feat: Sleep screen letterbox fill and image upscaling

Add configurable letterbox fill for sleep screen cover images that don't
match the display aspect ratio. Four fill modes are available: Solid
(single dominant edge shade), Blended (per-pixel edge colors), Gradient
(edge colors interpolated toward white/black), and None.

Enable upscaling of cover images smaller than the display in Fit mode by
modifying drawBitmap/drawBitmap1Bit to support both up and downscaling
via a unified block-fill approach.

Edge sampling data is cached to .crosspoint alongside the cover BMP to
avoid redundant bitmap scanning on subsequent sleeps. Cache is validated
against screen dimensions and auto-regenerated when stale.

New settings: Letterbox Fill (None/Solid/Blended/Gradient) and Gradient
Direction (To White/To Black).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/boot_sleep/SleepActivity.cpp

@@ -1,11 +1,15 @@
 #include "SleepActivity.h"
 
+#include <BitmapHelpers.h>
 #include <Epub.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
+#include <Serialization.h>
 #include <Txt.h>
 #include <Xtc.h>
 
+#include <algorithm>
+
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "components/UITheme.h"
@@ -13,6 +17,364 @@
 #include "images/Logo120.h"
 #include "util/StringUtils.h"
 
+namespace {
+
+// Number of source pixels along the image edge to average for the gradient color
+constexpr int EDGE_SAMPLE_DEPTH = 20;
+
+// Map a 2-bit quantized pixel value to an 8-bit grayscale value
+constexpr uint8_t val2bitToGray(uint8_t val2bit) { return val2bit * 85; }
+
+// Edge gradient data produced by sampleBitmapEdges and consumed by drawLetterboxGradients.
+// edgeA is the "first" edge (top or left), edgeB is the "second" edge (bottom or right).
+struct LetterboxGradientData {
+  uint8_t* edgeA = nullptr;
+  uint8_t* edgeB = nullptr;
+  int edgeCount = 0;
+  int letterboxA = 0;       // pixel size of the first letterbox area (top or left)
+  int letterboxB = 0;       // pixel size of the second letterbox area (bottom or right)
+  bool horizontal = false;  // true = top/bottom letterbox, false = left/right
+
+  void free() {
+    ::free(edgeA);
+    ::free(edgeB);
+    edgeA = nullptr;
+    edgeB = nullptr;
+  }
+};
+
+// Binary cache version for edge data files
+constexpr uint8_t EDGE_CACHE_VERSION = 1;
+
+// Load cached edge data from a binary file. Returns true if the cache was valid and loaded successfully.
+// Validates cache version and screen dimensions to detect stale data.
+bool loadEdgeCache(const std::string& path, int screenWidth, int screenHeight, LetterboxGradientData& data) {
+  FsFile file;
+  if (!Storage.openFileForRead("SLP", path, file)) return false;
+
+  uint8_t version;
+  serialization::readPod(file, version);
+  if (version != EDGE_CACHE_VERSION) {
+    file.close();
+    return false;
+  }
+
+  uint16_t cachedW, cachedH;
+  serialization::readPod(file, cachedW);
+  serialization::readPod(file, cachedH);
+  if (cachedW != static_cast<uint16_t>(screenWidth) || cachedH != static_cast<uint16_t>(screenHeight)) {
+    file.close();
+    return false;
+  }
+
+  uint8_t horizontal;
+  serialization::readPod(file, horizontal);
+  data.horizontal = (horizontal != 0);
+
+  uint16_t edgeCount;
+  serialization::readPod(file, edgeCount);
+  data.edgeCount = edgeCount;
+
+  int16_t lbA, lbB;
+  serialization::readPod(file, lbA);
+  serialization::readPod(file, lbB);
+  data.letterboxA = lbA;
+  data.letterboxB = lbB;
+
+  if (edgeCount == 0 || edgeCount > 2048) {
+    file.close();
+    return false;
+  }
+
+  data.edgeA = static_cast<uint8_t*>(malloc(edgeCount));
+  data.edgeB = static_cast<uint8_t*>(malloc(edgeCount));
+  if (!data.edgeA || !data.edgeB) {
+    data.free();
+    file.close();
+    return false;
+  }
+
+  if (file.read(data.edgeA, edgeCount) != static_cast<int>(edgeCount) ||
+      file.read(data.edgeB, edgeCount) != static_cast<int>(edgeCount)) {
+    data.free();
+    file.close();
+    return false;
+  }
+
+  file.close();
