BadNote/lib/editor/engine/brush.dart

// lib/editor/engine/brush.dart
//
// Data-driven, Krita-compatible brush model — the extensibility seam for the
// pen engine. Each [BrushKind] maps to an immutable [BrushProfile] that fully
// describes how a stroke is captured (pressure pre-warp) and rendered
// (perfect_freehand geometry params + caps/taper). Adding a brush = adding one
// const entry to [kBrushPresets]; no render-path branching.
//
// Mirrors Krita's sensor→curve design (Pixel brush: each property is driven by
// a sensor through a response curve). Here the response curve is a pure power
// law `p^gamma` applied to pressure BEFORE perfect_freehand (rnote's
// `PressureCurve`: Pow2 = quadratic, Sqrt = √p), and the geometry knobs are
// perfect_freehand's `thinning`/`streamline`/`smoothing`/caps. A future `.kpp`
// (Krita brush preset) importer can produce [BrushProfile]s from the same
// fields — see TODO(brush-kpp-import).
//
// Source spec: docs/research/pen-brush-spec.md §1 (rnote pressure curve) and §4
// (per-brush perfect_freehand option tables). The numbers below are lifted from
// that spec verbatim.

import 'dart:ui' show Color, BlendMode;

import 'package:freezed_annotation/freezed_annotation.dart';

/// The four selectable brushes. Extensible: add a kind here + a preset in
/// [kBrushPresets]. The eraser is NOT a brush — it stays a separate tool.
///
/// The `@JsonValue` names are the STABLE on-disk identifiers persisted in the
/// sidecar (`EditorStroke.brush`); they are decoupled from the Dart enum
/// identifiers so renaming a constant here never breaks existing sidecars. A
/// brush whose stored name is unknown (e.g. a future brush opened by an older
/// build) is read back as [fountainPen] (see `EditorStroke.brush`'s JsonKey).
enum BrushKind {
  /// Strong pressure→width (rnote Pow2 / quadratic), soft taper, solid ink.
  @JsonValue('fountainPen')
  fountainPen,

  /// Near-constant thin width; pressure carries OPACITY (the ballpoint "tell").
  @JsonValue('ballpoint')
  ballpoint,

  /// Broad, flat width, translucent, square (uncapped) ends.
  @JsonValue('highlighter')
  highlighter,

  /// Moderate width + opacity from pressure (rnote Sqrt / √p), scratchy.
  @JsonValue('pencil')
  pencil,
}

/// Immutable, const description of one brush.
///
/// The capture path reads [pressureGamma] (the rnote power-law warp applied via
/// `PressureCurve(gamma: pressureGamma)` BEFORE perfect_freehand) and the render
/// path reads the perfect_freehand geometry fields ([pfThinning], [pfStreamline],
/// [pfSmoothing], [simulatePressure]) plus the cap/taper flags.
///
/// [opacity] / [blendMultiply] drive the painters' compositing via
/// [resolveStrokePaint] (closes TODO(brush-opacity)): opacity is multiplied
/// into the stroke color's alpha (pressure-tied for ballpoint/pencil — see
/// [resolveStrokeOpacity]) and [blendMultiply] selects [BlendMode.multiply].
class BrushProfile {
  const BrushProfile({
    required this.kind,
    required this.baseWidthFraction,
    required this.pressureGamma,
    required this.pfThinning,
    required this.pfStreamline,
    required this.pfSmoothing,
    required this.simulatePressure,
    required this.capStart,
    required this.capEnd,
    required this.taper,
    required this.opacity,
    required this.blendMultiply,
  });

  /// Which brush this profile is for.
  final BrushKind kind;

  /// Suggested base stroke width as a fraction of page width (so it scales with
  /// zoom, matching `PenStroke.width`). The editors may override with their own
  /// configured pen/highlighter widths; this is the spec's nominal default
  /// (spec §4 diameters, expressed as a page-width fraction).
  final double baseWidthFraction;

  /// rnote `PressureCurve` exponent applied to raw pressure at CAPTURE, before
  /// perfect_freehand. `2.0` = Pow2 (quadratic, fountain pen); `0.5` = Sqrt
  /// (pencil); `1.0` = Linear (ballpoint / highlighter). Fed through the
  /// existing `PressureCurve(gamma: …)` — no new pow function (spec §1).
  final double pressureGamma;

  /// perfect_freehand `thinning`: how strongly (pre-warped) pressure modulates
  /// width. `0.0` = constant width (highlighter); high = wide dynamic range
  /// (fountain pen) (spec §4).
  final double pfThinning;

  /// perfect_freehand `streamline`: EMA low-pass on input positions (spec §4).
  final double pfStreamline;

