====== RGB UVC Gamma Adjustment ======

At this stage of the imaging pipeline the pixel data is represented in
full-range RGB color space with 12-bit precision per component, as produced by
the CMX stage ([[isp:gen5-cmx]]).

The purpose of this block is to apply **UVC gamma adjustment** in **RGB space**
by applying the **same LUT** to all three channels (baseline implementation).
This avoids the undesired desaturation artifacts caused by applying a nonlinear
transfer function to luma-only in YCbCr.

This block is intended to be implemented in FPGA logic and therefore avoids
runtime floating-point arithmetic.

After this stage completes, the next pipeline stage is the RGB-to-YCbCr
conversion stage ([[isp:gen5-rgb-to-yuv]]).

===== Input format =====
The input consists of RGB pixels with the following properties:

\[
 R12 \in [0..4095]\\
 G12 \in [0..4095]\\
 B12 \in [0..4095]
\]

All values are unsigned 12-bit integers.

===== Output format =====
The output format is identical to the input format:

\[
 R12^{\gamma} \in [0..4095]\\
 G12^{\gamma} \in [0..4095]\\
 B12^{\gamma} \in [0..4095]
\]

All three channels are modified by the gamma adjustment stage.

===== Step 1: UVC gamma transfer function =====
Gamma adjustment is applied **independently** to each RGB channel \(R12\),
\(G12\), and \(B12\) using the **same** transfer function / LUT.

The UVC gamma transfer function is defined in the following document:
[[https://wiki.sub2r.com/doku.php/isp:gamma|UVC Gamma Correction Specification]]

No additional interpretation or modification of the transfer function is
performed at this stage.

===== Step 2: LUT-based FPGA implementation =====
For FPGA implementation, the UVC gamma adjustment shall be realized using a
LUT with **4096 entries**, indexed by the 12-bit channel value.

Each LUT entry maps a 12-bit input channel value to a 12-bit output channel
value:

<code>
X_in  = i                       // i in range [0..4095]
X_out = LUT[i]                  // gamma-adjusted channel value
</code>

The same LUT instance (same contents) is used for all channels.

===== Step 3: LUT generation tool =====
To simplify and standardize LUT generation, the following online tool may be
used:
[[https://toolbox.sub2r.com/sub2r-tb-gamma.html|Sub2r Gamma LUT Generator]]

The tool generates FPGA-ready gamma tables compatible with a 12-bit input and
12-bit output pipeline.

===== Step 4: pixel reconstruction =====
For each pixel, the output components are constructed as:

\[
R12^{\gamma} = LUT[R12]\\
G12^{\gamma} = LUT[G12]\\
B12^{\gamma} = LUT[B12]
\]

No additional scaling, offsetting, or clamping is required beyond the LUT
bounds.

===== Integration notes =====
This block is placed as follows:

<code>
... -> CMX -> RGB UVC Gamma -> RGB-to-YCbCr -> ...
</code>

The downstream RGB-to-YCbCr block consumes \(R12^{\gamma}\), \(G12^{\gamma}\),
and \(B12^{\gamma}\) as its inputs.

{{:isp:screenshot_2026-01-27_063531.png?400|}}

===== Expected results =====
   * Gamma \(1.0\) is an identity gamma, that produces a LUT of a linear set of incremental values with one-to-one correspondence between input and output
   * Image doesn't get washed out too much when Gamma is increased (within the \(1.0\)-\(2.5\) range)
   * Slight hue drift is expected, but not to be an excessive one at extreme Gamma values
   * LUT table for a given Gamma should match the one produced by the [[https://toolbox.sub2r.com/sub2r-tb-gamma.html|online tool]]

===== Notes =====
   * This block operates entirely in **RGB space**.
   * Gamma adjustment is applied independently to **all three channels** using the **same** LUT (baseline implementation).
   * This stage assumes **full-range (computer-range) 12-bit RGB** coding.