Tone & Color

All tools on this page run in one GPU compute shader (shaders/shader.wgsl). Slider values are converted to shader values in image_processing.rs. "Linear" means scene-linear RGB with Rec.709 primaries; "display" means the 0 to 1 values after the tone mapper.

Luminance is always Rec.709 luma: Y = 0.2126·R + 0.7152·G + 0.0722·B.

Order of operations

The shader always runs the steps in this order, whatever order you moved the sliders in:

#	Step	Works in
1	Chromatic aberration correction	input
2	Convert non-RAW input to linear	linear
3	Add up mask values
4	Noise reduction	square-root luma
5	Sharpening	square-root luma
6	Clarity, Structure, Centré clarity	log2 luma
7	Dehaze	linear
8	Exposure	linear
9	Glow, Halation, Flares	linear
10	White balance	Bradford LMS
11	Centré exposure and color	linear
12	Whites, Shadows, Blacks, Contrast	gamma 2.2 luma
13	Highlights	linear luma
14	Color calibration	linear
15	HSL mixer	HSV of sRGB values
16	Hue	HSV of sRGB values
17	Saturation, Vibrance	linear
18	Color grading (global, then each mask)	linear
19	Vignette	linear
20	Tone mapper (Basic or AgX)	linear to display
21	Built-in film emulation	V-Log to display
22	Brightness	display
23	Curves (global, then each mask)	display
24	Custom LUT	display
25	Grain	display
26	Clipping warning overlay	display
27	Dither and output	display

Steps 4 to 9 are covered in Detail & Effects.

A few things follow from this order:

• Exposure comes before the tone mapper, so pushing it never clips hard with AgX; Brightness comes after, so it reshapes midtones without touching black and white.
• Color grading and vignette work on scene-linear light, so the tone mapper rolls them off naturally.
• Curves always see the finished, tone-mapped image.

Exposure

rgb = rgb · 2^(slider / 0.8)

The slider goes from −5 to +5, which is ±6.25 EV. It is a pure multiplication in linear light, exactly like changing the shutter speed. Baseline Exposure and highlight-preservation compensation (see RAW Processing) are added on top.

White balance

White balance is a von Kries adaptation in Bradford LMS space, the same method used to convert between illuminants in color management:

gains = LMS(as-shot white) / LMS(target white)
rgb'  = LMS→RGB · ( gains · RGB→LMS · rgb ) / luma(adapted white)

• Dividing by the luma of the adapted white keeps overall brightness constant when you change the temperature.
• Temperature and tint are converted to a chromaticity with Robertson's method (31 isotherms). Tint moves perpendicular to the Planckian locus.
• In relative mode, one unit of Temperature is 1.5 mired and one unit of Tint is 1.5 tint units, so ±100 is ±150 mired. Mired steps are perceptually even: the same slider move looks similar whether the photo was shot at 3000 K or 7000 K.
• Kelvin mode covers 2,000 to 50,000 K and tint ±150.
• In masks, white balance is a relative shift added in log-gain space.

Whites, Shadows and Blacks

These three work on the luminance of an edge-aware base layer and then put the local detail back, so they change tones without flattening texture. The base comes from guided filters on log2 luminance (radius 10 for the tonal base and 40 for the broad base, measured at a 1080 px short side), computed once per photo.

Shadows and Blacks lift or lower the tone curve with two curves that peak at different places, in gamma 2.2 (t = Y^(1/2.2)):

shadow_lift = (slider/120) · t · (1 − t)^4.5
black_lift  = (slider/40)  · t · (1 − t)^12

The (1 − t)^12 term makes Blacks act much closer to black than Shadows. Positive values also stretch contrast around a pivot of 0.2, and add back local detail so lifted shadows don't look flat. Detail is not boosted in near-black areas, so noise isn't amplified. Brightened shadows are desaturated very slightly (up to 12%).

Whites works on the broad base above a pivot of 0.4 (gamma 2.2). Positive values push the upper tones up with a soft shoulder; negative values compress them with a rational curve. Because it moves the base and keeps detail ratios, highlight texture survives.