+  Serial.printf("[%lu] [SLP] Loaded edge cache from %s (%d edges)\n", millis(), path.c_str(), edgeCount);
+  return true;
+}
+
+// Save edge data to a binary cache file for reuse on subsequent sleep screens.
+bool saveEdgeCache(const std::string& path, int screenWidth, int screenHeight, const LetterboxGradientData& data) {
+  if (!data.edgeA || !data.edgeB || data.edgeCount <= 0) return false;
+
+  FsFile file;
+  if (!Storage.openFileForWrite("SLP", path, file)) return false;
+
+  serialization::writePod(file, EDGE_CACHE_VERSION);
+  serialization::writePod(file, static_cast<uint16_t>(screenWidth));
+  serialization::writePod(file, static_cast<uint16_t>(screenHeight));
+  serialization::writePod(file, static_cast<uint8_t>(data.horizontal ? 1 : 0));
+  serialization::writePod(file, static_cast<uint16_t>(data.edgeCount));
+  serialization::writePod(file, static_cast<int16_t>(data.letterboxA));
+  serialization::writePod(file, static_cast<int16_t>(data.letterboxB));
+  file.write(data.edgeA, data.edgeCount);
+  file.write(data.edgeB, data.edgeCount);
+  file.close();
+
+  Serial.printf("[%lu] [SLP] Saved edge cache to %s (%d edges)\n", millis(), path.c_str(), data.edgeCount);
+  return true;
+}
+
+// Read the bitmap once to sample the first/last EDGE_SAMPLE_DEPTH rows or columns.
+// Returns edge color arrays in source pixel resolution.  Caller must call data.free() when done.
+// After sampling the bitmap is rewound via rewindToData().
+LetterboxGradientData sampleBitmapEdges(const Bitmap& bitmap, int imgX, int imgY, int pageWidth, int pageHeight,
+                                        float scale, float cropX, float cropY) {
+  LetterboxGradientData data;
+
+  const int cropPixX = static_cast<int>(std::floor(bitmap.getWidth() * cropX / 2.0f));
+  const int cropPixY = static_cast<int>(std::floor(bitmap.getHeight() * cropY / 2.0f));
+  const int visibleWidth = bitmap.getWidth() - 2 * cropPixX;
+  const int visibleHeight = bitmap.getHeight() - 2 * cropPixY;
+
+  if (visibleWidth <= 0 || visibleHeight <= 0) return data;
+
+  const int outputRowSize = (bitmap.getWidth() + 3) / 4;
+  auto* outputRow = static_cast<uint8_t*>(malloc(outputRowSize));
+  auto* rowBytes = static_cast<uint8_t*>(malloc(bitmap.getRowBytes()));
+  if (!outputRow || !rowBytes) {
+    ::free(outputRow);
+    ::free(rowBytes);
+    return data;
+  }
+
+  if (imgY > 0) {
+    // Top/bottom letterboxing -- sample per-column averages of first/last N rows
+    data.horizontal = true;
+    data.edgeCount = visibleWidth;
+    const int scaledHeight = static_cast<int>(std::round(static_cast<float>(visibleHeight) * scale));
+    data.letterboxA = imgY;
+    data.letterboxB = pageHeight - imgY - scaledHeight;
+    if (data.letterboxB < 0) data.letterboxB = 0;
+
+    const int sampleRows = std::min(EDGE_SAMPLE_DEPTH, visibleHeight);
+
+    auto* accumTop = static_cast<uint32_t*>(calloc(visibleWidth, sizeof(uint32_t)));
+    auto* accumBot = static_cast<uint32_t*>(calloc(visibleWidth, sizeof(uint32_t)));
+    data.edgeA = static_cast<uint8_t*>(malloc(visibleWidth));
+    data.edgeB = static_cast<uint8_t*>(malloc(visibleWidth));
+
+    if (!accumTop || !accumBot || !data.edgeA || !data.edgeB) {
+      ::free(accumTop);
+      ::free(accumBot);
+      data.free();
+      ::free(outputRow);
+      ::free(rowBytes);
+      return data;
+    }
+
+    for (int bmpY = 0; bmpY < bitmap.getHeight(); bmpY++) {
+      if (bitmap.readNextRow(outputRow, rowBytes) != BmpReaderError::Ok) break;
+      const int logicalY = bitmap.isTopDown() ? bmpY : bitmap.getHeight() - 1 - bmpY;
+      if (logicalY < cropPixY || logicalY >= bitmap.getHeight() - cropPixY) continue;
+      const int outY = logicalY - cropPixY;
+
+      const bool inTop = (outY < sampleRows);
+      const bool inBot = (outY >= visibleHeight - sampleRows);
+      if (!inTop && !inBot) continue;
+
+      for (int bmpX = cropPixX; bmpX < bitmap.getWidth() - cropPixX; bmpX++) {
+        const int outX = bmpX - cropPixX;
+        const uint8_t val = (outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8))) & 0x3;
