feat(home): Add cover image thumbnail to Continue Reading card

Display book cover image as background in the Continue Reading card on
the home screen, improving visual identification of the current book.

Key changes:
- Add thumbnail generation (thumb.bmp) for EPUB and XTC/XTCH files
  - Uses same dithering/scaling algorithms as sleep screen covers
  - Target size: 240x400 (half screen) for optimal Continue Reading card fit
- Add JpegToBmpConverter::jpegFileToBmpStreamWithSize() for custom target sizes
- Add GfxRenderer::copyStoredBwBuffer() and freeStoredBwBuffer() for
  framebuffer caching to maintain fast navigation performance
- Add UTF-8 safe string truncation for Korean/CJK text in title/author display
- Draw white boxes behind title/author text for readability over cover image
- Increase HomeActivityTask stack size to 4096 for cover image rendering
- Add bounds checking in XTC thumbnail generation to prevent buffer overflow

lib/Epub/Epub.cpp

@@ -328,6 +328,69 @@ bool Epub::generateCoverBmp() const {
   return false;
 }
 
+std::string Epub::getThumbBmpPath() const { return cachePath + "/thumb.bmp"; }
+
+bool Epub::generateThumbBmp() const {
+  // Already generated, return true
+  if (SdMan.exists(getThumbBmpPath().c_str())) {
+    return true;
+  }
+
+  if (!bookMetadataCache || !bookMetadataCache->isLoaded()) {
+    Serial.printf("[%lu] [EBP] Cannot generate thumb BMP, cache not loaded\n", millis());
+    return false;
+  }
+
+  const auto coverImageHref = bookMetadataCache->coreMetadata.coverItemHref;
+  if (coverImageHref.empty()) {
+    Serial.printf("[%lu] [EBP] No known cover image for thumbnail\n", millis());
+    return false;
+  }
+
+  if (coverImageHref.substr(coverImageHref.length() - 4) == ".jpg" ||
+      coverImageHref.substr(coverImageHref.length() - 5) == ".jpeg") {
+    Serial.printf("[%lu] [EBP] Generating thumb BMP from JPG cover image\n", millis());
+    const auto coverJpgTempPath = getCachePath() + "/.cover.jpg";
+
+    FsFile coverJpg;
+    if (!SdMan.openFileForWrite("EBP", coverJpgTempPath, coverJpg)) {
+      return false;
+    }
+    readItemContentsToStream(coverImageHref, coverJpg, 1024);
+    coverJpg.close();
+
+    if (!SdMan.openFileForRead("EBP", coverJpgTempPath, coverJpg)) {
+      return false;
+    }
+
+    FsFile thumbBmp;
+    if (!SdMan.openFileForWrite("EBP", getThumbBmpPath(), thumbBmp)) {
+      coverJpg.close();
+      return false;
+    }
+    // Use smaller target size for Continue Reading card (half of screen: 240x400)
+    constexpr int THUMB_TARGET_WIDTH = 240;
+    constexpr int THUMB_TARGET_HEIGHT = 400;
+    const bool success =
+        JpegToBmpConverter::jpegFileToBmpStreamWithSize(coverJpg, thumbBmp, THUMB_TARGET_WIDTH, THUMB_TARGET_HEIGHT);
+    coverJpg.close();
+    thumbBmp.close();
+    SdMan.remove(coverJpgTempPath.c_str());
+
+    if (!success) {
+      Serial.printf("[%lu] [EBP] Failed to generate thumb BMP from JPG cover image\n", millis());
+      SdMan.remove(getThumbBmpPath().c_str());
+    }
+    Serial.printf("[%lu] [EBP] Generated thumb BMP from JPG cover image, success: %s\n", millis(),
+                  success ? "yes" : "no");
+    return success;
+  } else {
+    Serial.printf("[%lu] [EBP] Cover image is not a JPG, skipping thumbnail\n", millis());
+  }
+
+  return false;
+}
+
 uint8_t* Epub::readItemContentsToBytes(const std::string& itemHref, size_t* size, const bool trailingNullByte) const {
   if (itemHref.empty()) {
     Serial.printf("[%lu] [EBP] Failed to read item, empty href\n", millis());

