id: brilliance
label: op.brilliance
order: 70

doc: |
  Brilliance — shadows up and highlights down at once.

  Apple's control, and a genuinely different idea from contrast: it applies
  the opposite correction at each end of the range while leaving mid-tones
  alone. The result reads as "more light in the scene" rather than "less
  contrast", because local relationships survive where a plain contrast
  reduction flattens them.

  It overlaps with highlights/shadows deliberately — one control doing both
  in a fixed relationship is easier to reach for than two controls needing
  to be balanced against each other.

placement: |
  After the tone curve has had the final word on tone, and before the colour
  controls, which should act on the tones the photographer has settled.

params:
  brilliance:
    label: param.brilliance
    kind: amount

uniforms:
  amount:
    value: brilliance / 100 * 0.5
    doc: |
      Half a stop at each end at full travel — the two ends move apart by a
      stop in total, which is a strong but not destructive flattening.

helpers: [luminance, tone_position, apply_tone_gain]

wgsl: |
  let luma = luminance(c);
  let pos = tone_position(luma);

  // Opposite corrections at the two ends: shadows up, highlights down, both
  // tapering to nothing at the mid-point. This is what separates brilliance
  // from a contrast control — mid-tones keep their local relationships, so
  // the image gains apparent light rather than losing structure.
  let lift = (1.0 - smoothstep(0.0, 0.5, pos)) * amount;
  let pull = smoothstep(0.5, 1.0, pos) * amount;

  // A mild saturation compensation. Flattening the tonal range washes colour
  // out; without this, brilliance looks faded at useful settings.
  let gain = exp2(lift - pull);
  c = c * gain;

  let luma_after = luminance(c);
  c = mix(vec3<f32>(luma_after), c, 1.0 + max(amount, 0.0) * 0.15);
  c = max(c, vec3<f32>(0.0));

tests:
  - name: it_starts_neutral
    expect_active: false

  - name: full_travel_is_half_a_stop_at_each_end
    set: { brilliance: 100 }
    expect: { amount: 0.5 }

  - name: the_two_ends_move_in_opposite_directions
    why: |
      The property that makes this brilliance rather than a contrast slider.
      Both terms scaling the same way would flatten without lifting.
    expect_wgsl: ["let gain = exp2(lift - pull);"]