feat(image): implement linear-time guided filter for robust document shadow removal

README.md

@@ -46,9 +46,9 @@ Why **PDFSeal**? In English, **"Seal"** has a charming double meaning:
 - **Smart Compress**: Reduce file size by up to 90% across 4 fine-tuned modes: *Balanced (300 DPI)* for crisp readability; *Extreme (150 DPI)* for tight email caps; *Lossless Object Stream Compression* for structural optimization without rasterization; and **Target Size Bisection** (specify exact thresholds like ≤ 2MB, auto-tuning resolution and quality factors in-browser). All modes are guarded by a Universal Anti-Inflation Size Guard.
 - **Watermark & Protect**: High-DPI transparent canvas watermark stamping engine. Supports multilingual typography, custom font sizing, opacity, tilt angles (-90° to 90°), repetitive grid tiling or centered placement, quick color swatches (Official Red, Navy Blue), and custom hex values, with direct chaining into owner-level permission encryption.
 - **Dynamic Page Numbering**: Professional document pagination. Supports customizable format macros (`{n}`, `{total}`, `Page {n} of {total}`, etc.), 6 header/footer alignment anchors, odd/even mirror flipping, and cover page skipping (starting page 1 on sheet 2). Features proprietary **Whiteout Masking** (full-ribbon or local bounding box) with canvas color auto-sampling to cleanly conceal misaligned legacy page numbers.
-- **Sign & Stamp**: Engineered for rigorous contract signing. Supports freehand drawing, typography-generated signatures, and uploaded seal images. Proprietary local background flattening automatically strips camera shadows and paper whiteout, with instant ink recoloring (Original, Ink Black, Deep Blue, Stamp Red). One-click batch initial placement across all, even, odd, or non-terminal pages, backed by a persistent local seal library and positioning presets.
+- **Sign & Stamp**: Engineered for rigorous contract signing. Supports freehand drawing, typography-generated signatures, and uploaded seal images. Proprietary local background flattening automatically strips camera shadows and paper whiteout, with instant ink recoloring (Original, Ink Black, Deep Blue, Stamp Red). Features **Smart Auto-Trim Bounding Boxes** to crop away transparent whitespace for pixel-perfect placement, and automatic aspect-ratio preservation for official circular and square seals. One-click batch initial placement across all, even, odd, or non-terminal pages, backed by a persistent local seal library and positioning presets.
 - **PDF to Image**: High-fidelity rasterization of PDF pages into crisp PNG or JPG images. Choose between *Standard 150 DPI* for web distribution and *Print-Grade 300 DPI* for archival clarity. Download individual pages directly from thumbnail cards or export the complete set as a zipped archive in one click.
-- **Image to PDF**: Convert batches of PNG, JPG, WebP, and BMP images into a unified multi-page PDF. Offers standardized A4 dimensions (with auto-orientation and proportional centered margins) or adaptive native-image sizing, ideal for expense receipt collation and identity document archiving.
+- **Image to PDF (with Scanner Enhancement)**: Convert batches of PNG, JPG, WebP, and BMP images into a unified multi-page PDF. Includes a proprietary **In-Browser Document Scanner Enhancement Engine (CamScanner-Style)**: $O(W \times H)$ illuminance normalization flattens phone/wrist shadows and whitens yellowish paper backgrounds, with 3 optimized filter modes (*Auto Color* preserving red seals, *Crisp B&W* high-contrast binary text, and *Clean Grayscale*), live real-time thumbnail updates, and interactive **Hold to Compare** original view. Offers standardized A4 dimensions (with auto-orientation and proportional centered margins) or adaptive native-image sizing.
 - **Protect & Encrypt**: Modern high-grade AES-256 client-side encryption. Protect documents with an **Open Password (User Password)** to prevent unauthorized reading, and set an **Owner Password** to enforce granular operational restrictions (prohibiting unauthorized printing, text/image extraction, form alterations, and annotating), securing distributed contracts without sending keys to the cloud.
 - **Unlock & Strip Restrictions**: Permanently remove encryption locks and permission restrictions. After validating the document password, the client engine restructures the internal object streams to permanently purge encryption dictionaries, yielding a clean, unrestricted, plaintext PDF.
 - **Scrub Metadata**: Essential privacy purification for government, corporate audits, and sensitive disclosures. Deeply audits and permanently purges hidden forensic metadata and underlying attributes: document titles, author names, company names, software fingerprints (Word, WPS, InDesign), timestamps, camera GPS coordinates, private application dictionaries (PieceInfo), and embedded thumbnail caches.
@@ -66,7 +66,7 @@ Why **PDFSeal**? In English, **"Seal"** has a charming double meaning:
 - **Battle-Tested Presets**:
   - 📁 **Tender & Sensitive Document Sanitizer**: Metadata scrubbing $\rightarrow$ Balanced compression $\rightarrow$ Anti-leak watermark stamping.
   - 🛡️ **Privacy & Content Redaction**: Metadata scrubbing $\rightarrow$ Keyword/regex PII redaction $\rightarrow$ AES permission encryption.
-  - 🧾 **Expense Receipts Auto-Packer**: Multi-image to A4 PDF conversion $\rightarrow$ Compression sized for reimbursement portals.
+  - 🧾 **Expense Receipts Auto-Packer**: Multi-image to A4 PDF conversion (with automatic shadow removal & paper whitening) $\rightarrow$ Audit page numbering $\rightarrow$ Smart compression $\rightarrow$ EXPENSE REPORT watermark.
   - 📑 **Contract Batch Stamping & Archive**: Permission unlock $\rightarrow$ Last page signature stamping $\rightarrow$ Security watermark.
 - **Visual Drag & Drop Pipeline Builder**:
   - Reorder, configure, add, or remove steps with live node compatibility port validation.