lib/Epub/Epub.h

@@ -43,6 +43,8 @@ class Epub {
   const std::string& getAuthor() const;
   std::string getCoverBmpPath() const;
   bool generateCoverBmp() const;
+  std::string getThumbBmpPath() const;
+  bool generateThumbBmp() const;
   uint8_t* readItemContentsToBytes(const std::string& itemHref, size_t* size = nullptr,
                                    bool trailingNullByte = false) const;
   bool readItemContentsToStream(const std::string& itemHref, Print& out, size_t chunkSize) const;

lib/GfxRenderer/GfxRenderer.cpp

@@ -181,13 +181,14 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
   for (int bmpY = 0; bmpY < bitmap.getHeight(); bmpY++) {
     // The BMP's (0, 0) is the bottom-left corner (if the height is positive, top-left if negative).
     // Screen's (0, 0) is the top-left corner.
-    int screenY = y + (bitmap.isTopDown() ? bmpY : bitmap.getHeight() - 1 - bmpY);
-    if (isScaled) {
-      screenY = std::floor(screenY * scale);
-    }
+    const int bmpYOffset = bitmap.isTopDown() ? bmpY : bitmap.getHeight() - 1 - bmpY;
+    int screenY = y + (isScaled ? static_cast<int>(std::floor(bmpYOffset * scale)) : bmpYOffset);
     if (screenY >= getScreenHeight()) {
       break;
     }
+    if (screenY < 0) {
+      continue;
+    }
 
     if (bitmap.readRow(outputRow, rowBytes, bmpY) != BmpReaderError::Ok) {
       Serial.printf("[%lu] [GFX] Failed to read row %d from bitmap\n", millis(), bmpY);
@@ -197,13 +198,13 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
     }
 
     for (int bmpX = 0; bmpX < bitmap.getWidth(); bmpX++) {
-      int screenX = x + bmpX;
-      if (isScaled) {
-        screenX = std::floor(screenX * scale);
-      }
+      int screenX = x + (isScaled ? static_cast<int>(std::floor(bmpX * scale)) : bmpX);
       if (screenX >= getScreenWidth()) {
         break;
       }
+      if (screenX < 0) {
+        continue;
+      }
 
       const uint8_t val = outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8)) & 0x3;
 
@@ -435,6 +436,61 @@ void GfxRenderer::restoreBwBuffer() {
   Serial.printf("[%lu] [GFX] Restored and freed BW buffer chunks\n", millis());
 }
 
+bool GfxRenderer::copyStoredBwBuffer() {
+  // Check if all chunks are allocated
+  for (const auto& bwBufferChunk : bwBufferChunks) {
+    if (!bwBufferChunk) {
+      return false;
+    }
+  }
+
+  uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
+  if (!frameBuffer) {
+    return false;
+  }
+
+  for (size_t i = 0; i < BW_BUFFER_NUM_CHUNKS; i++) {
+    const size_t offset = i * BW_BUFFER_CHUNK_SIZE;
+    memcpy(frameBuffer + offset, bwBufferChunks[i], BW_BUFFER_CHUNK_SIZE);
+  }
+
+  return true;
+}
+
+void GfxRenderer::freeStoredBwBuffer() { freeBwBufferChunks(); }
+
+/**
+ * Copy stored BW buffer to framebuffer without freeing the stored chunks.
+ * Use this when you want to restore the buffer but keep it for later reuse.
+ * Returns true if buffer was copied successfully.
+ */
+bool GfxRenderer::copyStoredBwBuffer() {
+  // Check if all chunks are allocated
+  for (const auto& bwBufferChunk : bwBufferChunks) {
+    if (!bwBufferChunk) {
+      return false;
+    }
+  }
+
+  uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
+  if (!frameBuffer) {
+    return false;
+  }
+
+  for (size_t i = 0; i < BW_BUFFER_NUM_CHUNKS; i++) {
+    const size_t offset = i * BW_BUFFER_CHUNK_SIZE;
+    memcpy(frameBuffer + offset, bwBufferChunks[i], BW_BUFFER_CHUNK_SIZE);
+  }
+
+  return true;
+}
+
+/**
+ * Free the stored BW buffer chunks manually.
+ * Use this when you no longer need the stored buffer.
+ */
+void GfxRenderer::freeStoredBwBuffer() { freeBwBufferChunks(); }
+
 /**
  * Cleanup grayscale buffers using the current frame buffer.
  * Use this when BW buffer was re-rendered instead of stored/restored.

