diff --git a/src/dec/alpha.c b/src/dec/alpha.c
index 47c70395..4a6d25e5 100644
--- a/src/dec/alpha.c
+++ b/src/dec/alpha.c
@@ -23,12 +23,14 @@
 //------------------------------------------------------------------------------
 // ALPHDecoder object.
 
-ALPHDecoder* ALPHNew(void) {
+// Allocates a new alpha decoder instance.
+static ALPHDecoder* ALPHNew(void) {
   ALPHDecoder* const dec = (ALPHDecoder*)WebPSafeCalloc(1ULL, sizeof(*dec));
   return dec;
 }
 
-void ALPHDelete(ALPHDecoder* const dec) {
+// Clears and deallocates an alpha decoder instance.
+static void ALPHDelete(ALPHDecoder* const dec) {
   if (dec != NULL) {
     VP8LDelete(dec->vp8l_dec_);
     dec->vp8l_dec_ = NULL;
@@ -44,17 +46,19 @@ void ALPHDelete(ALPHDecoder* const dec) {
 // Returns false in case of error in alpha header (data too short, invalid
 // compression method or filter, error in lossless header data etc).
 static int ALPHInit(ALPHDecoder* const dec, const uint8_t* data,
-                    size_t data_size, int width, int height, uint8_t* output) {
+                    size_t data_size, const VP8Io* const src_io,
+                    uint8_t* output) {
   int ok = 0;
   const uint8_t* const alpha_data = data + ALPHA_HEADER_LEN;
   const size_t alpha_data_size = data_size - ALPHA_HEADER_LEN;
   int rsrv;
+  VP8Io* const io = &dec->io_;
 
-  assert(width > 0 && height > 0);
-  assert(data != NULL && output != NULL);
+  assert(data != NULL && output != NULL && src_io != NULL);
 
-  dec->width_ = width;
-  dec->height_ = height;
+  dec->width_ = src_io->width;
+  dec->height_ = src_io->height;
+  assert(dec->width_ > 0 && dec->height_ > 0);
 
   if (data_size <= ALPHA_HEADER_LEN) {
     return 0;
@@ -80,6 +84,21 @@ static int ALPHInit(ALPHDecoder* const dec, const uint8_t* data,
     ok = VP8LDecodeAlphaHeader(dec, alpha_data, alpha_data_size, output);
   }
   VP8FiltersInit();
+
+  // Copy the necessary parameters from src_io to io
+  VP8InitIo(io);
+  WebPInitCustomIo(NULL, io);
+  io->opaque = output;      // output plane
+  io->width = src_io->width;
+  io->height = src_io->height;
+
+  io->use_cropping = src_io->use_cropping;
+  io->crop_left = src_io->crop_left;
+  io->crop_right = src_io->crop_right;
+  io->crop_top = src_io->crop_top;
+  io->crop_bottom = src_io->crop_bottom;
+  // No need to copy the scaling parameters.
+
   return ok;
 }
 
@@ -116,6 +135,27 @@ static int ALPHDecode(VP8Decoder* const dec, int row, int num_rows) {
   return 1;
 }
 
+static int AllocateAlphaPlane(VP8Decoder* const dec, const VP8Io* const io) {
+  const int stride = io->width;
+  const int height = io->height;
+  const uint64_t alpha_size = (uint64_t)stride * height;
+  assert(dec->alpha_plane_mem_ == NULL);
+  dec->alpha_plane_mem_ =
+      (uint8_t*)WebPSafeMalloc(alpha_size, sizeof(*dec->alpha_plane_));
+  if (dec->alpha_plane_mem_ == NULL) return 0;
+  dec->alpha_plane_ = dec->alpha_plane_mem_;
+  return 1;
+}
+
+void WebPDeallocateAlphaMemory(VP8Decoder* const dec) {
+  assert(dec != NULL);
+  WebPSafeFree(dec->alpha_plane_mem_);
+  dec->alpha_plane_mem_ = NULL;
+  dec->alpha_plane_ = NULL;
+  ALPHDelete(dec->alph_dec_);
+  dec->alph_dec_ = NULL;
+}
+
 //------------------------------------------------------------------------------
 // Main entry point.
 
