RH Subtractive Mixer

🎛️ Subtractive Color Grading in CMY Density Space

RH Subtractive Mixer is a highly specialized film emulation tool that performs true subtractive color mixing by converting your image into CMY Dye Density Space.

Unlike standard RGB tools that mix light additively (where adding color increases brightness), this tool simulates physical film dye. When you add color density, you subtract light, resulting in incredibly rich, deep, and thick colors that maintain a cinematic density.

It also features a mathematical Luminance Lock that re-calculates the transmittance of the new dye mixture and automatically adjusts the exposure to perfectly preserve the original perceptual brightness of your image.

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📖 Table of Contents

• 1. Core Concept
• 2. Density Configuration
• 3. Subtractive Shifts (CMY & RGB)
• 4. Subtractive Saturation
• 5. Under the Hood — Density Math & Luma Lock
• 6. Node Placement

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1. Core Concept 💡

The tool works by inverting the incoming log signal into Density for Cyan, Magenta, and Yellow dyes:

• Cyan Dye absorbs Red light.
• Magenta Dye absorbs Green light.
• Yellow Dye absorbs Blue light.

The tool isolates the pure hue density of every pixel (by subtracting the neutral gray density). This ensures that when you shift colors, you are only affecting saturated objects and leaving your neutral white balance perfectly untouched.

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2. Lock Luminance🔒

• Default: Checked (1)
• When you add heavy subtractive shifts or saturation, physical dyes get dark. This checkbox calculates the exact Rec.709 perceptual luminance of the original pixel, compares it to the new dark pixel, and perfectly lifts the density offset to match the original brightness. The result is intensely colorful but perfectly illuminated.

🎚️ Shift Sensitivity

A global dampener for all shift and saturation sliders.

• Default: 0.5
• If you find the color shifts are too aggressive, lower this value to give your sliders much finer, smoother control.

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3. Subtractive Shifts (CMY & RGB) 🎨

These six sliders allow you to push colors toward neighboring hues by physically adding dye density to the mixture. Because it operates on pure hues, it will never tint your grays.

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4. Subtractive Saturation 🩸

🎨 Subtractive Saturation

Instead of pushing colors left or right, this slider takes the existing pure hue density of the pixel and multiplies it.

• Range: -1.0 to 5.0
• Pushing this slider up adds thick, rich dye exactly where the color already exists. Combined with Lock Luminance, this is the ultimate tool for achieving incredibly dense, filmic saturation without the "video-like" brightness that comes from standard saturation tools.

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5. Under the Hood — Density Math & Luma Lock 🔬

CMY Density Inversion

The conversion to density is a linear inversion of the input signal, where the intensity of the incoming light is subtracted from the designated density offset.

Pure Hue Isolation

To prevent the CMY shifts from tinting the gray balance, the tool calculates the "Neutral Density" (the underlying gray component) of the pixel and isolates it from the color processing. All sliders operate exclusively on the remaining pure color density values.

Luma Lock Transmittance

If Lock Luminance is enabled, the tool converts the densities back into physical Transmittance, calculates the perceptual Luma, and finds the ratio between the original image and the graded image. It then applies a proprietary adjustment to the final density to perfectly restore the original brightness without shifting the hue.

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6. Node Placement 🗂️

Expected pipeline:

• Scene-referred input
• DaVinci Wide Gamut / Intermediate (DWG/DI)

Recommended placement:

• After primary balance and exposure, but before any output display transform or LUT.
• This tool is highly dependent on the exposure of the image (due to the Density Offset parameter), so it is best placed after your primary exposure nodes are locked in.