+        const uint8_t gray = val2bitToGray(val);
+        if (inTop) accumTop[outX] += gray;
+        if (inBot) accumBot[outX] += gray;
+      }
+    }
+
+    for (int i = 0; i < visibleWidth; i++) {
+      data.edgeA[i] = static_cast<uint8_t>(accumTop[i] / sampleRows);
+      data.edgeB[i] = static_cast<uint8_t>(accumBot[i] / sampleRows);
+    }
+    ::free(accumTop);
+    ::free(accumBot);
+
+  } else if (imgX > 0) {
+    // Left/right letterboxing -- sample per-row averages of first/last N columns
+    data.horizontal = false;
+    data.edgeCount = visibleHeight;
+    const int scaledWidth = static_cast<int>(std::round(static_cast<float>(visibleWidth) * scale));
+    data.letterboxA = imgX;
+    data.letterboxB = pageWidth - imgX - scaledWidth;
+    if (data.letterboxB < 0) data.letterboxB = 0;
+
+    const int sampleCols = std::min(EDGE_SAMPLE_DEPTH, visibleWidth);
+
+    auto* accumLeft = static_cast<uint32_t*>(calloc(visibleHeight, sizeof(uint32_t)));
+    auto* accumRight = static_cast<uint32_t*>(calloc(visibleHeight, sizeof(uint32_t)));
+    data.edgeA = static_cast<uint8_t*>(malloc(visibleHeight));
+    data.edgeB = static_cast<uint8_t*>(malloc(visibleHeight));
+
+    if (!accumLeft || !accumRight || !data.edgeA || !data.edgeB) {
+      ::free(accumLeft);
+      ::free(accumRight);
+      data.free();
+      ::free(outputRow);
+      ::free(rowBytes);
+      return data;
+    }
+
+    for (int bmpY = 0; bmpY < bitmap.getHeight(); bmpY++) {
+      if (bitmap.readNextRow(outputRow, rowBytes) != BmpReaderError::Ok) break;
+      const int logicalY = bitmap.isTopDown() ? bmpY : bitmap.getHeight() - 1 - bmpY;
+      if (logicalY < cropPixY || logicalY >= bitmap.getHeight() - cropPixY) continue;
+      const int outY = logicalY - cropPixY;
+
+      // Sample left edge columns
+      for (int bmpX = cropPixX; bmpX < cropPixX + sampleCols; bmpX++) {
+        const uint8_t val = (outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8))) & 0x3;
+        accumLeft[outY] += val2bitToGray(val);
+      }
+      // Sample right edge columns
+      for (int bmpX = bitmap.getWidth() - cropPixX - sampleCols; bmpX < bitmap.getWidth() - cropPixX; bmpX++) {
+        const uint8_t val = (outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8))) & 0x3;
+        accumRight[outY] += val2bitToGray(val);
+      }
+    }
+
+    for (int i = 0; i < visibleHeight; i++) {
+      data.edgeA[i] = static_cast<uint8_t>(accumLeft[i] / sampleCols);
+      data.edgeB[i] = static_cast<uint8_t>(accumRight[i] / sampleCols);
+    }
+    ::free(accumLeft);
+    ::free(accumRight);
+  }
+
+  ::free(outputRow);
+  ::free(rowBytes);
+  bitmap.rewindToData();
+  return data;
+}
+
+// Draw dithered fills in the letterbox areas using the sampled edge colors.
+// fillMode selects the fill algorithm: SOLID (single dominant shade), BLENDED (per-pixel edge color),
+// or GRADIENT (per-pixel edge color interpolated toward targetColor).
+// targetColor is the color the gradient fades toward (255=white, 0=black); only used in GRADIENT mode.
+// Must be called once per render pass (BW, GRAYSCALE_LSB, GRAYSCALE_MSB).
+void drawLetterboxFill(GfxRenderer& renderer, const LetterboxGradientData& data, float scale, uint8_t fillMode,
+                       int targetColor) {
+  if (!data.edgeA || !data.edgeB || data.edgeCount <= 0) return;
+
+  const bool isSolid = (fillMode == CrossPointSettings::SLEEP_SCREEN_LETTERBOX_FILL::LETTERBOX_SOLID);
+  const bool isGradient = (fillMode == CrossPointSettings::SLEEP_SCREEN_LETTERBOX_FILL::LETTERBOX_GRADIENT);
+
+  // For SOLID mode, compute the dominant (average) shade for each edge once
+  uint8_t solidColorA = 0, solidColorB = 0;
+  if (isSolid) {
+    uint32_t sumA = 0, sumB = 0;
+    for (int i = 0; i < data.edgeCount; i++) {
+      sumA += data.edgeA[i];
+      sumB += data.edgeB[i];
+    }
+    solidColorA = static_cast<uint8_t>(sumA / data.edgeCount);
+    solidColorB = static_cast<uint8_t>(sumB / data.edgeCount);
+  }
+
+  // Helper: compute gray value for a pixel given the edge color and interpolation factor t (0..1)