@@ -132,41 +172,42 @@ const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
   }
 
   if (!dec->is_alpha_decoded_) {
-    int ok;
     if (dec->alph_dec_ == NULL) {    // Initialize decoder.
-      assert(dec->alpha_plane_ != NULL);
       dec->alph_dec_ = ALPHNew();
       if (dec->alph_dec_ == NULL) return NULL;
+      if (!AllocateAlphaPlane(dec, io)) goto Error;
       if (!ALPHInit(dec->alph_dec_, dec->alpha_data_, dec->alpha_data_size_,
-                    io->width, io->height, dec->alpha_plane_)) {
-        ALPHDelete(dec->alph_dec_);
-        dec->alph_dec_ = NULL;
-        return NULL;
+                    io, dec->alpha_plane_)) {
+        goto Error;
       }
       // if we allowed use of alpha dithering, check whether it's needed at all
       if (dec->alph_dec_->pre_processing_ != ALPHA_PREPROCESSED_LEVELS) {
-        dec->alpha_dithering_ = 0;  // disable dithering
+        dec->alpha_dithering_ = 0;       // disable dithering
       } else {
-        num_rows = height;          // decode everything in one pass
+        num_rows = io->height - row;     // decode everything in one pass
       }
     }
 
-    if (row + num_rows > height) {
-      num_rows = height - row;
-    }
-
-    ok = ALPHDecode(dec, row, num_rows);
     assert(dec->alph_dec_ != NULL);
-    if (!ok || dec->is_alpha_decoded_) {
+    assert(row + num_rows <= io->height);
+    if (!ALPHDecode(dec, row, num_rows)) goto Error;
+
+    if (dec->is_alpha_decoded_) {   // finished?
       ALPHDelete(dec->alph_dec_);
       dec->alph_dec_ = NULL;
+      if (dec->alpha_dithering_ > 0) {
+        if (!WebPDequantizeLevels(dec->alpha_plane_, width, height, width,
+                                  dec->alpha_dithering_)) {
+          goto Error;
+        }
+      }
     }
-    if (ok && dec->is_alpha_decoded_ && dec->alpha_dithering_ > 0) {
-      ok = WebPDequantizeLevels(dec->alpha_plane_, width, height, width,
-                                dec->alpha_dithering_);
-    }
-    if (!ok) return NULL;  // Error.
   }
+
   // Return a pointer to the current decoded row.
   return dec->alpha_plane_ + row * width;
+
+ Error:
+  WebPDeallocateAlphaMemory(dec);
+  return NULL;
 }
diff --git a/src/dec/alphai.h b/src/dec/alphai.h
index b4170229..ea4ce2f4 100644
--- a/src/dec/alphai.h
+++ b/src/dec/alphai.h
@@ -40,11 +40,8 @@ struct ALPHDecoder {
 //------------------------------------------------------------------------------
 // internal functions. Not public.
 
-// Allocates a new alpha decoder instance.
-ALPHDecoder* ALPHNew(void);
-
-// Clears and deallocates an alpha decoder instance.
-void ALPHDelete(ALPHDecoder* const dec);
+// Deallocate memory associated to dec->alpha_plane_ decoding
+void WebPDeallocateAlphaMemory(VP8Decoder* const dec);
 
 //------------------------------------------------------------------------------
 
diff --git a/src/dec/vp8.c b/src/dec/vp8.c
index 214b8273..955c784d 100644
--- a/src/dec/vp8.c
+++ b/src/dec/vp8.c
@@ -656,8 +656,7 @@ void VP8Clear(VP8Decoder* const dec) {
     return;
   }
   WebPGetWorkerInterface()->End(&dec->worker_);
-  ALPHDelete(dec->alph_dec_);
-  dec->alph_dec_ = NULL;
+  WebPDeallocateAlphaMemory(dec);
   WebPSafeFree(dec->mem_);
   dec->mem_ = NULL;
   dec->mem_size_ = 0;
@@ -666,4 +665,3 @@ void VP8Clear(VP8Decoder* const dec) {
 }
 
 //------------------------------------------------------------------------------
-
diff --git a/src/dec/vp8i.h b/src/dec/vp8i.h
index 6b9afd75..ef14f4ec 100644
--- a/src/dec/vp8i.h
+++ b/src/dec/vp8i.h
@@ -258,9 +258,10 @@ struct VP8Decoder {
   struct ALPHDecoder* alph_dec_;  // alpha-plane decoder object
   const uint8_t* alpha_data_;     // compressed alpha data (if present)
   size_t alpha_data_size_;
-  int is_alpha_decoded_;  // true if alpha_data_ is decoded in alpha_plane_
-  uint8_t* alpha_plane_;  // output. Persistent, contains the whole data.
-  int alpha_dithering_;   // derived from decoding options (0=off, 100=full).
+  int is_alpha_decoded_;      // true if alpha_data_ is decoded in alpha_plane_
+  uint8_t* alpha_plane_mem_;  // memory allocated for alpha_plane_
+  uint8_t* alpha_plane_;      // output. Persistent, contains the whole data.
+  int alpha_dithering_;       // derived from decoding options (0=off, 100=full)
 };
 
 //------------------------------------------------------------------------------