At this stage of the imaging pipeline the pixel data is represented in full-range YCbCr 4:4:4 color space with 12-bit precision per component.
The purpose of this block is to apply UVC Contrast adjustment by modifying the luma component relative to a neutral midpoint, while preserving chroma.
The UVC Contrast control is defined in terms of an HSL-based model in UVC Contrast
Conceptually, UVC Contrast scales the distance of a pixel’s lightness from a neutral midpoint.
In a YCbCr pipeline using full-range coding, this behavior maps directly to a linear scaling of the luma component around the mid-level, while chroma components remain unchanged.
This document specifies that mapping explicitly for FPGA implementation.
The input consists of YCbCr pixels with the following properties:
\[ Y12 \in [0..4095]\\ C_b12 \in [0..4095]\text{, neutral at }2048\\ C_r12 \in [0..4095]\text{, neutral at }2048 \]
All values are unsigned 12-bit integers using full-range (computer-range) coding.
The output format is identical to the input format:
\[ Y12^{\text{contrast}} \in [0..4095]\\ C_b12^{\text{contrast}} = C_b12\\ C_r12^{\text{contrast}} = C_r12 \]
Only the luma component is modified by the contrast stage.
The UVC Contrast control provides a contrast factor \(C\), defined and scaled as specified in UVC Contrast
That document defines: * the control range, * the neutral (unity) position, * and the mapping from control value to contrast factor.
Contrast is applied by scaling the luma value relative to the neutral midpoint of the full-range Y domain.
For 12-bit full-range luma, the neutral midpoint is:
\[ Y_{\text{mid}} = 2048 \]
The contrast-adjusted luma is computed as:
\[ Y12^{\text{contrast}} = \mathrm{clamp12}\left( (Y12 - Y_{\text{mid}}) \times C + Y_{\text{mid}} \right) \]
Where: * \(Y12\) is the input luma value, * \(C\) is the contrast factor, * and `clamp12(x)` clamps the result to the range \([0..4095]\).
The chroma components are passed through unchanged:
\[ C_b12^{\text{contrast}} = C_b12\\ C_r12^{\text{contrast}} = C_r12 \]
* Contrast adjustment shall be implemented using signed arithmetic. * The subtraction from and addition to the midpoint must be performed at
sufficient bit width to avoid overflow before clamping.
* The contrast factor \(C\) is expected to be implemented as a fixed-point
value derived from the UVC control.
* Clamping is applied after scaling and offsetting. * Implementations may use a multiplier or a small LUT-based approximation,
depending on resource and performance constraints.
* This block operates entirely in YCbCr space. * Contrast adjustment affects only luma, preserving chroma relationships. * A contrast factor of unity leaves the image unchanged. * Contrast values greater than unity increase dynamic range around mid-level;
values less than unity reduce contrast.
* Extreme contrast values may cause luma clipping, which is expected behavior. * This stage assumes full-range (computer-range) YCbCr coding.