+  // GRADIENT interpolates from edgeColor toward targetColor; SOLID and BLENDED return edgeColor directly.
+  auto computeGray = [&](int edgeColor, float t) -> int {
+    if (isGradient) return edgeColor + static_cast<int>(static_cast<float>(targetColor - edgeColor) * t);
+    return edgeColor;
+  };
+
+  if (data.horizontal) {
+    // Top letterbox
+    if (data.letterboxA > 0) {
+      const int imgTopY = data.letterboxA;
+      for (int screenY = 0; screenY < imgTopY; screenY++) {
+        const float t = static_cast<float>(imgTopY - screenY) / static_cast<float>(imgTopY);
+        for (int screenX = 0; screenX < renderer.getScreenWidth(); screenX++) {
+          int edgeColor;
+          if (isSolid) {
+            edgeColor = solidColorA;
+          } else {
+            int srcCol = static_cast<int>(screenX / scale);
+            srcCol = std::max(0, std::min(srcCol, data.edgeCount - 1));
+            edgeColor = data.edgeA[srcCol];
+          }
+          const int gray = computeGray(edgeColor, t);
+          renderer.drawPixelGray(screenX, screenY, quantizeNoiseDither(gray, screenX, screenY));
+        }
+      }
+    }
+
+    // Bottom letterbox
+    if (data.letterboxB > 0) {
+      const int imgBottomY = renderer.getScreenHeight() - data.letterboxB;
+      for (int screenY = imgBottomY; screenY < renderer.getScreenHeight(); screenY++) {
+        const float t = static_cast<float>(screenY - imgBottomY + 1) / static_cast<float>(data.letterboxB);
+        for (int screenX = 0; screenX < renderer.getScreenWidth(); screenX++) {
+          int edgeColor;
+          if (isSolid) {
+            edgeColor = solidColorB;
+          } else {
+            int srcCol = static_cast<int>(screenX / scale);
+            srcCol = std::max(0, std::min(srcCol, data.edgeCount - 1));
+            edgeColor = data.edgeB[srcCol];
+          }
+          const int gray = computeGray(edgeColor, t);
+          renderer.drawPixelGray(screenX, screenY, quantizeNoiseDither(gray, screenX, screenY));
+        }
+      }
+    }
+  } else {
+    // Left letterbox
+    if (data.letterboxA > 0) {
+      const int imgLeftX = data.letterboxA;
+      for (int screenX = 0; screenX < imgLeftX; screenX++) {
+        const float t = static_cast<float>(imgLeftX - screenX) / static_cast<float>(imgLeftX);
+        for (int screenY = 0; screenY < renderer.getScreenHeight(); screenY++) {
+          int edgeColor;
+          if (isSolid) {
+            edgeColor = solidColorA;
+          } else {
+            int srcRow = static_cast<int>(screenY / scale);
+            srcRow = std::max(0, std::min(srcRow, data.edgeCount - 1));
+            edgeColor = data.edgeA[srcRow];
+          }
+          const int gray = computeGray(edgeColor, t);
+          renderer.drawPixelGray(screenX, screenY, quantizeNoiseDither(gray, screenX, screenY));
+        }
+      }
+    }
+
+    // Right letterbox
+    if (data.letterboxB > 0) {
+      const int imgRightX = renderer.getScreenWidth() - data.letterboxB;
+      for (int screenX = imgRightX; screenX < renderer.getScreenWidth(); screenX++) {
+        const float t = static_cast<float>(screenX - imgRightX + 1) / static_cast<float>(data.letterboxB);
+        for (int screenY = 0; screenY < renderer.getScreenHeight(); screenY++) {
+          int edgeColor;
+          if (isSolid) {
+            edgeColor = solidColorB;
+          } else {
+            int srcRow = static_cast<int>(screenY / scale);
+            srcRow = std::max(0, std::min(srcRow, data.edgeCount - 1));
+            edgeColor = data.edgeB[srcRow];
+          }
+          const int gray = computeGray(edgeColor, t);
+          renderer.drawPixelGray(screenX, screenY, quantizeNoiseDither(gray, screenX, screenY));
+        }
+      }
+    }
+  }
+}
+
+}  // namespace
+
 void SleepActivity::onEnter() {
   Activity::onEnter();
   GUI.drawPopup(renderer, "Entering Sleep...");
@@ -121,7 +483,7 @@ void SleepActivity::renderDefaultSleepScreen() const {
   renderer.displayBuffer(HalDisplay::HALF_REFRESH);
 }
 