  /// perfect_freehand `smoothing`: outline corner-softening (spec §4).
  final double pfSmoothing;

  /// perfect_freehand `simulatePressure`: when true, fakes pressure from
  /// velocity. All four presets ship `false` so REAL stylus pressure (already
  /// pre-warped by [pressureGamma]) drives width (spec §4). The render path
  /// still falls back to simulation when the device reports NO usable pressure.
  final bool simulatePressure;

  /// Round cap on the start of the stroke (false = square end, highlighter).
  final bool capStart;

  /// Round cap on the end of the stroke (false = square end, highlighter).
  final bool capEnd;

  /// Whether the ends taper to a point (fountain pen) (spec §4).
  final bool taper;

  /// Per-stroke opacity in [0,1]; `1.0` = solid. For fountain pen / highlighter
  /// this flat value is used; ballpoint/pencil derive opacity from pressure
  /// instead (spec §3/§4) — see [resolveStrokeOpacity]. Applied by the painters
  /// via [resolveStrokePaint] (multiplied into the stroke color's alpha).
  final double opacity;

  /// Whether the brush composites with [BlendMode.multiply] (highlighter
  /// build-up / marker feel). Applied by [resolveStrokePaint].
  final bool blendMultiply;
}

/// The 4 brush presets, populated from the spec §4 tables.
///
/// Widths are the spec's logical-px diameters re-expressed as page-width
/// fractions against the project's ~1000px logical page (the existing
/// pen/highlighter widths are 0.006 / 0.02). Fountain pen ≈ pen (0.006),
/// highlighter ≈ 0.02 so the existing pen/highlighter visuals are PRESERVED as
/// the fountainPen/highlighter presets (no regression).
const Map<BrushKind, BrushProfile> kBrushPresets = {
  // Fountain pen — spec §4: size~6, thinning 0.9, smoothing 0.55,
  // streamline 0.45, simulatePressure false, taper on, pressure pre-warped to
  // p² (Pow2 / quadratic = pressureGamma 2.0). Solid ink (opacity 1.0).
  BrushKind.fountainPen: BrushProfile(
    kind: BrushKind.fountainPen,
    baseWidthFraction: 0.006,
    pressureGamma: 2.0,
    // Was 0.9 — too aggressive on short CJK strokes (width collapses mid-glyph).
    pfThinning: 0.65,
    pfStreamline: 0.4,
    pfSmoothing: 0.5,
    simulatePressure: false,
    capStart: true,
    capEnd: true,
    // Light taper only; full taper made Chinese characters look frayed.
    taper: false,
    opacity: 1.0,
    blendMultiply: false,
  ),
  // Ballpoint — Krita-inspired "ink pen": near-constant width, SOLID opacity.
  // Pressure modulates WIDTH slightly (thinning 0.15), NOT alpha — translucent
  // srcOver stacking looked like accidental multiply when strokes overlapped.
  BrushKind.ballpoint: BrushProfile(
    kind: BrushKind.ballpoint,
    baseWidthFraction: 0.0022,
    pressureGamma: 1.0,
    pfThinning: 0.15,
    pfStreamline: 0.55,
    pfSmoothing: 0.5,
    simulatePressure: false,
    capStart: true,
    capEnd: true,
    taper: false,
    opacity: 1.0,
    blendMultiply: false,
  ),
  // Highlighter — spec §4: size~22, thinning 0.0 (constant width),
  // smoothing 0.4, streamline 0.5, square (uncapped) ends, translucent +
  // multiply build-up. opacity 0.35 / blendMultiply true are APPLIED via
  // resolveStrokePaint: the 0.35 is multiplied INTO the color's existing alpha
  // (the capture path ships a 0x80 / 50% translucent color), and the stroke
  // composites with BlendMode.multiply (cross-stroke overlap darkens = marker).
  BrushKind.highlighter: BrushProfile(
    kind: BrushKind.highlighter,
    baseWidthFraction: 0.02,
    pressureGamma: 1.0,
    pfThinning: 0.0,
    pfStreamline: 0.5,
    pfSmoothing: 0.4,
    simulatePressure: false,
    capStart: false,
    capEnd: false,
    taper: false,
    opacity: 0.35,
    blendMultiply: true,
  ),
  // Pencil — Krita-inspired: soft graphite, moderate translucency via √p, but
  // NEVER multiply blend (only highlighter uses multiply). Cap ~0.88 so overlaps
  // darken gently under srcOver without turning into marker blobs.
  BrushKind.pencil: BrushProfile(
    kind: BrushKind.pencil,
    baseWidthFraction: 0.003,
    pressureGamma: 0.5,
    pfThinning: 0.45,
    pfStreamline: 0.35,
    pfSmoothing: 0.45,
    simulatePressure: false,
    capStart: true,
    capEnd: true,
    taper: false,
    opacity: 0.88,
    blendMultiply: false,
  ),
};

