RH Chroma Gate
dctlsRH Chroma Gate
📊 Zone-Aware Saturation Shaping
Saturate Sunsets, Not Skin. Conventional saturation tools hit everything the same way — push greens and you push the dark greens in shadow just as hard as the bright greens in the sky. RH Chroma Gate separates saturation control by both hue and luminance zone, so you can make completely different saturation decisions for the same colour at different brightness levels, all in a single node.
Key Features
-
6 Hue Vectors × 3 Luminance Zones
-
Adjustable Zone Pivots & Softness
-
Zone Mask Preview
-
Reuleaux (HRO) Colour Model
RH Chroma Gate is a dual-axis saturation control tool. It lets you independently scale the saturation of each of the six hue primaries (Red, Yellow, Green, Cyan, Blue, Magenta) in three distinct luminance zones Shadows, Midtones, and Highlights simultaneously.
Where conventional saturation tools treat the entire tonal range uniformly, RH Chroma Gate lets you make completely different saturation decisions for the same colour at different brightness levels. For example: push the saturation of reds in the highlights (to pump up a sunset sky) while pulling back the saturation of reds in the shadows (to avoid over-saturated shadow skin tones) in a single node.
📖 Table of Contents
- 1. Core Concept
- 2. Zone Configuration
- 3. Per-Vector Per-Zone Controls
- 4. Under the Hood — Reuleaux (HRO) Color Model
- 5. Node Placement
- 6. Usage Tips
1. Core Concept 💡
For each pixel, the tool:
- Converts the pixel to the Reuleaux (HRO) colour space to extract hue, saturation (Rho), and intensity.
- Generates three zone masks (Shadow, Mid, Highlight) using smooth blending around the zone pivots.
- Computes six hue vector weights using proprietary weighting functions centred at each primary's nominal hue angle.
- Calculates a per-pixel saturation multiplier by combining the hue weights and zone weights
- Applies the multiplier to the pixel's in HRO space.
- Reconstructs the output from the modified HRO values.
The result is a 18-parameter (6 hues × 3 zones) saturation surface that is continuous in both hue and luminance, with no hard zone boundaries.
2. Zone Configuration 🗺️
Shadow Pivot
Sets the upper boundary of the shadow zone.
- Default:
0.33 - Range:
0.0 – 1.0 - Pixels with Rec.709 luma below this value are treated as shadows. The shadow mask fades smoothly over the Zone Softness range.
Highlight Pivot
Sets the lower boundary of the highlight zone.
- Default:
0.66 - Range:
0.0 – 1.0 - Pixels with luma above this value are treated as highlights.
Zone Softness
Controls how gradually the zone masks blend at their boundaries.
- Default:
0.1 - Range:
0.01 – 0.5 - Lower values create sharper zone transitions (more isolation between zones).
- Higher values create smoother, more diffuse transitions.
- A minimum of
0.001is enforced internally to prevent division-by-zero artefacts.
Show Zone Masks
A diagnostic checkbox that outputs the three zone masks as a false-colour image:
- Red = Highlight zone
- Green = Mid zone
- Blue = Shadow zone
Use this before making any saturation adjustments to verify the zone coverage matches the content you intend to affect.
3. Per-Vector Per-Zone Controls 🎛️
Six hue vectors × three luminance zones = 18 saturation controls. Each slider controls the saturation multiplier for one hue family within one luminance zone.
Default value: 1.0 (no change) for all sliders.
| Control | Effect |
|---|---|
1.0 | No change — exact null (identity). |
< 1.0 | Reduces saturation for that hue in that zone (e.g., 0.0 = fully desaturate). |
> 1.0 | Boosts saturation for that hue in that zone (e.g., 2.0 = doubled chroma). |
The six vectors and their available zone controls:
| Vector | Shadow | Mid | Highlight |
|---|---|---|---|
| Red | Red Shadow Sat | Red Mid Sat | Red High Sat |
| Yellow | Yel Shadow Sat | Yel Mid Sat | Yel High Sat |
| Green | Grn Shadow Sat | Grn Mid Sat | Grn High Sat |
| Cyan | Cyn Shadow Sat | Cyn Mid Sat | Cyn High Sat |
| Blue | Blu Shadow Sat | Blu Mid Sat | Blu High Sat |
| Magenta | Mag Shadow Sat | Mag Mid Sat | Mag High Sat |
How the final saturation multiplier is calculated:
For each pixel, the tool computes a blended saturation multiplier. This multiplier is calculated by combining the influence of the pixel's current hue (its proximity to each of the six primary colour vectors) with its luminance (how much it falls into the shadow, midtone, or highlight zones). This results in a seamless, weighted adjustment driven dynamically by the combination of all 18 controls.
4. Under the Hood — Reuleaux (HRO) Color Model 🔬
All saturation operations are performed in the proprietary Reuleaux (HRO) colour space:
- Opponent Axes: The RGB values are projected onto a set of orthogonal colour axes.
- Brightness & Intensity: Separately tracks the scene's brightness and intensity, independently of colour.
- Hue & Saturation: Calculates hue as a continuous angle and saturation as chroma relative to brightness.
Saturation scaling operates on the saturation component only the hue angle and intensity are not modified. This means adjusting saturation has no hue rotation or luminance side-effect.
The use of standard Rec.709 luma for zone computation rather than the raw intensity value ensures the zone boundaries align with perceived brightness, making the shadow/highlight separation behave predictably for standard grading workflows.
5. Node Placement 🗂️
Expected pipeline:
- Scene-referred input
- DaVinci Wide Gamut / Intermediate (DWG/DI)
Recommended placement:
- After primary balance corrections.
- Before or after creative look development, depending on intent.
- Particularly useful after a photochemical or creative transform to clean up any over-saturation introduced in specific tonal ranges.
This tool expects scene-referred input. In display-referred (Rec.709) space, the luma zone thresholds will correspond to different apparent brightness levels and may require re-tuning of the pivot values.
6. Usage Tips 💬
- Use Show Zone Masks first: Confirm the shadow, mid, and highlight zones are correctly placed before touching any saturation slider. A common mistake is leaving the pivots at default when the footage's tonal range requires different settings.
- Start by reducing, not boosting: It is generally safer to pull back saturation in problem areas (e.g., over-saturated shadows or specular highlights) before boosting elsewhere.
- Red Mid for skin tones: Reducing Red Mid Sat slightly (e.g.,
0.85) is an effective, non-destructive way to lower the saturation of midtone skin without affecting highlight warmth or shadow coolness. - Yel High + Grn High for sky/foliage balance: Skies can pick up both cyan and blue, and foliage both green and yellow. Controlling these two vectors independently in the highlight and midtone zones gives precise control over outdoor scene saturation.
- Zone Softness at 0.15–0.2 for most footage: The default
0.1produces fairly tight zones. Widening the softness slightly produces more naturalistic transitions and is recommended when the footage has significant noise in the shadows. - Combine with RH Hue Compressor: Use Hue Compressor upstream to normalise any hue drift, then use Chroma Gate to set the saturation behaviour across zones. These two tools are complementary and work well together in series.