-void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
+void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap, const std::string& edgeCachePath) const {
   int x, y;
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
@@ -129,45 +491,79 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
 
   Serial.printf("[%lu] [SLP] bitmap %d x %d, screen %d x %d\n", millis(), bitmap.getWidth(), bitmap.getHeight(),
                 pageWidth, pageHeight);
-  if (bitmap.getWidth() > pageWidth || bitmap.getHeight() > pageHeight) {
-    // image will scale, make sure placement is right
-    float ratio = static_cast<float>(bitmap.getWidth()) / static_cast<float>(bitmap.getHeight());
-    const float screenRatio = static_cast<float>(pageWidth) / static_cast<float>(pageHeight);
 
-    Serial.printf("[%lu] [SLP] bitmap ratio: %f, screen ratio: %f\n", millis(), ratio, screenRatio);
-    if (ratio > screenRatio) {
-      // image wider than viewport ratio, scaled down image needs to be centered vertically
-      if (SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
-        cropX = 1.0f - (screenRatio / ratio);
-        Serial.printf("[%lu] [SLP] Cropping bitmap x: %f\n", millis(), cropX);
-        ratio = (1.0f - cropX) * static_cast<float>(bitmap.getWidth()) / static_cast<float>(bitmap.getHeight());
-      }
-      x = 0;
-      y = std::round((static_cast<float>(pageHeight) - static_cast<float>(pageWidth) / ratio) / 2);
-      Serial.printf("[%lu] [SLP] Centering with ratio %f to y=%d\n", millis(), ratio, y);
-    } else {
-      // image taller than viewport ratio, scaled down image needs to be centered horizontally
-      if (SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
-        cropY = 1.0f - (ratio / screenRatio);
-        Serial.printf("[%lu] [SLP] Cropping bitmap y: %f\n", millis(), cropY);
-        ratio = static_cast<float>(bitmap.getWidth()) / ((1.0f - cropY) * static_cast<float>(bitmap.getHeight()));
-      }
-      x = std::round((static_cast<float>(pageWidth) - static_cast<float>(pageHeight) * ratio) / 2);
-      y = 0;
-      Serial.printf("[%lu] [SLP] Centering with ratio %f to x=%d\n", millis(), ratio, x);
+  // Always compute aspect-ratio-preserving scale and position (supports both larger and smaller images)
+  float ratio = static_cast<float>(bitmap.getWidth()) / static_cast<float>(bitmap.getHeight());
+  const float screenRatio = static_cast<float>(pageWidth) / static_cast<float>(pageHeight);
+
+  Serial.printf("[%lu] [SLP] bitmap ratio: %f, screen ratio: %f\n", millis(), ratio, screenRatio);
+  if (ratio > screenRatio) {
+    // image wider than viewport ratio, needs to be centered vertically
+    if (SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
+      cropX = 1.0f - (screenRatio / ratio);
+      Serial.printf("[%lu] [SLP] Cropping bitmap x: %f\n", millis(), cropX);
+      ratio = (1.0f - cropX) * static_cast<float>(bitmap.getWidth()) / static_cast<float>(bitmap.getHeight());
     }
+    x = 0;
+    y = std::round((static_cast<float>(pageHeight) - static_cast<float>(pageWidth) / ratio) / 2);
+    Serial.printf("[%lu] [SLP] Centering with ratio %f to y=%d\n", millis(), ratio, y);
   } else {
-    // center the image
-    x = (pageWidth - bitmap.getWidth()) / 2;
-    y = (pageHeight - bitmap.getHeight()) / 2;
+    // image taller than or equal to viewport ratio, needs to be centered horizontally
+    if (SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
+      cropY = 1.0f - (ratio / screenRatio);
+      Serial.printf("[%lu] [SLP] Cropping bitmap y: %f\n", millis(), cropY);
+      ratio = static_cast<float>(bitmap.getWidth()) / ((1.0f - cropY) * static_cast<float>(bitmap.getHeight()));
+    }
+    x = std::round((static_cast<float>(pageWidth) - static_cast<float>(pageHeight) * ratio) / 2);
+    y = 0;
+    Serial.printf("[%lu] [SLP] Centering with ratio %f to x=%d\n", millis(), ratio, x);
   }
 