lib/GfxRenderer/GfxRenderer.h

@@ -88,8 +88,10 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
-  bool storeBwBuffer();  // Returns true if buffer was stored successfully
-  void restoreBwBuffer();
+  bool storeBwBuffer();       // Returns true if buffer was stored successfully
+  void restoreBwBuffer();     // Restore and free the stored buffer
+  bool copyStoredBwBuffer();  // Copy stored buffer to framebuffer without freeing
+  void freeStoredBwBuffer();  // Free the stored buffer manually
   void cleanupGrayscaleWithFrameBuffer() const;
 
   // Low level functions

lib/JpegToBmpConverter/JpegToBmpConverter.cpp

@@ -356,7 +356,7 @@ void writeBmpHeader8bit(Print& bmpOut, const int width, const int height) {
 }
 
 // Helper function: Write BMP header with 2-bit color depth
-void JpegToBmpConverter::writeBmpHeader(Print& bmpOut, const int width, const int height) {
+static void writeBmpHeader2bit(Print& bmpOut, const int width, const int height) {
   // Calculate row padding (each row must be multiple of 4 bytes)
   const int bytesPerRow = (width * 2 + 31) / 32 * 4;  // 2 bits per pixel, round up
   const int imageSize = bytesPerRow * height;
@@ -427,9 +427,10 @@ unsigned char JpegToBmpConverter::jpegReadCallback(unsigned char* pBuf, const un
   return 0;  // Success
 }
 
-// Core function: Convert JPEG file to 2-bit BMP
-bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
-  Serial.printf("[%lu] [JPG] Converting JPEG to BMP\n", millis());
+// Internal implementation with configurable target size
+bool JpegToBmpConverter::jpegFileToBmpStreamInternal(FsFile& jpegFile, Print& bmpOut, int targetWidth,
+                                                     int targetHeight) {
+  Serial.printf("[%lu] [JPG] Converting JPEG to BMP (target: %dx%d)\n", millis(), targetWidth, targetHeight);
 
   // Setup context for picojpeg callback
   JpegReadContext context = {.file = jpegFile, .bufferPos = 0, .bufferFilled = 0};
@@ -464,10 +465,10 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
   uint32_t scaleY_fp = 65536;
   bool needsScaling = false;
 
-  if (USE_PRESCALE && (imageInfo.m_width > TARGET_MAX_WIDTH || imageInfo.m_height > TARGET_MAX_HEIGHT)) {
+  if (targetWidth > 0 && targetHeight > 0 && (imageInfo.m_width > targetWidth || imageInfo.m_height > targetHeight)) {
     // Calculate scale to fit within target dimensions while maintaining aspect ratio
-    const float scaleToFitWidth = static_cast<float>(TARGET_MAX_WIDTH) / imageInfo.m_width;
-    const float scaleToFitHeight = static_cast<float>(TARGET_MAX_HEIGHT) / imageInfo.m_height;
+    const float scaleToFitWidth = static_cast<float>(targetWidth) / imageInfo.m_width;
+    const float scaleToFitHeight = static_cast<float>(targetHeight) / imageInfo.m_height;
     const float scale = (scaleToFitWidth < scaleToFitHeight) ? scaleToFitWidth : scaleToFitHeight;
 
     outWidth = static_cast<int>(imageInfo.m_width * scale);
@@ -484,7 +485,7 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
     needsScaling = true;
 
