observer/flutter_app/lib/detection/visual_prior.dart

import 'dart:math' as math;
import 'dart:typed_data';

import 'detection_result.dart';

/// 运行时视觉先验：对低置信度野鸡框做多线索过滤，降低户外误报。
///
/// 仅对 score < [maxScore]（0.35）的 pheasant 框生效；高分框与
/// suspect（生境预警）不参与过滤，避免误杀。
///
/// 线索：
/// - 颜色：绿色主导（草/叶）、蓝色主导（天空/水）、平坦低饱和（键盘/石头/文字）
/// - 位置：中心在画面上部 15%（天空区）——野鸡是地栖动物，不会出现在天空
///
/// 采样在原始 planes 上进行（后台 isolate 内，不依赖 UI 线程）。
class VisualPrior {
  static const double maxScore = 0.35;
  static const double skyTopRatio = 0.15;

  // 颜色判定阈值（与 tflite_detector 的 YUV 有限范围展开一致）
  static const double greenDiff = 20;
  static const double blueDiff = 10;
  static const double flatRange = 10;

  // 采样点中满足条件的比例超过即拒绝
  static const double greenRatio = 0.5;
  static const double blueRatio = 0.4;
  static const double flatRatio = 0.6;

  static List<DetectionResult> filter(
    List<DetectionResult> results, {
    required List<Uint8List> planes,
    required List<int> strides,
    required int width,
    required int height,
    required bool isBgra,
    required bool rgbaOrder,
  }) {
    if (results.isEmpty || width <= 0 || height <= 0) return results;
    final kept = <DetectionResult>[];
    for (final r in results) {
      final lowConfPheasant = r.label == 'pheasant' && r.score < maxScore;
      if (lowConfPheasant &&
          _reject(r, planes, strides, width, height, isBgra, rgbaOrder)) {
        continue;
      }
      kept.add(r);
    }
    return kept;
  }

  static bool _reject(DetectionResult r, List<Uint8List> planes,
      List<int> strides, int width, int height, bool isBgra, bool rgbaOrder) {
    // 位置线索：detectRaw 输出为图像坐标系，centerY 直接可判天空区
    if (r.centerY < skyTopRatio) return true;

    // 颜色线索：框中心 ±20% 区域 5×5 采样（小框采样点重合也没关系）
    final cx = (r.centerX * width).round().clamp(0, width - 1).toInt();
    final cy = (r.centerY * height).round().clamp(0, height - 1).toInt();
    final halfW = math.max(1.0, r.width * width * 0.2);
    final halfH = math.max(1.0, r.height * height * 0.2);

    var green = 0, blue = 0, flat = 0, total = 0;
    for (var gy = -2; gy <= 2; gy++) {
      for (var gx = -2; gx <= 2; gx++) {
        final px = (cx + gx * halfW / 2).round().clamp(0, width - 1).toInt();
        final py = (cy + gy * halfH / 2).round().clamp(0, height - 1).toInt();
        final (r_, g_, b_) =
            _pixel(planes, strides, px, py, width, height, isBgra, rgbaOrder);
        total++;
        final mn = math.min(r_, math.min(g_, b_));
        final mx = math.max(r_, math.max(g_, b_));
        if (g_ - r_ > greenDiff && g_ - b_ > greenDiff) green++;
        if (b_ > r_ + blueDiff) blue++;
        if (mx - mn < flatRange) flat++;
      }
    }
    if (total == 0) return false;
    if (green / total > greenRatio) return true;
    if (blue / total > blueRatio) return true;
    if (flat / total > flatRatio) return true;
    return false;
  }

  /// 读取单像素 RGB（0~255）。
  /// 8888 单平面按实际字节序取通道：BGRA=[b,g,r,a]（iOS 插件）、
  /// RGBA=[r,g,b,a]（Android 自写原生通道）——字节序写死会让 Android
  /// 低分框采样到 R/B 互换的颜色（橙色野鸡身被误判成"蓝色"）整批误杀；
  /// YUV：y 平面 + 4:2:0 半分辨率 U/V（NV12 交错或 I420 分离）。
  static (double, double, double) _pixel(List<Uint8List> planes,
      List<int> strides, int x, int y, int width, int height, bool isBgra,
      bool rgbaOrder) {
    if (isBgra) {
      final src = planes[0];
      final i = y * strides[0] + x * 4;
      final rOff = rgbaOrder ? 0 : 2;
      final bOff = rgbaOrder ? 2 : 0;
      return (src[i + rOff].toDouble(), src[i + 1].toDouble(),
          src[i + bOff].toDouble());
    }
    final yy =
        (planes[0][y * strides[0] + x] - 16.0) * (255.0 / 219.0);
    final nv12 = planes.length == 2;
    final ux = (x ~/ 2).clamp(0, width ~/ 2 - 1).toInt();
    final uy = (y ~/ 2).clamp(0, height ~/ 2 - 1).toInt();
    final uvStride = strides[1];
    final un = ((nv12
                ? planes[1][uy * uvStride + ux * 2].toDouble()
                : planes[1][uy * uvStride + ux].toDouble()) -
            128.0) *
        (255.0 / 224.0);
    final vn = ((nv12
                ? planes[1][uy * uvStride + ux * 2 + 1].toDouble()
                : planes[2][uy * uvStride + ux].toDouble()) -
            128.0) *
        (255.0 / 224.0);
    final r = yy + 1.402 * vn;
    final g = yy - 0.344136 * un - 0.714136 * vn;
    final b = yy + 1.772 * un;
    return (r, g, b);
  }
}