   Serial.printf("[%lu] [SLP] drawing to %d x %d\n", millis(), x, y);
+
+  // Compute the scale factor (same formula as drawBitmap) so we can map screen coords to source coords
+  const float effectiveWidth = (1.0f - cropX) * bitmap.getWidth();
+  const float effectiveHeight = (1.0f - cropY) * bitmap.getHeight();
+  const float scale =
+      std::min(static_cast<float>(pageWidth) / effectiveWidth, static_cast<float>(pageHeight) / effectiveHeight);
+
+  // Determine letterbox fill settings
+  const uint8_t fillMode = SETTINGS.sleepScreenLetterboxFill;
+  const bool wantFill = (fillMode != CrossPointSettings::SLEEP_SCREEN_LETTERBOX_FILL::LETTERBOX_NONE);
+  const int targetColor =
+      (SETTINGS.sleepScreenGradientDir == CrossPointSettings::SLEEP_SCREEN_GRADIENT_DIR::GRADIENT_TO_BLACK) ? 0 : 255;
+
+  static const char* fillModeNames[] = {"none", "solid", "blended", "gradient"};
+  const char* fillModeName = (fillMode < 4) ? fillModeNames[fillMode] : "unknown";
+
+  // Load cached edge data or sample from bitmap (first pass over bitmap, then rewind)
+  LetterboxGradientData gradientData;
+  const bool hasLetterbox = (x > 0 || y > 0);
+  if (hasLetterbox && wantFill) {
+    bool cacheLoaded = false;
+    if (!edgeCachePath.empty()) {
+      cacheLoaded = loadEdgeCache(edgeCachePath, pageWidth, pageHeight, gradientData);
+    }
+    if (!cacheLoaded) {
+      Serial.printf("[%lu] [SLP] Letterbox detected (x=%d, y=%d), sampling edges for %s fill\n", millis(), x, y,
+                    fillModeName);
+      gradientData = sampleBitmapEdges(bitmap, x, y, pageWidth, pageHeight, scale, cropX, cropY);
+      if (!edgeCachePath.empty() && gradientData.edgeA) {
+        saveEdgeCache(edgeCachePath, pageWidth, pageHeight, gradientData);
+      }
+    }
+  }
+
   renderer.clearScreen();
 
