fix: Replace full-image cache buffer with streaming band buffer to reduce memory usage (#2230)

lib/Epub/Epub/blocks/ImageBlock.cpp

@@ -1,5 +1,6 @@
 #include "ImageBlock.h"
 
+#include <FontCacheManager.h>
 #include <GfxRenderer.h>
 #include <Logging.h>
 #include <Serialization.h>
@@ -55,10 +56,22 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
 
   LOG_DBG("IMG", "Loading from cache: %s (%dx%d)", cachePath.c_str(), cachedWidth, cachedHeight);
 
-  // Read and render row by row to minimize memory usage
+  // Read several rows per SD access. A full-page image is re-rendered on every
+  // grayscale strip pass (~14x per page), and a one-row-per-read loop here means
+  // cachedHeight (~728) tiny reads through the storage mutex + SdFat each time —
+  // the dominant cost of displaying an image page. Batching rows into a ~4KB
+  // buffer cuts that to ~20 reads per pass without holding the whole image.
   const int bytesPerRow = (cachedWidth + 3) / 4;  // 2 bits per pixel, 4 pixels per byte
-  uint8_t* rowBuffer = (uint8_t*)malloc(bytesPerRow);
-  if (!rowBuffer) {
+  int rowsPerRead = 4096 / bytesPerRow;
+  if (rowsPerRead < 1) rowsPerRead = 1;
+  if (rowsPerRead > cachedHeight) rowsPerRead = cachedHeight;
+  uint8_t* readBuffer = (uint8_t*)malloc((size_t)rowsPerRead * bytesPerRow);
+  if (!readBuffer) {
+    // Fall back to a single-row buffer under memory pressure.
+    rowsPerRead = 1;
+    readBuffer = (uint8_t*)malloc(bytesPerRow);
+  }
+  if (!readBuffer) {
     LOG_ERR("IMG", "Failed to allocate row buffer");
     return false;
   }
@@ -66,16 +79,31 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
   DirectPixelWriter pw;
   pw.init(renderer);
 
+  int rowsInBuffer = 0;
+  int bufferRow = 0;
   for (int row = 0; row < cachedHeight; row++) {
-    if (cacheFile.read(rowBuffer, bytesPerRow) != bytesPerRow) {
-      LOG_ERR("IMG", "Cache read error at row %d", row);
-      free(rowBuffer);
-      return false;
+    if (bufferRow >= rowsInBuffer) {
+      const int toRead = (cachedHeight - row < rowsPerRead) ? (cachedHeight - row) : rowsPerRead;
+      const size_t bytes = (size_t)toRead * bytesPerRow;
+      if (cacheFile.read(readBuffer, bytes) != static_cast<int>(bytes)) {
+        LOG_ERR("IMG", "Cache read error at row %d", row);
+        free(readBuffer);
+        return false;
+      }
+      rowsInBuffer = toRead;
+      bufferRow = 0;
     }
 
+    const uint8_t* rowBuffer = readBuffer + (size_t)bufferRow * bytesPerRow;
+    bufferRow++;
+
     const int destY = y + row;
     pw.beginRow(destY);
-    for (int col = 0; col < cachedWidth; col++) {
+    // On a grayscale strip pass only a narrow column window of the image is in
+    // the active band; skip the rest instead of unpacking+clipping every pixel.
+    int colStart, colEnd;
+    pw.bandColRange(x, cachedWidth, colStart, colEnd);
+    for (int col = colStart; col < colEnd; col++) {
       const int byteIdx = col >> 2;            // col / 4
       const int bitShift = 6 - (col & 3) * 2;  // MSB first within byte
       uint8_t pixelValue = (rowBuffer[byteIdx] >> bitShift) & 0x03;
@@ -84,7 +112,7 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
     }
   }
 
-  free(rowBuffer);
+  free(readBuffer);
   LOG_DBG("IMG", "Cache render complete");
   return true;
 }
@@ -92,6 +120,14 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
 }  // namespace
 
 void ImageBlock::render(GfxRenderer& renderer, const int x, const int y) {
+  // The font-prewarm scan pass only accumulates glyphs; an image contributes
+  // none, and its DirectPixelWriter output bypasses the renderer's scan-mode
+  // suppression, so it would otherwise do a full (discarded) cache render every
+  // page view. Skip it here. The image still draws in the real BW/grayscale
+  // passes; on first view this just moves the one-time decode to the BW pass.
+  FontCacheManager* fcm = renderer.getFontCacheManager();
+  if (fcm && fcm->isScanning()) return;
+
   LOG_DBG("IMG", "Rendering image at %d,%d: %s (%dx%d)", x, y, imagePath.c_str(), width, height);
 
   const int screenWidth = renderer.getScreenWidth();