Split-toning

Blend two CAT16 plus RGB mixer corrections across brightness keyframes.

The split-toning module applies two independent color transforms:

• one keyed on the dark tones,
• one keyed on the bright tones.

Each keyframe combines:

• a chromatic adaptation defined by a correlated color temperature in CAT16,
• a 3×3 RGB channel mixer working in the pipeline working RGB space.

The two transforms are then interpolated according to the scene luminance of each pixel. This makes the module suitable both for creative split toning and for technical correction of mixed illuminants, for example cool bounce light in the shadows combined with a warmer direct illuminant in the highlights.

Scene-referred placement

The module is designed to run in linear scene-referred RGB and is inserted before color primaries in the pixelpipe.

The keying metric is the scene luminance (Y) derived from the current working RGB profile. This means the transition between the two keyframes is driven by brightness rather than by hue.

General principles

The user interface contains two tabs:

• dark (shown as shadows),
• bright (shown as highlights).

Each tab defines one brightness keyframe in EV. By default:

• the dark keyframe sits at -16 EV,
• the bright keyframe sits at 0 EV.

For each keyframe, the module first builds a full 3×3 transform:

1. a CAT16 chromatic adaptation derived from the selected temperature,
2. followed by the selected RGB mixer matrix.

The full transform is then blended over brightness as follows:

• below the dark keyframe, the dark transform fades linearly from identity up to full strength,
• between dark and bright, the module linearly interpolates from the dark transform to the bright transform,
• above the bright keyframe, the bright transform fades linearly back to identity.

The fade-out distance outside the [dark ; bright] segment is equal to the distance between the two keyframes.

This is important because the module does not blend colors after correction. It blends the full transform matrix itself, which generally gives more natural transitions than mixing already-corrected RGB values.

Preview

At the top of the module, a preview strip shows a linear gray gradient processed through the current split-toning settings.

This preview is normalized around the current keyframes so you can read at a glance:

• where the dark transform starts to appear,
• where the interpolation happens,
• where the bright transform fades out.

The preview colors are projected through the current display profile, so they reflect the actual monitor rendering rather than raw working-space RGB.

Module controls

Each tab contains three sections:

• a brightness keyframe,
• a CAT16 temperature,
• one of four RGB mixer GUI modes.

Brightness

brightness

Set the brightness keyframe in EV for the current tab.

The slider includes an area picker. Sample any region in the image and the module will measure the average luminance of that area, convert it to scene (Y), then map it to EV to place the keyframe automatically.

Use this to anchor the dark keyframe on a representative shadow or the bright keyframe on a representative highlight.

Temperature

temperature

Set the color temperature of the illuminant assumed for the current keyframe.

Internally, this builds a CAT16 chromatic adaptation in the current working RGB space. Lower temperatures bias the correction toward warmer light assumptions, higher temperatures toward cooler ones.

Because the temperature is only one part of the full transform, it can be used on its own for illuminant correction or combined with the channel mixer for more stylized color remapping.

Mode

mode

Choose how the RGB mixer matrix is exposed in the GUI.

The following modes are available:

• Complete,
• Simple,
• Primaries,
• White-preserving.

These are only different parameterizations of the same backend 3×3 mixer. Switching mode does not select a different processing algorithm. It only changes how the matrix is edited.

A 3×3 matrix has 9 coefficients, so a mode exposing 9 controls reaches every mixer, and a mode exposing 6 describes a restricted family. The 3 coefficients a 6-control mode does not expose are not dropped, they are solved to keep something invariant – which is the property you are really choosing between:

Unlike color calibration, this module always mixes in the pipeline working RGB, never in a CAT space – the CAT16 adaptation here is a separate step applied before the mixer, not the space the mixer runs in. The neutral is therefore always (1,1,1), and Simple and White-preserving turn out to describe exactly the same family of matrices, just through different controls. Choose between them by which set of sliders matches the correction you have in mind, not by what they can reach.

When the current matrix does not satisfy a mode’s constraint, that mode is refused and the GUI falls back to Complete, rather than showing controls that do not describe the matrix actually in use.

Complete mode

Complete is the unconstrained 3×3 mixer representation.

For each output row:

• output red,
• output green,
• output blue,

you can set the contribution of:

• input R,
• input G,
• input B.

