// 示意
Stack(
  children: [
    Image.file(file),
    Positioned(
      bottom: 20,
      left: 20,
      child: Column(
        children: lines.map((l) => Text(l, style: style)).toList(),
      ),
    ),
  ],
)
// 截圖導(dǎo)出
final boundary = key.currentContext!.findRenderObject() as RenderRepaintBoundary;
final image = await boundary.toImage(pixelRatio: 3.0);

import 'package:image/image.dart' as img;

final font = img.arial14;   // 內(nèi)置字體，只有英文
img.drawString(
  imageFile,
  'Hello Watermark',
  font: font,
  x: 20,
  y: imageFile.height - 40,
  color: img.ColorRgb8(255, 255, 255),
);

final font = await img.BitmapFont.fromZip(await rootBundle.load('assets/fonts/chinese.zip'));
img.drawString(imageFile, '項(xiàng)目名稱', font: font, x: 20, y: 100);

image_utils.Image → RGBA 像素 → ui.Image → Canvas 繪制 → JPEG 輸出

dependencies:
  image: ^4.0.0   # 用于圖片編碼/解碼

原始圖片字節(jié) → image_utils.Image
     ↓
轉(zhuǎn)為 ui.Image（避免二次編解碼）
     ↓
用 Canvas + TextPainter 繪制多行文字
     ↓
錄制 Picture → 轉(zhuǎn)回 ui.Image
     ↓
導(dǎo)出 RGBA 字節(jié) → 編碼為 JPEG

import 'dart:typed_data';
import 'dart:ui' as ui;
import 'package:flutter/material.dart';
import 'package:image/image.dart' as image_utils;

class WatermarkUtils {
  // 緩存 TextStyle，同字號(hào)復(fù)用同一個(gè)對(duì)象
  static final Map<String, TextStyle> _textStyleCache = {};

  // 緩存 TextPainter，相同文字+字號(hào)+寬度直接復(fù)用
  static final Map<String, TextPainter> _textPainterCache = {};

  // 復(fù)用 Paint 對(duì)象，避免重復(fù)創(chuàng)建
  static final Paint _imagePaint = Paint()
    ..filterQuality = FilterQuality.medium;

  /// 給 image_utils.Image 添加多行水印，返回 JPEG 字節(jié)
  static Future<Uint8List> addWatermark({
    required image_utils.Image imageFile,
    required List<String> lines,
  }) async {
    // 水印從下往上排，先把順序反轉(zhuǎn)
    final watermarkLines = lines.reversed.toList();

    // 字體大小按圖片短邊的 1/38 計(jì)算，自適應(yīng)不同分辨率
    final int imageWidth =
        imageFile.width > imageFile.height ? imageFile.height : imageFile.width;
    final int fontSize = imageWidth ~/ 38;

    // Step 1: image_utils.Image → ui.Image（直接用像素，跳過(guò)編碼）
    final ui.Image originalImage =
        await _createUIImageFromImageUtils(imageFile);

    // Step 2: 用 Canvas 繪制原圖 + 水印文字
    final recorder = ui.PictureRecorder();
    final canvas = Canvas(recorder);

    canvas.drawImage(originalImage, Offset.zero, _imagePaint);

    _drawWatermarkTexts(
      canvas,
      watermarkLines,
      imageFile.height,
      fontSize,
      imageWidth,
    );

    // Step 3: 錄制結(jié)束，生成帶水印的 ui.Image
    final watermarkedImage = await recorder
        .endRecording()
        .toImage(originalImage.width, originalImage.height);

    // Step 4: 導(dǎo)出 RGBA 字節(jié)
    final ByteData? byteData =
        await watermarkedImage.toByteData(format: ui.ImageByteFormat.rawRgba);

    watermarkedImage.dispose();
    originalImage.dispose();

    if (byteData == null) throw Exception('圖片數(shù)據(jù)轉(zhuǎn)換失敗');

    // Step 5: RGBA → JPEG
    return _rgbaToJPEG(
        byteData.buffer.asUint8List(), imageFile.width, imageFile.height);
  }

  // ──────────────────────────────────────────
  //  私有方法
  // ──────────────────────────────────────────

