DarkRoom/core/dr-pipeline/ops/contrast.yaml

id: contrast
label: op.contrast
order: 30
attributes: [tone]

doc: |
  Contrast — an S-curve about a fixed mid-point.

  Pushes tones away from middle grey (positive) or toward it (negative),
  pivoting where the eye reads "neither light nor dark". In linear light
  that point is 0.18, not 0.5: a scene-referred value of 0.5 is roughly a
  stop and a half above middle grey, and pivoting there would darken almost
  every photograph.

  The curve is applied in a perceptual domain rather than directly to linear
  values. Applied linearly, an S-curve crushes shadows far harder than it
  lifts highlights, because linear light devotes most of its range to the
  brightest stop.

placement: |
  After the capture corrections, before the region controls: it is the broad
  tonal statement those controls then refine.

params:
  contrast:
    label: param.contrast
    kind: amount

uniforms:
  amount:
    value: contrast / 100
    doc: The shader's curve expects -1..1; the descriptor speaks -100..100.

helpers: [luminance, apply_tone_gain]

define:
  contrast_curve: |
    // The steepening S, on a 0..1 perceptual position.
    //
    // Blends toward a smoothstep, which has zero gradient at both ends, so the
    // curve cannot invert however hard it is pushed — the failure that makes
    // naive gain-about-a-pivot unusable past moderate settings. Only the
    // positive direction comes here: flattening is not a curve at all (see
    // the fragment).
    fn contrast_curve(x: f32, amount: f32) -> f32 {
        let clamped = clamp(x, 0.0, 1.0);
        let s = clamped * clamped * (3.0 - 2.0 * clamped);
        return mix(clamped, s, amount);
    }

wgsl: |
  if (amount < 0.0) {
      // **Flattening mixes toward middle grey; it does not scale.**
      //
      // Every tone moves the same fraction of the way to 0.18, which at -1
      // collapses the picture to grey — the meaningful limit of 'no contrast'.
      // In luminance this is exactly what the ratio form below would compute,
      // but the ratio form reaches it by multiplying: a pixel at 0.001 has to
      // be lifted to 0.09, a gain of ninety, and in the deepest shadows the
      // channels are sensor noise, not a colour. After white balance the red
      // and blue noise sits above the green (their multipliers are nearly
      // twice its), so ninety times that noise is magenta — every black in the
      // frame turned pink. Mixing adds the lift as a neutral, so a black goes
      // to grey and its noise stays the size it was.
      //
      // The grey is (1, 1, 1) scaled, because this runs after white balance
      // in the camera's space, where that is what neutral is.
      c = mix(c, vec3<f32>(0.18), -amount);
  } else {
      let luma = luminance(c);
      // Only up to twice middle grey, which is the curve's whole domain. Above
      // it the curve's value is 1 and its slope 0, so leaving those tones
      // alone is the continuous continuation — where scaling them to the
      // curve's top, as this once did through a clamp, pinned every highlight
      // in the photograph to 0.36 at the smallest touch of the slider.
      if (luma > 0.0001 && luma < 0.36) {
          // Work on luminance and rescale the colour by the ratio, rather than
          // curving each channel independently. Per-channel contrast shifts hue
          // wherever the channels differ — the classic symptom being skies going
          // cyan as contrast rises. Safe here where it was not for flattening:
          // the S only ever pulls a shadow down, so the gain is at most one
          // below the pivot and noise is never amplified.
          //
          // MIDDLE_GREY is 0.18: the linear value the eye reads as mid-tone.
          // The curve operates on luma/(2*0.18) so that middle grey lands at
          // the curve's own 0.5 pivot.
          let pos = luma / 0.36;
          let curved = contrast_curve(pos, amount);
          // Not `target`: that is a WGSL reserved keyword, and using it produces a
          // parse error in generated code rather than anywhere a reader would look.
          let curved_luma = curved * 0.36;
          c = apply_tone_gain(c, curved_luma / luma);
      }
  }
  c = max(c, vec3<f32>(0.0));

tests:
  - name: neutral_does_nothing
    expect: { amount: 0.0 }
    expect_active: false

  - name: the_amount_is_normalised_to_unit_range
    set: { contrast: 100 }
    expect: { amount: 1.0 }

  - name: the_negative_direction_normalises_too
    set: { contrast: -100 }
    expect: { amount: -1.0 }

  - name: contrast_works_on_luminance_not_per_channel
    why: |
      Curving each channel separately shifts hue; the ratio form is what keeps
      a blue sky blue as contrast rises.
    expect_wgsl: ["luminance(c)", "apply_tone_gain"]

  - name: the_pivot_is_middle_grey_not_half
    why: |
      Pivoting at 0.5 in linear light would darken nearly every image:
      scene-referred 0.5 is well above what the eye calls mid-tone.
    expect_wgsl: ["0.36"]

  - name: a_division_by_luminance_is_guarded
    why: |
      A black pixel has zero luminance; dividing by it would produce NaN and
      propagate through everything downstream.
    expect_wgsl: ["luma > 0.0001"]

  - name: flattening_mixes_toward_grey_rather_than_scaling
    why: |
      Lifting a shadow by a luminance ratio multiplies its noise by the same
      ratio — ninety at the bottom of a night photograph — and after white
      balance that noise is magenta. A mix adds the lift as a neutral.
    expect_wgsl: ["mix(c, vec3<f32>(0.18), -amount)"]

  - name: highlights_are_not_pinned_to_the_top_of_the_curve
    why: |
      The curve covers 0..0.36. A clamp into that range scaled every brighter
      pixel down to 0.36; tones above it are left as they are.
    expect_wgsl: ["luma < 0.36"]

  - name: the_curve_cannot_invert
    why: |
      A gain-about-a-pivot form produces a non-monotonic curve past moderate
      settings, which inverts tones. smoothstep cannot.
    expect_helper_wgsl: { contrast_curve: ["3.0 - 2.0 * clamped"] }