     Serial.printf("[%lu] [JPG] Pre-scaling %dx%d -> %dx%d (fit to %dx%d)\n", millis(), imageInfo.m_width,
-                  imageInfo.m_height, outWidth, outHeight, TARGET_MAX_WIDTH, TARGET_MAX_HEIGHT);
+                  imageInfo.m_height, outWidth, outHeight, targetWidth, targetHeight);
   }
 
   // Write BMP header with output dimensions
@@ -493,7 +494,7 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
     writeBmpHeader8bit(bmpOut, outWidth, outHeight);
     bytesPerRow = (outWidth + 3) / 4 * 4;
   } else {
-    writeBmpHeader(bmpOut, outWidth, outHeight);
+    writeBmpHeader2bit(bmpOut, outWidth, outHeight);
     bytesPerRow = (outWidth * 2 + 31) / 32 * 4;
   }
 
@@ -736,3 +737,14 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
   Serial.printf("[%lu] [JPG] Successfully converted JPEG to BMP\n", millis());
   return true;
 }
+
+// Core function: Convert JPEG file to 2-bit BMP (uses default target size)
+bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
+  return jpegFileToBmpStreamInternal(jpegFile, bmpOut, TARGET_MAX_WIDTH, TARGET_MAX_HEIGHT);
+}
+
+// Convert with custom target size (for thumbnails)
+bool JpegToBmpConverter::jpegFileToBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth,
+                                                     int targetMaxHeight) {
+  return jpegFileToBmpStreamInternal(jpegFile, bmpOut, targetMaxWidth, targetMaxHeight);
+}

lib/JpegToBmpConverter/JpegToBmpConverter.h

@@ -5,11 +5,13 @@ class Print;
 class ZipFile;
 
 class JpegToBmpConverter {
-  static void writeBmpHeader(Print& bmpOut, int width, int height);
   // [COMMENTED OUT] static uint8_t grayscaleTo2Bit(uint8_t grayscale, int x, int y);
   static unsigned char jpegReadCallback(unsigned char* pBuf, unsigned char buf_size,
                                         unsigned char* pBytes_actually_read, void* pCallback_data);
+  static bool jpegFileToBmpStreamInternal(class FsFile& jpegFile, Print& bmpOut, int targetWidth, int targetHeight);
 
  public:
   static bool jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut);
+  // Convert with custom target size (for thumbnails)
+  static bool jpegFileToBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
 };