README.zh-CN.md

@@ -46,9 +46,9 @@
 - **智能多模压缩 (Smart Compress)**：最高压缩 90% 体积。内置 4 档精细模式：*均衡模式 (300 DPI)* 兼顾画质与体积；*极限模式 (150 DPI)* 极限瘦身适合邮件附件；*无损对象流压缩* 纯净精简内部结构；以及**目标体积智能逼近**（输入硬性指标如 ≤ 2MB，纯本地二分搜索算法自动微调清晰度逼近上限）。全模式均受通用防膨胀体积护栏保护。
 - **水印与安全防伪 (Watermark & Protect)**：高 DPI 透明 Canvas 水印压印引擎。支持多语言排版、自定义字号、不透明度、倾斜角度（-90° 至 90°）、平铺或单点居中印压、快捷印章色（印章红、商务蓝等）与 Hex 色值自定义，支持在压印完成后直接连带配置所有者权限加密锁。
 - **动态页码编排 (Dynamic Page Numbering)**：专业级文档统一页码规整。支持灵活宏表达式（`{n}`、`{total}`、`Page {n} of {total}`、`第 {n} 页 共 {total} 页`）、6 大页眉/页脚对齐锚点、奇偶页镜像翻转、跳过首页封面（第 2 页起码为 1）。独创**旧页码白底遮罩覆盖**（整行色带或局部小框），支持画布智能吸色匹配背景底色，彻底遮蔽原有错乱页码。
-- **电子签名与盖章 (Sign & Stamp)**：专为严肃公文与合同签署打造。支持手绘手写签名、输入文字生成签名、本地印章与签名照片上传。自研盒式低通平整算法自动去除近景拍摄阴影与纸张白底，支持原色/墨黑/深蓝/公章红重着色。支持一键将简签（Initials）批量盖印至全部页面、偶数页、奇数页或末页，配备 LocalStorage 离线印章库与预设位置。
+- **电子签名与盖章 (Sign & Stamp)**：专为严肃公文与合同签署打造。支持手绘手写签名、输入文字生成签名、本地印章与签名照片上传。自研盒式低通平整算法自动去除近景拍摄阴影与纸张白底，支持原色/墨黑/深蓝/公章红重着色。新增**智能透明留白裁切 (Auto-Trim)** 紧致字迹定界框杜绝边缘误触，自适应按真实自然比例放置圆形/方形公章防压扁变形。支持一键将简签批量盖印至全部页面、偶数页、奇数页或末页，配备 LocalStorage 离线印章库与预设位置。
 - **PDF 转图片 (PDF to Image)**：逐页将 PDF 渲染为清晰 PNG 或 JPG 图像。支持*标准 150 DPI* 与*印刷级 300 DPI* 超清画质。可在页面缩略图上直接单页下载预览，亦可一键将全册所有页面完整打包为 ZIP 归档。
-- **图片转 PDF (Image to PDF)**：将 PNG、JPG、WebP、BMP 等图片快速批量拼版合成为多页 PDF。提供标准 A4 统一画幅（支持纵横自适应与等比居中排版）以及原图比例自适应贴合模式，满足票据报销拼版与证件扫描归档。
+- **图片转 PDF / 扫描件增强 (Image to PDF)**：将 PNG、JPG、WebP、BMP 等图片快速批量拼版合成为多页 PDF。内置独创纯前端 **CamScanner 级文档扫描件增强引擎**：基于高效率 $O(W \times H)$ 照度归一化算法，一键抹平手机与手腕投影、消除纸面发黄发灰暗斑并提亮为纯白底色，提供 3 档专业滤镜（*智能彩色*保留公章红印与彩字、*高清黑白*高反差自适应锐化字迹、*清晰灰度*），支持卡片缩略图实时增白与**按住查看原图对比**。提供标准 A4 统一画幅与原图比例自适应贴合模式。
 - **加密与权限保护 (Protect & Encrypt)**：现代高强度 AES-256 原生算法加密。支持设置**打开密码 (User Password)** 限制未授权查阅，同时支持设置**所有者管理密码 (Owner Password)** 细粒度锁定操作权限（严禁未授权打印、禁止复制文本与图像、禁止表单修改与批注），确保外发合同机密安全。
 - **解密与解除限制 (Unlock & Strip Restrictions)**：永久剥离密码保护与操作权限限制。输入正确密码后，底层引擎重构文档对象流，永久清除加密字典及打印、复制、编辑权限锁，输出一份完全自由干净的明文 PDF。
 - **元数据清理 (Scrub Metadata)**：深入扫描并物理擦除 PDF 内部隐藏的所有敏感属性与底层指纹：文档标题、作者名称、制作机构、编辑软件（Word / WPS / Acrobat 等指纹）、创建与修改时间戳、相机 GPS 地理坐标、私有应用字典（PieceInfo）以及内嵌缩略图缓存，100% 杜绝痕迹泄漏。
@@ -66,7 +66,7 @@
 - **久经考验的预设流程**：
   - 📁 **招投标与涉密文档预处理**：元数据清理 → 均衡压缩 → 防泄漏水印。
   - 🛡️ **隐私脱敏与安全归档**：元数据清理 → 敏感信息/PII 规则脱敏 → AES 权限加密。
-  - 🧾 **报销票据自动打包**：多图转 A4 PDF → 按报销门户要求压缩体积。
+  - 🧾 **报销票据自动打包**：多图转 A4 PDF（自动去阴影与背景增白）→ 编排审计页码 → 按报销门户要求压缩体积 → 加印 EXPENSE REPORT 凭据水印。
   - 📑 **合同批量盖章与归档**：权限解锁 → 末页签名盖章 → 安全水印。
 - **可视化拖拽流水线编辑器**：
   - 重新排序、配置参数、增删步骤，实时节点兼容性端口校验。