Contrast

A per-channel S-curve in gamma 2.2 with the pivot at 0.5 (about 0.218 linear, so slightly above middle grey):

p = rgb^(1/2.2)
s = 2^(1.25 · slider/100)                         0.42 to 2.38
p' = 0.5·(2p)^s              if p < 0.5
p' = 1 − 0.5·(2(1 − p))^s    otherwise

Because it works per channel, more contrast also increases saturation a little, like a film curve. Linear values above 1.0 are left alone, so the tone mapper can still compress them.

Highlights

Highlights compresses or expands tones above a low pivot (luma 0.06), blended in between 0.06 and 0.28, on the tonal base:

• Negative (recovery): compressed = d / (1 + 2.2k · d / (1 + 0.35d)), a rational shoulder that compresses very bright (above 1.0) areas strongly while mid highlights move less. Local and mid-scale detail are boosted a little, so cloud texture comes back instead of turning flat grey.
• Positive: brightens the upper tones with a curve that eases off for very bright values.
• Blown areas: with recovery, pixels above 1.0 are pulled toward neutral (up to 40%), so areas with no color information left turn white instead of a flat, saturated blob.

The result is applied as a luminance ratio, so hues don't shift.

Color calibration

Global only. The hue sliders rotate the camera primaries toward their neighbours (up to 25%), as in a 3x3 primary mixing matrix:

Primary	Positive values	Negative values
Red	toward green	toward blue
Green	toward blue	toward red
Blue	toward red	toward green

The saturation sliders scale chroma weighted by how much each primary contributes to the pixel. Shadows Tint adds up to ±6.25% magenta or green below luma 0.3.

HSL mixer

Hue and saturation are read from the HSV of the sRGB-encoded color; luminance is measured as linear luma. There are eight bands:

Band	Center	Width
Red	358°	35°
Orange	25°	45°
Yellow	60°	40°
Green	115°	90°
Aqua	180°	60°
Blue	225°	60°
Purple	280°	55°
Magenta	330°	50°

• Each band's weight is a Gaussian of the hue distance, and the weights are normalized to add up to 1. So there are no gaps between bands, and two neighbouring bands never double up.
• Hue sliders shift by up to ±60°. Saturation multiplies saturation. Luminance scales luma, and the hue and saturation changes keep luma constant.
• Near-grey pixels are protected: hue and saturation changes fade in between 5% and 20% saturation, so neutral areas don't pick up color.

Saturation and Vibrance

rgb = mix(Y, rgb, (1 + saturation) · (1 + vibrance_factor))

• Saturation scales chroma evenly while keeping luma.
• Positive Vibrance works mainly on low-saturation colors ((1 − S)^1.25) and is halved around orange skin hues (about 25°), so faces don't turn orange.
• Negative Vibrance reduces the most saturated colors first.

Color grading

Color grading adds a color offset in linear light to three tonal zones:

shadow edge    = 0.1 + max(0, −balance)·0.5
highlight edge = 0.5 − max(0,  balance)·0.5
feather        = 0.2 · blending

Shadows, midtones and highlights masks are smooth steps around these edges; midtones is whatever is left. Each zone adds (color − 0.5) · saturation and a luminance offset, with zone strengths of 0.1 / 0.6 / 0.8 for color and 0.5 / 0.8 / 1.0 for luminance (shadows / midtones / highlights). Global applies everywhere at full strength.

• Negative balance extends the shadow zone upward; positive extends the highlight zone downward.
• Because the offsets are added in linear light before the tone mapper, a strong shadow tint stays subtle in deep blacks and builds up smoothly.

Vignette

uv = (position − center) · 2,  shaped by |uv|^(1 − roundness)
m  = smoothstep(midpoint − feather/2, midpoint + feather/2, distance)
negative amount:  rgb · (1 + amount·m)          darkens
positive amount:  mix(rgb, white, amount·m)     fades to white

With roundness 0 the shape is a circle (corrected for the aspect ratio). It's applied in linear light before tone mapping, which makes the falloff look like natural lens vignetting.

Tone mappers

The tone mapper converts scene-linear light (which can be far above 1.0) into display values.