lib/Xtc/Xtc.cpp

@@ -293,6 +293,270 @@ bool Xtc::generateCoverBmp() const {
   return true;
 }
 
+std::string Xtc::getThumbBmpPath() const { return cachePath + "/thumb.bmp"; }
+
+bool Xtc::generateThumbBmp() const {
+  // Already generated
+  if (SdMan.exists(getThumbBmpPath().c_str())) {
+    return true;
+  }
+
+  if (!loaded || !parser) {
+    Serial.printf("[%lu] [XTC] Cannot generate thumb BMP, file not loaded\n", millis());
+    return false;
+  }
+
+  if (parser->getPageCount() == 0) {
+    Serial.printf("[%lu] [XTC] No pages in XTC file\n", millis());
+    return false;
+  }
+
+  // Setup cache directory
+  setupCacheDir();
+
+  // Get first page info for cover
+  xtc::PageInfo pageInfo;
+  if (!parser->getPageInfo(0, pageInfo)) {
+    Serial.printf("[%lu] [XTC] Failed to get first page info\n", millis());
+    return false;
+  }
+
+  // Get bit depth
+  const uint8_t bitDepth = parser->getBitDepth();
+
+  // Calculate target dimensions for thumbnail (fit within 240x400 Continue Reading card)
+  constexpr int THUMB_TARGET_WIDTH = 240;
+  constexpr int THUMB_TARGET_HEIGHT = 400;
+
+  // Calculate scale factor
+  float scaleX = static_cast<float>(THUMB_TARGET_WIDTH) / pageInfo.width;
+  float scaleY = static_cast<float>(THUMB_TARGET_HEIGHT) / pageInfo.height;
+  float scale = (scaleX < scaleY) ? scaleX : scaleY;
+
+  // Only scale down, never up
+  if (scale >= 1.0f) {
+    // Page is already small enough, just use cover.bmp
+    // Copy cover.bmp to thumb.bmp
+    if (generateCoverBmp()) {
+      FsFile src, dst;
+      if (SdMan.openFileForRead("XTC", getCoverBmpPath(), src)) {
+        if (SdMan.openFileForWrite("XTC", getThumbBmpPath(), dst)) {
+          uint8_t buffer[512];
+          while (src.available()) {
+            size_t bytesRead = src.read(buffer, sizeof(buffer));
+            dst.write(buffer, bytesRead);
+          }
+          dst.close();
+        }
+        src.close();
+      }
+      Serial.printf("[%lu] [XTC] Copied cover to thumb (no scaling needed)\n", millis());
+      return SdMan.exists(getThumbBmpPath().c_str());
+    }
+    return false;
+  }
+
+  uint16_t thumbWidth = static_cast<uint16_t>(pageInfo.width * scale);
+  uint16_t thumbHeight = static_cast<uint16_t>(pageInfo.height * scale);
+
+  Serial.printf("[%lu] [XTC] Generating thumb BMP: %dx%d -> %dx%d (scale: %.3f)\n", millis(), pageInfo.width,
+                pageInfo.height, thumbWidth, thumbHeight, scale);
+
+  // Allocate buffer for page data
+  size_t bitmapSize;
+  if (bitDepth == 2) {
+    bitmapSize = ((static_cast<size_t>(pageInfo.width) * pageInfo.height + 7) / 8) * 2;
+  } else {
+    bitmapSize = ((pageInfo.width + 7) / 8) * pageInfo.height;
+  }
+  uint8_t* pageBuffer = static_cast<uint8_t*>(malloc(bitmapSize));
+  if (!pageBuffer) {
+    Serial.printf("[%lu] [XTC] Failed to allocate page buffer (%lu bytes)\n", millis(), bitmapSize);
+    return false;
+  }
+
+  // Load first page (cover)
+  size_t bytesRead = const_cast<xtc::XtcParser*>(parser.get())->loadPage(0, pageBuffer, bitmapSize);
+  if (bytesRead == 0) {
+    Serial.printf("[%lu] [XTC] Failed to load cover page for thumb\n", millis());
+    free(pageBuffer);
+    return false;
+  }
+
+  // Create thumbnail BMP file - use 2-bit format like EPUB covers
+  FsFile thumbBmp;
+  if (!SdMan.openFileForWrite("XTC", getThumbBmpPath(), thumbBmp)) {
+    Serial.printf("[%lu] [XTC] Failed to create thumb BMP file\n", millis());
+    free(pageBuffer);
+    return false;
+  }
+
+  // Write 2-bit BMP header (same format as JpegToBmpConverter)
+  const uint32_t rowSize = (thumbWidth * 2 + 31) / 32 * 4;  // 2 bits per pixel, aligned
+  const uint32_t imageSize = rowSize * thumbHeight;
