
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===== In RGB color space ===== | ===== In RGB color space ===== | ||
- | The mathematically correct way to do the brightness is to first convert into HSL color space, adjust the L (luma) component, and then convert back into RGB. That gives the correct result but costs way too many transistors and cycles. So we've got improvise! | + | The mathematically correct way to do the brightness is to first convert into HSL color space, adjust the L (luma) component, and then convert back into RGB. That gives the correct result but costs way too many transistors and cycles. So we've got to improvise! |
- | Here's the algorithm to follow, assuming \(R, G, B \in [0..4095] and br \in [-1024..+1023]\): | + | ==== Algorithm ==== |
+ | Here's the algorithm to follow, assuming \(R, G, B \in [0..4095]\) and \(br \in [-1024..+1023]\): | ||
- calculate luminosity | - calculate luminosity | ||
- | - figure out the slope for each components based on whether luma is below or above 50% | ||
- adjust the brightness additively and clamp the value to range \([0..4095]\) | - adjust the brightness additively and clamp the value to range \([0..4095]\) | ||
+ | - figure out the slope for each components based on whether luma is below or above 50% and set chroma components to values that correspond to that 50% luma | ||
+ | - figure out if the new luma is going to cross the 50% boundary and if so - "flip the slopes" | ||
- recalculate RGB components | - recalculate RGB components | ||
+ | |||
+ | ==== Calculation reference ==== | ||
To calculate luminosity we just find the max and min of the triplet and get a simple average: | To calculate luminosity we just find the max and min of the triplet and get a simple average: | ||
\[L = \frac{min(R, | \[L = \frac{min(R, | ||
+ | |||
+ | Brightness adjustment is a trivial addition, clamping the value to its proper limits: | ||
+ | |||
+ | \[ | ||
+ | L` = L + br \\ | ||
+ | L` \in [0..4095] | ||
+ | \] | ||
The slope \(k_R\) for the red component calculation depends on whether \(L\) is above or below the middle: | The slope \(k_R\) for the red component calculation depends on whether \(L\) is above or below the middle: | ||
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\begin{cases} | \begin{cases} | ||
R / L & \text{if} \; L \leq 2047 \\ | R / L & \text{if} \; L \leq 2047 \\ | ||
- | \frac{R - 2047}{L - 2047} & \text{if} \; L > 2047 | + | \frac{R - 4095}{L - 4095} & \text{if} \; L > 2047 |
\end{cases} | \end{cases} | ||
\] | \] | ||
- | Similarly find the \(k_G\) and \(k_B\) for green and blue components respectively. | + | Finding the " |
- | + | \[ | |
- | Brightness adjustment is a trivial addition, clamping the value to its proper limits: | + | R = |
+ | \begin{cases} | ||
+ | k_R * 2047 & \text{if} \; L \leq 2047 \\ | ||
+ | 4095 - k_R * 2047 & \text{if} \; L > 2047 | ||
+ | \end{cases} | ||
+ | \] | ||
+ | If we are crossing the middle luma boundary as the result of this adjustment - flip the slope: | ||
\[ | \[ | ||
- | L` = L + br \\ | + | k_R = 2 - k_R |
- | L` \in [0..4095] | + | |
\] | \] | ||
- | Apply the new \(L`\) to R component and clamp the result: | + | Applying |
\[ | \[ | ||
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\] | \] | ||
- | Similarly perform calculations for green and blue components. | + | \(G\) and \(B\) calculations are similar to \(R\). |
===== In HSL color space ===== | ===== In HSL color space ===== |