   const bool hasGreyscale = bitmap.hasGreyscale() &&
                             SETTINGS.sleepScreenCoverFilter == CrossPointSettings::SLEEP_SCREEN_COVER_FILTER::NO_FILTER;
 
+  // Draw letterbox fill (BW pass)
+  if (gradientData.edgeA) {
+    drawLetterboxFill(renderer, gradientData, scale, fillMode, targetColor);
+  }
+
   renderer.drawBitmap(bitmap, x, y, pageWidth, pageHeight, cropX, cropY);
 
   if (SETTINGS.sleepScreenCoverFilter == CrossPointSettings::SLEEP_SCREEN_COVER_FILTER::INVERTED_BLACK_AND_WHITE) {
@@ -180,18 +576,26 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
     bitmap.rewindToData();
     renderer.clearScreen(0x00);
     renderer.setRenderMode(GfxRenderer::GRAYSCALE_LSB);
+    if (gradientData.edgeA) {
+      drawLetterboxFill(renderer, gradientData, scale, fillMode, targetColor);
+    }
     renderer.drawBitmap(bitmap, x, y, pageWidth, pageHeight, cropX, cropY);
     renderer.copyGrayscaleLsbBuffers();
 
     bitmap.rewindToData();
     renderer.clearScreen(0x00);
     renderer.setRenderMode(GfxRenderer::GRAYSCALE_MSB);
+    if (gradientData.edgeA) {
+      drawLetterboxFill(renderer, gradientData, scale, fillMode, targetColor);
+    }
     renderer.drawBitmap(bitmap, x, y, pageWidth, pageHeight, cropX, cropY);
     renderer.copyGrayscaleMsbBuffers();
 
     renderer.displayGrayBuffer();
     renderer.setRenderMode(GfxRenderer::BW);
   }
+
+  gradientData.free();
 }
 
 void SleepActivity::renderCoverSleepScreen() const {
@@ -261,12 +665,18 @@ void SleepActivity::renderCoverSleepScreen() const {
     return (this->*renderNoCoverSleepScreen)();
   }
 
+  // Derive edge cache path from cover BMP path (e.g. cover.bmp -> cover_edges.bin)
+  std::string edgeCachePath;
+  if (coverBmpPath.size() > 4) {
+    edgeCachePath = coverBmpPath.substr(0, coverBmpPath.size() - 4) + "_edges.bin";
+  }
+
   FsFile file;
   if (Storage.openFileForRead("SLP", coverBmpPath, file)) {
     Bitmap bitmap(file);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
       Serial.printf("[SLP] Rendering sleep cover: %s\n", coverBmpPath.c_str());
-      renderBitmapSleepScreen(bitmap);
+      renderBitmapSleepScreen(bitmap, edgeCachePath);
       return;
     }
   }

src/activities/boot_sleep/SleepActivity.h

@@ -1,4 +1,6 @@
 #pragma once
+#include <string>
+
 #include "../Activity.h"
 
 class Bitmap;
@@ -13,6 +15,6 @@ class SleepActivity final : public Activity {
   void renderDefaultSleepScreen() const;
   void renderCustomSleepScreen() const;
   void renderCoverSleepScreen() const;
-  void renderBitmapSleepScreen(const Bitmap& bitmap) const;
+  void renderBitmapSleepScreen(const Bitmap& bitmap, const std::string& edgeCachePath = "") const;
   void renderBlankSleepScreen() const;
 };