+  const uint32_t fileSize = 14 + 40 + 16 + imageSize;  // 16 bytes for 4-color palette
+
+  // File header
+  thumbBmp.write('B');
+  thumbBmp.write('M');
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&fileSize), 4);
+  uint32_t reserved = 0;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&reserved), 4);
+  uint32_t dataOffset = 14 + 40 + 16;  // 2-bit palette has 4 colors (16 bytes)
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&dataOffset), 4);
+
+  // DIB header
+  uint32_t dibHeaderSize = 40;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&dibHeaderSize), 4);
+  int32_t widthVal = thumbWidth;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&widthVal), 4);
+  int32_t heightVal = -static_cast<int32_t>(thumbHeight);  // Negative for top-down
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&heightVal), 4);
+  uint16_t planes = 1;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&planes), 2);
+  uint16_t bitsPerPixel = 2;  // 2-bit for 4 grayscale levels
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&bitsPerPixel), 2);
+  uint32_t compression = 0;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&compression), 4);
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&imageSize), 4);
+  int32_t ppmX = 2835;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&ppmX), 4);
+  int32_t ppmY = 2835;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&ppmY), 4);
+  uint32_t colorsUsed = 4;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&colorsUsed), 4);
+  uint32_t colorsImportant = 4;
+  thumbBmp.write(reinterpret_cast<const uint8_t*>(&colorsImportant), 4);
+
+  // Color palette (4 colors for 2-bit, same as JpegToBmpConverter)
+  uint8_t palette[16] = {
+      0x00, 0x00, 0x00, 0x00,  // Color 0: Black
+      0x55, 0x55, 0x55, 0x00,  // Color 1: Dark gray (85)
+      0xAA, 0xAA, 0xAA, 0x00,  // Color 2: Light gray (170)
+      0xFF, 0xFF, 0xFF, 0x00   // Color 3: White
+  };
+  thumbBmp.write(palette, 16);
+
+  // Allocate row buffer for 2-bit output
+  const size_t dstRowSize = (thumbWidth * 2 + 7) / 8;
+  uint8_t* rowBuffer = static_cast<uint8_t*>(malloc(rowSize));
+  if (!rowBuffer) {
+    free(pageBuffer);
+    thumbBmp.close();
+    return false;
+  }
+
+  // Fixed-point scale factor (16.16)
+  uint32_t scaleInv_fp = static_cast<uint32_t>(65536.0f / scale);
+
+  // Pre-calculate plane info for 2-bit mode
+  const size_t planeSize = (bitDepth == 2) ? ((static_cast<size_t>(pageInfo.width) * pageInfo.height + 7) / 8) : 0;
+  const uint8_t* plane1 = (bitDepth == 2) ? pageBuffer : nullptr;
+  const uint8_t* plane2 = (bitDepth == 2) ? pageBuffer + planeSize : nullptr;
+  const size_t colBytes = (bitDepth == 2) ? ((pageInfo.height + 7) / 8) : 0;
+  const size_t srcRowBytes = (bitDepth == 1) ? ((pageInfo.width + 7) / 8) : 0;
+
+  for (uint16_t dstY = 0; dstY < thumbHeight; dstY++) {
+    memset(rowBuffer, 0xFF, rowSize);  // Start with all white (color 3)
+
+    // Calculate source Y range with bounds checking
+    uint32_t srcYStart = (static_cast<uint32_t>(dstY) * scaleInv_fp) >> 16;
+    uint32_t srcYEnd = (static_cast<uint32_t>(dstY + 1) * scaleInv_fp) >> 16;
+    if (srcYStart >= pageInfo.height) srcYStart = pageInfo.height - 1;
+    if (srcYEnd > pageInfo.height) srcYEnd = pageInfo.height;
+    if (srcYEnd <= srcYStart) srcYEnd = srcYStart + 1;
+    if (srcYEnd > pageInfo.height) srcYEnd = pageInfo.height;
+
+    for (uint16_t dstX = 0; dstX < thumbWidth; dstX++) {
+      // Calculate source X range with bounds checking
+      uint32_t srcXStart = (static_cast<uint32_t>(dstX) * scaleInv_fp) >> 16;
+      uint32_t srcXEnd = (static_cast<uint32_t>(dstX + 1) * scaleInv_fp) >> 16;
+      if (srcXStart >= pageInfo.width) srcXStart = pageInfo.width - 1;