/// Resolve the [BrushProfile] for [kind] (always present; const map).
BrushProfile brushProfileFor(BrushKind kind) => kBrushPresets[kind]!;

// ---- Compositing (opacity + blend) — closes TODO(brush-opacity) -------------
//
// perfect_freehand produces a single closed fill polygon per stroke; the
// painters then fill it with ONE Paint. These helpers resolve that Paint's
// alpha + blend mode from the stroke's [BrushProfile] so the four brushes feel
// distinct (the ballpoint/highlighter/pencil "soul"), while geometry stays in
// the freehand path. Both render paths (PenCanvas + the PDF
// `_PageOverlayPainter`) call [resolveStrokePaint] so they can never diverge.

/// Resolve the EFFECTIVE per-stroke opacity in [0,1] for [profile], given the
/// stroke's AVERAGE pressure [pressureAvg].
///
/// Krita-inspired (not a full brush engine): ballpoint stays essentially solid
/// (width carries the pressure feel); pencil uses a soft √p curve capped below
/// 1 so light strokes stay grey without multiply-style mud; fountain/highlighter
/// use the flat profile opacity. Per-dab / textured brushes remain deferred.
double resolveStrokeOpacity(BrushProfile profile, {double pressureAvg = 0.5}) {
  final p = pressureAvg.clamp(0.0, 1.0);
  switch (profile.kind) {
    // Solid ink — tiny residual so "hover contact" can't punch full black holes
    // into overlapping strokes, but no 0.55 floor translucency stacking.
    case BrushKind.ballpoint:
      return (0.92 + 0.08 * p).clamp(0.0, 1.0);
    // Soft graphite: √p darkens quickly under pressure, capped by profile.
    case BrushKind.pencil:
      final soft = 0.50 + 0.38 * _sqrt01(p);
      return soft.clamp(0.0, profile.opacity);
    case BrushKind.fountainPen:
    case BrushKind.highlighter:
      return profile.opacity.clamp(0.0, 1.0);
  }
}

double _sqrt01(double v) {
  if (v <= 0) return 0;
  if (v >= 1) return 1;
  var x = v;
  for (var i = 0; i < 8; i++) {
    x = 0.5 * (x + v / x);
  }
  return x;
}

/// Multiply [opacity] (0..1) into [argb]'s existing alpha channel and return the
/// new ARGB int. Keeps any alpha the capture path already baked in (e.g. the
/// highlighter's 0x80 translucent capture) so this composes WITHOUT
/// double-counting — the profile opacity scales whatever alpha the color has.
int applyOpacityToArgb(int argb, double opacity) {
  final baseAlpha = (argb >> 24) & 0xFF;
  final scaled = (baseAlpha * opacity.clamp(0.0, 1.0)).round().clamp(0, 255);
  return (scaled << 24) | (argb & 0x00FFFFFF);
}

/// The fully-resolved fill [Color] + [BlendMode] for one stroke, so every
/// painter can configure its `Paint` identically. [argb] is the stroke's stored
/// color; [pressureAvg] is the mean point pressure (`pressure ?? 0.5`).
///
/// - [color]: stroke color with `profile`-resolved opacity multiplied into its
///   alpha (pressure-tied for ballpoint/pencil; flat for fountain/highlighter).
/// - [blendMode]: [BlendMode.multiply] for the highlighter (marker build-up:
///   cross-stroke overlap darkens), [BlendMode.srcOver] otherwise. The stroke
///   is still drawn ONCE per render (single fill polygon) so its OWN self-
///   overlap never darkens — that single-draw invariant lives in the painters.
class ResolvedStrokePaint {
  const ResolvedStrokePaint({required this.color, required this.blendMode});

  final Color color;
  final BlendMode blendMode;
}

/// Resolve the paint config for a stroke drawn with [kind]. See
/// [ResolvedStrokePaint]. TODO(brush-texture): pencil paper-grain texture is
/// still deferred — opacity is enough for this increment.
ResolvedStrokePaint resolveStrokePaint(
  BrushKind kind,
  int argb, {
  double pressureAvg = 0.5,
}) {
  final profile = brushProfileFor(kind);
  final opacity = resolveStrokeOpacity(profile, pressureAvg: pressureAvg);
  return ResolvedStrokePaint(
    color: Color(applyOpacityToArgb(argb, opacity)),
    blendMode: profile.blendMultiply ? BlendMode.multiply : BlendMode.srcOver,
  );
}