Basic (default for new edits)

• RAW: sRGB encode → ^(1/1.1) → 75% smoothstep S-curve. Middle grey (0.18) lands at 0.49. Anything at or above 1.0 is clipped per channel.
• Non-RAW: just the sRGB encoding, so an unedited JPEG looks exactly like the original.

AgX (based on Troy Sobotka's AgX)

1. Negative values are removed by an offset.
2. An "inset" matrix moves the colors toward the white point in a Rec.2020-based space, with a small rotation of each primary.
3. Each channel is encoded as log2 from −15.2 to +5 EV around middle grey.
4. A sigmoid curve is applied, then a 2.4 gamma.
5. An "outset" matrix restores most of the saturation.

Scene value	AgX display value
0.001	0.00
0.18 (middle grey)	0.51
0.5	0.75
1.0	0.92
about 1.40	1.00

AgX has about half a stop of headroom above 1.0 and rolls very bright colors toward white instead of skewing their hue (a blown orange sky stays orange-white instead of turning flat yellow).

Film emulation

The built-in film looks are scene-referred 3D LUTs:

1. The linear image (before tone mapping) is encoded with Panasonic V-Log (0.18 lands at 0.423).
2. The LUT is sampled with tetrahedral interpolation (the cube is split into six tetrahedra; smoother than trilinear, no hue shifts along the grey axis).
3. The result is mixed with the tone-mapped image by the LUT intensity. At 100% the film LUT replaces the tone mapper completely.

Your own LUTs (.cube, .3dl, HALD images) are treated as display-referred: they're applied after Curves, to the finished image.

Brightness

Brightness runs after the tone mapper, on display values. It uses a rational curve that keeps 0 and 1 fixed:

k  = 2^(−1.2 · slider/0.8)
Y' = Y / (Y + (1 − Y)·k)

Chroma is scaled a bit less than luma and is reduced near white, so brightening doesn't produce oversaturated highlights.

Curves

• Points are in 0 to 255 display values, at most 16 per channel.
• Interpolation is a monotone cubic (Fritsch–Carlson). A curve that goes up between two points never dips in between, so there's no overshoot or banding from wiggly curves.
• The Luma curve is a master curve applied to R, G and B separately (like "RGB" in other editors), so it also affects saturation. Then the R, G and B curves are applied.
• Curves that are a straight line are skipped.

Grain

Two octaves of gradient (Perlin-style) noise, mixed by Roughness, added equally to R, G and B (monochrome grain). Its size scales with the image, so grain looks the same in the preview and the export. It fades out in pure blacks and whites.

Clipping warnings and output

• The clipping overlay marks any channel above 0.998 red and below 0.002 blue, on the final display values.
• 8-bit output gets ±½ step of dither noise against banding. 16-bit TIFF exports are written without dither from 16-bit float data.

Slider scaling

How UI values map to shader values:

Slider	UI range	Shader value
Exposure, Brightness	−5 to 5	÷0.8 (±6.25)
Contrast, Saturation, Vibrance, Dehaze	±100	÷100
Highlights, Shadows	±100	÷120
Whites, Blacks	±100	÷40
Clarity, Structure	±100	÷125
Temperature, Tint (relative)	±100	1.5 mired per unit
Sharpness	±100	÷50
HSL hue	±100	up to ±60°
HSL saturation, luminance	±100	÷100
Color grading saturation, luminance	0 to 100, ±100	÷500
Color grading balance	±100	÷200
Calibration hue / saturation	±100	÷400 / ÷120
Vignette amount, midpoint, roundness, feather		÷100
Grain amount / size	0 to 100	÷400 / ÷50
Chromatic aberration	±100	÷10000 (±1% scale of red or blue)

Masks

Most adjustments inside a mask are added to the global value before the operation runs:

value at pixel = global + Σ (mask value × mask influence)

So a mask at 50% influence with Exposure +1 gives exactly +0.5 EV, not a 50% blend of two images, and overlapping masks add up. White balance adds in log-gain space. Color grading and curves are different: each mask's grade or curve is blended over the result (mix(rgb, curve(rgb), influence)), one mask after the other.

Global only: tone mapper, LUTs, vignette, grain, color calibration, chromatic aberration and Centré. Up to 32 masks are used.

See also

• Detail & Effects
• Basic adjustments, Color, Curves

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