+      if (srcXEnd > pageInfo.width) srcXEnd = pageInfo.width;
+      if (srcXEnd <= srcXStart) srcXEnd = srcXStart + 1;
+      if (srcXEnd > pageInfo.width) srcXEnd = pageInfo.width;
+
+      // Area averaging: sum grayscale values (0-255 range)
+      uint32_t graySum = 0;
+      uint32_t totalCount = 0;
+
+      for (uint32_t srcY = srcYStart; srcY < srcYEnd && srcY < pageInfo.height; srcY++) {
+        for (uint32_t srcX = srcXStart; srcX < srcXEnd && srcX < pageInfo.width; srcX++) {
+          uint8_t grayValue = 255;  // Default: white
+
+          if (bitDepth == 2) {
+            // XTH 2-bit mode: pixel value 0-3
+            // Bounds check for column index
+            if (srcX < pageInfo.width) {
+              const size_t colIndex = pageInfo.width - 1 - srcX;
+              const size_t byteInCol = srcY / 8;
+              const size_t bitInByte = 7 - (srcY % 8);
+              const size_t byteOffset = colIndex * colBytes + byteInCol;
+              // Bounds check for buffer access
+              if (byteOffset < planeSize) {
+                const uint8_t bit1 = (plane1[byteOffset] >> bitInByte) & 1;
+                const uint8_t bit2 = (plane2[byteOffset] >> bitInByte) & 1;
+                const uint8_t pixelValue = (bit1 << 1) | bit2;
+                // Convert 2-bit (0-3) to grayscale: 0=black, 3=white
+                // pixelValue: 0=white, 1=light gray, 2=dark gray, 3=black (XTC polarity)
+                grayValue = (3 - pixelValue) * 85;  // 0->255, 1->170, 2->85, 3->0
+              }
+            }
+          } else {
+            // 1-bit mode
+            const size_t byteIdx = srcY * srcRowBytes + srcX / 8;
+            const size_t bitIdx = 7 - (srcX % 8);
+            // Bounds check for buffer access
+            if (byteIdx < bitmapSize) {
+              const uint8_t pixelBit = (pageBuffer[byteIdx] >> bitIdx) & 1;
+              // XTC polarity: 1=black, 0=white
+              grayValue = pixelBit ? 0 : 255;
+            }
+          }
+
+          graySum += grayValue;
+          totalCount++;
+        }
+      }
+
+      // Calculate average grayscale and quantize to 2-bit
+      uint8_t avgGray = (totalCount > 0) ? static_cast<uint8_t>(graySum / totalCount) : 255;
+
+      // Quantize to 4 levels (same thresholds as JpegToBmpConverter)
+      uint8_t twoBit;
+      if (avgGray < 43) {
+        twoBit = 0;  // Black
+      } else if (avgGray < 128) {
+        twoBit = 1;  // Dark gray
+      } else if (avgGray < 213) {
+        twoBit = 2;  // Light gray
+      } else {
+        twoBit = 3;  // White
+      }
+
+      // Pack 2-bit value into row buffer (MSB first)
+      const size_t byteIndex = (dstX * 2) / 8;
+      const size_t bitOffset = 6 - ((dstX * 2) % 8);
+      // Bounds check for row buffer access
+      if (byteIndex < rowSize) {
+        rowBuffer[byteIndex] &= ~(0x03 << bitOffset);   // Clear bits
+        rowBuffer[byteIndex] |= (twoBit << bitOffset);  // Set bits
+      }
+    }
+
+    // Write row (already padded to 4-byte boundary by rowSize)
+    thumbBmp.write(rowBuffer, rowSize);
+  }
+
+  free(rowBuffer);
+  thumbBmp.close();
+  free(pageBuffer);
+
+  Serial.printf("[%lu] [XTC] Generated thumb BMP (%dx%d): %s\n", millis(), thumbWidth, thumbHeight,
+                getThumbBmpPath().c_str());
+  return true;
+}
+
 uint32_t Xtc::getPageCount() const {
   if (!loaded || !parser) {
     return 0;

lib/Xtc/Xtc.h

@@ -62,6 +62,9 @@ class Xtc {
   // Cover image support (for sleep screen)
   std::string getCoverBmpPath() const;
   bool generateCoverBmp() const;
+  // Thumbnail support (for Continue Reading card)
+  std::string getThumbBmpPath() const;
+  bool generateThumbBmp() const;
 
   // Page access
   uint32_t getPageCount() const;