  /// image_utils.Image → ui.Image（不經(jīng)過(guò) JPEG 編解碼）
  static Future<ui.Image> _createUIImageFromImageUtils(
      image_utils.Image img) async {
    final bytes = img.getBytes(order: image_utils.ChannelOrder.rgba);
    final buffer = await ui.ImmutableBuffer.fromUint8List(bytes);
    final descriptor = ui.ImageDescriptor.raw(
      buffer,
      width: img.width,
      height: img.height,
      pixelFormat: ui.PixelFormat.rgba8888,
    );
    final codec = await descriptor.instantiateCodec();
    final frameInfo = await codec.getNextFrame();

    descriptor.dispose();
    codec.dispose();

    return frameInfo.image;
  }

  /// 從底部向上逐行繪制水印文字
  static void _drawWatermarkTexts(
    Canvas canvas,
    List<String> lines,
    int imageHeight,
    int fontSize,
    int imageWidth,
  ) {
    double startY = imageHeight - (fontSize * 2.0);
    const double lineGap = 20;
    final double maxWidth = imageWidth - 40.0;

    for (final line in lines) {
      if (line.isNotEmpty) {
        final rect = _drawText(canvas, line, startY, fontSize,
            maxWidth: maxWidth);
        startY = rect.top - lineGap;
      }
    }
  }

  /// 繪制單行文字，返回繪制區(qū)域 Rect（用于計(jì)算下一行位置）
  static Rect _drawText(Canvas canvas, String text, double y, int fontSize,
      {double maxWidth = double.infinity}) {
    if (text.isEmpty) return Rect.zero;

    final cacheKey = '${text}_${fontSize}_${maxWidth.toInt()}';
    TextPainter? painter = _textPainterCache[cacheKey];

    if (painter == null) {
      final styleKey = fontSize.toString();
      TextStyle? style = _textStyleCache[styleKey];

      if (style == null) {
        style = TextStyle(
          color: Colors.white,
          fontSize: fontSize.toDouble(),
          shadows: [
            Shadow(
              offset: const Offset(1, 1),
              blurRadius: 3.0,
              color: Colors.black.withOpacity(0.5),
            ),
          ],
        );
        _textStyleCache[styleKey] = style;
      }

      painter = TextPainter(
        text: TextSpan(text: text, style: style),
        textDirection: TextDirection.ltr,
        maxLines: 2,
        textAlign: TextAlign.left,
      )..layout(maxWidth: maxWidth);

      // 緩存上限 50 條，超出時(shí)清理一半
      if (_textPainterCache.length >= 50) {
        final keys = _textPainterCache.keys.take(25).toList();
        for (final k in keys) {
          _textPainterCache.remove(k);
        }
      }
      _textPainterCache[cacheKey] = painter;
    }

    final offset = Offset(20, y - painter.height);
    painter.paint(canvas, offset);

    return Rect.fromLTWH(20, y - painter.height, painter.width, painter.height);
  }

  /// RGBA 字節(jié) → JPEG Uint8List
  static Uint8List _rgbaToJPEG(Uint8List rgba, int width, int height) {
    final img = image_utils.Image.fromBytes(
      width: width,
      height: height,
      bytes: rgba.buffer,
      numChannels: 4,
    );
    return Uint8List.fromList(image_utils.encodeJpg(img, quality: 95));
  }

  /// 手動(dòng)清理緩存（內(nèi)存敏感場(chǎng)景可調(diào)用）
  static void clearCache() {
    _textStyleCache.clear();
    _textPainterCache.clear();
  }
}

// 準(zhǔn)備水印文字，每個(gè)元素一行
final lines = [
  '項(xiàng)目：XX大廈改造工程',
  '位置：3號(hào)樓-東立面',
  '時(shí)間：2024-06-18 14:32',
  '拍攝人：張三',
];

// imageFile 是通過(guò) image.decodeJpg() 解碼的 image_utils.Image
final Uint8List result = await WatermarkUtils.addWatermark(
  imageFile: imageFile,
  lines: lines,
);

// 寫(xiě)入文件
await File('/path/to/output.jpg').writeAsBytes(result);

final int fontSize = imageWidth ~/ 38;