src/locales/de.json

@@ -530,6 +530,20 @@
   "img2pdf_orient_auto": "Automatisch",
   "img2pdf_orient_portrait": "Hochformat",
   "img2pdf_orient_landscape": "Querformat",
+  "img2pdf_enhance_title": "Scanner-Verbesserung",
+  "img2pdf_enhance_desc": "Schatten glätten & Papier aufhellen",
+  "img2pdf_filter_label": "Filtermodus",
+  "img2pdf_filter_color": "Auto-Farbe",
+  "img2pdf_filter_bw": "Klares S/W",
+  "img2pdf_filter_grayscale": "Graustufen",
+  "img2pdf_shadow_label": "Schattenentfernung",
+  "img2pdf_shadow_low": "Leicht",
+  "img2pdf_shadow_medium": "Ausgewogen",
+  "img2pdf_shadow_high": "Stark",
+  "img2pdf_badge_enhanced": "Verbessert",
+  "img2pdf_hold_compare": "Gedrückt halten für Original",
+  "img2pdf_original": "Original",
+  "img2pdf_compare": "Vergleich",
   "pipeline_title": "Pipeline & Stapelverarbeitung",
   "pipeline_subtitle": "Verketten Sie Bereinigung, Komprimierung, Wasserzeichen, Stempeln und Zusammenführen. 100% lokal im Browserspeicher ohne Uploads.",
   "pro_desktop_btn": "Pro / Desktop-App",

src/locales/en.json

@@ -530,6 +530,20 @@
   "img2pdf_orient_auto": "Auto",
   "img2pdf_orient_portrait": "Portrait",
   "img2pdf_orient_landscape": "Landscape",
+  "img2pdf_enhance_title": "Scanner Enhancement",
+  "img2pdf_enhance_desc": "Flatten phone shadows & whiten paper",
+  "img2pdf_filter_label": "Enhance Filter",
+  "img2pdf_filter_color": "Auto Color",
+  "img2pdf_filter_bw": "Crisp B&W",
+  "img2pdf_filter_grayscale": "Grayscale",
+  "img2pdf_shadow_label": "Shadow Removal",
+  "img2pdf_shadow_low": "Subtle",
+  "img2pdf_shadow_medium": "Balanced",
+  "img2pdf_shadow_high": "Strong",
+  "img2pdf_badge_enhanced": "Enhanced",
+  "img2pdf_hold_compare": "Hold to see original",
+  "img2pdf_original": "Original",
+  "img2pdf_compare": "Hold Compare",
   "pipeline_title": "Automated Pipeline & Batch",
   "pipeline_subtitle": "Chain sanitization, compression, watermarking, stamping, and merging. Batch process multiple documents 100% locally in browser memory with zero uploads.",
   "pro_desktop_btn": "Upgrade Pro / Desktop",

