Curves Editor

Freehand transfer curves on a monotone spline, per channel — RGB, CIELAB (L*, a*, b*, Chroma) and Saturation — with a live before/after histogram, presets, and a mask-aware, instant-apply pipeline.

1. Using the tool

The grid is your transfer curve: the horizontal axis is input tone, the vertical axis is output. Each channel keeps its own curve, and the channels you aren't editing are drawn as dim overlays so you always see the whole transform.

Shaping the curve

Add a point — left-click an empty spot inside the grid; it starts dragging immediately. (You can also drop one straight from the image — see below.)
Move a point — drag it. Interior points are free in output and clamped in input between their two neighbours, so the curve can't fold back on itself.
Endpoints are edge-locked — the black point (bottom-left) slides along the left or bottom edge, the white point (top-right) along the right or top edge, so they double as black/white-point handles. They can't be deleted.
Delete a point — right-click it, double-click it, or hover/select it and press Delete.
Hover a point — it highlights and shows its exact in / out values right next to it.
Stacked points — clicking a spot where points overlap cycles through them, so a handle buried under another (e.g. an endpoint pushed into a corner) is still reachable on the next click.
Point X / Y — with a point selected, type its exact input/output values (0–1) into the spin-boxes.

Working from the image

Hover the preview — vertical value-lines appear on both the curve grid and the histogram at the hovered pixel's R/G/B (or K) levels, and the status line reads out the cursor position and those values. You can see exactly where a feature's tones sit on your curve before you touch a handle.
Double-click the preview — drops a control point on the active curve at that pixel's brightness (nudged slightly if it lands on an existing point), so you can anchor the curve to a tone you picked straight off the image.
Ctrl+click the grid — sets an inflection pivot and redistributes the interior handles evenly around it, a quick way to seed a symmetric contrast move.

Channels, presets & preview

Control	What it does
Channel	K (brightness / RGB linked), R/G/B, L, a, b, Chroma, or Saturation* — each with an independent curve; the inactive ones draw as dim overlays.
Presets	Built-in shapes (S-curves, lift/crush shadows, highlight roll-off, …) plus your own saved multi-channel presets.
Pop Out	Detach the editor into its own resizable window (position/size remembered).
Histogram	Before (dim) vs after (bright) for R/G/B and luminance, with log scaling and x-axis zoom/pan.
Toggle Preview / Apply / Reset	Show the result on the canvas, commit it to the document, or return every channel to the identity line.
Grip (bottom-left)	Drag to the canvas to mint a Curves shortcut with all channel curves baked in, or drop on an image to apply headlessly.

2. Technical details

A curve is a monotone spline through your control points, baked to a lookup table and applied either per RGB channel or per colour-space component. Everything runs on float data in [0,1].

2.1 The curve: a monotone PCHIP spline

The control points are interpolated with a PCHIP (Piecewise Cubic Hermite) spline — a shape-preserving monotone cubic. Unlike a natural cubic it never overshoots or rings between points, so the curve does exactly what the handles say and the transfer stays monotone (no tonal inversions).

2.2 From curve to lookup table

The spline is sampled into a lookup table — 65 536 entries for the final apply (256 for the live preview) — and every pixel is mapped through it:

\mathrm{out}(x)=\mathrm{LUT}\left[\,\mathrm{round}(x\,(N-1))\,\right],\qquad N=65536

Because the LUT is monotone, the before→after histogram can be remapped exactly by resampling the input CDF onto the output axis — mass-preserving and gap-free, so the displayed “after” histogram can't show a false comb where the curve expands tones.

2.3 Channel modes

RGB. K applies one LUT to all three channels (linked brightness); R/G/B curve a single channel. When both are set, the per-channel curve is composed after K:

\mathrm{LUT}_{R}'=\mathrm{LUT}_{R}\circ\mathrm{LUT}_{K}

CIELAB (L*, a*, b*, Chroma). The image is converted sRGB→XYZ (D65) with

[ X ] [ 0.412456 0.357576 0.180438 ] [ R ] [ Y ] = [ 0.212673 0.715152 0.072175 ] [ G ] [ Z ] [ 0.019334 0.119192 0.950304 ] [ B ]

then XYZ→Lab about the D65 white point (Xn, Yn, Zn) = (0.95047, 1, 1.08883):

L^{*}=116\,f\!\left(\frac{Y}{Y_n}\right)-16

a^{*}=500\left(f\!\left(\frac{X}{X_n}\right)-f\!\left(\frac{Y}{Y_n}\right)\right),\qquad b^{*}=200\left(f\!\left(\frac{Y}{Y_n}\right)-f\!\left(\frac{Z}{Z_n}\right)\right)

f(t)=\sqrt[3]{t}\ \ (t>\delta^{3}),\qquad f(t)=\frac{t}{3\delta^{2}}+\frac{4}{29}\ \ (t\leq\delta^{3}),\qquad \delta=\frac{6}{29}

The selected component is normalised to [0,1], curved, and de-normalised (L*/100, (a*+128)/255, (b*+128)/255). For Chroma the curve scales the colour's radius in the a*–b* plane while keeping its hue:

C=\sqrt{a^{*2}+b^{*2}}\;,\qquad (a^{*},b^{*})\;\to\;(a^{*},b^{*})\cdot\frac{C_{\mathrm{new}}}{C}

Saturation curves the S channel of HSV and converts back.

2.4 Order of operations

When several channels are active they are applied in a fixed order so the result is deterministic: first the RGB stage (K composed into R/G/B), then the Lab stage (L*, a*, b*, Chroma via one round-trip), then Saturation (via HSV). Mono images support K (applied directly) and L* (through the Lab lightness nonlinearity); the colour-only channels are skipped.

2.5 Inflection pivot

Ctrl+clicking the grid (or double-clicking the image) sets a pivot u and redistributes the interior handles evenly on either side of it along the identity line — a quick way to seed a symmetric contrast curve around a chosen tone.

2.6 Masking & apply

With an active mask the result is blended back against the source over the mask:

\mathrm{out}=\mathrm{mask}\cdot\mathrm{out}+(1-\mathrm{mask})\cdot\mathrm{src}

The preview runs on a downsampled copy; the full-resolution result is computed in a background thread, keyed by a signature of every channel's points plus the active mask, so Apply is usually instant (it commits the already-computed result) and falls back to a synchronous compute only if you apply before the worker finishes.
The grip/headless preset is kind="curves_multi" and carries every channel's points, so a dropped shortcut reproduces the entire multi-channel transform — not just the active curve.
After Apply, all curves reset to the identity line on the already-transformed image so you can keep stacking adjustments.