Each row also provides a normalize checkbox. When enabled, that row is normalized by the sum of its coefficients.

This is the most general representation and is always available.

Simple mode

Simple is an exact geometric representation of a restricted subset of normalized mixer matrices. It exposes 6 parameters:

global hue rotation

A rigid rotation of the normalized chroma plane.

chroma (u,v) axes orientation

The orientation of the principal axes used to express the chroma transform.

u stretch and v stretch

Signed gains along the two chroma axes. A value of 1 is neutral. 0 collapses the selected axis to achromatic.

achromatic coupling amount and achromatic coupling hue

Couple a chroma direction into the achromatic axis while preserving the black-white axis.

This mode is exact only when all three output rows are normalized and have non-zero sums. If the current matrix does not satisfy those constraints, the GUI falls back to Complete.

Primaries mode

Primaries is another exact GUI representation, this time expressed as a generalized primaries basis in working RGB. It exposes 9 parameters:

• white hue,
• white purity,
• red hue,
• red purity,
• green hue,
• green purity,
• blue hue,
• blue purity,
• gain.

This mode is exact only when the current 3×3 mixer can be interpreted as a non-degenerate affine basis change with non-zero affine sums. If not, the GUI falls back to Complete.

It is the only mode of the four that can tint the neutral or change the overall gain of the keyframe. Use it when that is what you want; use one of the 6-control modes when it is not.

White-preserving mode

White-preserving is the primaries picture with the white nailed down. It exposes 6 parameters, 2 per primary:

red, green and blue rotation

Turn that primary around the neutral in the chromaticity plane. This is a per-primary hue rotation: you can swing the reds of a keyframe without touching its blues, which the single global rotation of Simple cannot do.

red, green and blue saturation

Scale the distance between that primary and the neutral. 0% leaves it alone, -100% collapses it onto the neutral, +100% doubles its distance.

All six controls at 0 is the identity matrix. Setting the three saturation sliders together to the same value is a plain saturation control for that keyframe: +100% doubles the colorfulness of the keyed tones, and moving them down together desaturates them. A single primary can be taken all the way down to -100%, but all three at once stop at about -86%: a keyframe with every primary sitting on the neutral maps everything to one color and has no transform left to describe. Past that limit the sliders snap back to the last usable setting.

The property the mode guarantees is that the neutral of the keyframe is left exactly where it is, no matter what the six sliders do. The 3 column magnitudes are solved from that requirement rather than exposed, which is what lets 6 controls describe every neutral-preserving mixer and nothing else. In a split-toning context this gives a clean division of labour: the CAT16 temperature is what tints the neutral of a keyframe, and the mixer then reshapes the colors around that neutral without moving it. If you want the split to come from the temperature alone, this mode guarantees the mixer will not add a cast of its own on top.

This mode requires the current matrix to leave the neutral unchanged, every column to have a non-zero sum, and the 3 primaries not to be collinear – two primaries turned onto the same direction flatten the model and are refused. If those are not met, the GUI falls back to Complete.

Practical guidance

This module works best when the two keyframes reflect real exposure zones in the scene.

A practical workflow is often:

1. pick a representative shadow area on the dark tab to set the dark brightness keyframe,
2. pick a representative highlight area on the bright tab to set the bright keyframe,
3. use the temperature sliders first to correct mixed illuminants,
4. refine the look with the RGB mixer of each tab,
5. use the preview strip to check the transition region between both anchors.

For technical correction:

• use the dark keyframe to neutralize colored bounce light in the shadows,
• use the bright keyframe to compensate the direct illuminant in the highlights,
• keep the mixer close to identity and rely mostly on CAT16 temperature when you want the most physically grounded result.

For creative grading:

• place the two keyframes closer together to concentrate the transition in the midtones,
• push them farther apart to make the blend more gradual,
• use different mixer modes for exploration, but remember they all write the same backend matrix,
• prefer White-preserving (or Simple) when you want the split to come purely from the keyed blend, since neither can move the neutral of a keyframe.

If the image starts to feel synthetic, simplify the transform first:

• reduce the mixer excursions,
• rely more on temperature,
• widen the distance between the two brightness anchors.

This module is strongest when used as a broad tonal and illuminant shaper, not as a local hue-isolation tool.