src/locales/es.json

@@ -530,6 +530,20 @@
   "img2pdf_orient_auto": "Automático",
   "img2pdf_orient_portrait": "Vertical",
   "img2pdf_orient_landscape": "Horizontal",
+  "img2pdf_enhance_title": "Mejora de Escáner",
+  "img2pdf_enhance_desc": "Eliminar sombras y blanquear papel",
+  "img2pdf_filter_label": "Filtro de Mejora",
+  "img2pdf_filter_color": "Color Automático",
+  "img2pdf_filter_bw": "B/N Nítido",
+  "img2pdf_filter_grayscale": "Escala de Grises",
+  "img2pdf_shadow_label": "Eliminación de sombras",
+  "img2pdf_shadow_low": "Suave",
+  "img2pdf_shadow_medium": "Equilibrado",
+  "img2pdf_shadow_high": "Fuerte",
+  "img2pdf_badge_enhanced": "Mejorado",
+  "img2pdf_hold_compare": "Mantén presionado para original",
+  "img2pdf_original": "Original",
+  "img2pdf_compare": "Comparar",
   "pipeline_title": "Pipeline y Lotes",
   "pipeline_subtitle": "Encadene saneamiento, compresión, marcas de agua, sellado y combinación. Procesamiento por lotes 100% local sin subidas.",
   "pro_desktop_btn": "Pro / App de escritorio",

src/locales/fr.json

@@ -530,6 +530,20 @@
   "img2pdf_orient_auto": "Automatique",
   "img2pdf_orient_portrait": "Portrait",
   "img2pdf_orient_landscape": "Paysage",
+  "img2pdf_enhance_title": "Amélioration Scanner",
+  "img2pdf_enhance_desc": "Supprimer les ombres et blanchir le papier",
+  "img2pdf_filter_label": "Filtre d'amélioration",
+  "img2pdf_filter_color": "Couleur Auto",
+  "img2pdf_filter_bw": "N&B Net",
+  "img2pdf_filter_grayscale": "Niveaux de gris",
+  "img2pdf_shadow_label": "Suppression des ombres",
+  "img2pdf_shadow_low": "Léger",
+  "img2pdf_shadow_medium": "Équilibré",
+  "img2pdf_shadow_high": "Fort",
+  "img2pdf_badge_enhanced": "Amélioré",
+  "img2pdf_hold_compare": "Maintenir pour original",
+  "img2pdf_original": "Original",
+  "img2pdf_compare": "Comparer",
   "pipeline_title": "Pipeline & Traitement par lots",
   "pipeline_subtitle": "Enchaînez anonymisation, compression, filigrane, tampon et fusion. Traitement par lots 100% local en mémoire sans téléversement.",
   "pro_desktop_btn": "Pro / Application bureau",

src/locales/zh.json

@@ -530,6 +530,20 @@
   "img2pdf_orient_auto": "自适应",
   "img2pdf_orient_portrait": "纵向",
   "img2pdf_orient_landscape": "横向",
+  "img2pdf_enhance_title": "文档扫描增强 (去阴影 / 增白)",
+  "img2pdf_enhance_desc": "抹平手机暗影，纸面纯白提亮",
+  "img2pdf_filter_label": "增强滤镜",
+  "img2pdf_filter_color": "智能彩色 (保留印章)",
+  "img2pdf_filter_bw": "高清黑白",
+  "img2pdf_filter_grayscale": "清晰灰度",
+  "img2pdf_shadow_label": "去阴影强度",
+  "img2pdf_shadow_low": "弱",
+  "img2pdf_shadow_medium": "均衡",
+  "img2pdf_shadow_high": "强力",
+  "img2pdf_badge_enhanced": "已增强",
+  "img2pdf_hold_compare": "按住查看原图",
+  "img2pdf_original": "原图",
+  "img2pdf_compare": "按住对比",
   "pipeline_title": "自动化流水线 & 批处理",
   "pipeline_subtitle": "一键串联脱敏、压缩、加水印、盖章与合并，对多份文档进行流水线批量处理。100% 浏览器本地运算，零上传，极速安全。",
   "pro_desktop_btn": "升级 Pro / 独立桌面版",