Alpha decoding: significantly reduce memory usage

Simply get rid of an intermediate buffer of size width x height, by
using the fact that stride == width in this case.

Change-Id: I92376a2561a3beb6e723e8bcf7340c7f348e02c2
(cherry picked from commit edccd19436)

src/dec/alpha.c

@@ -20,20 +20,10 @@
 extern "C" {
 #endif
 
-// TODO(skal): move to dsp/ ?
-static void CopyPlane(const uint8_t* src, int src_stride,
-                      uint8_t* dst, int dst_stride, int width, int height) {
-  while (height-- > 0) {
-    memcpy(dst, src, width);
-    src += src_stride;
-    dst += dst_stride;
-  }
-}
-
 //------------------------------------------------------------------------------
 // Decodes the compressed data 'data' of size 'data_size' into the 'output'.
 // The 'output' buffer should be pre-allocated and must be of the same
-// dimension 'height'x'stride', as that of the image.
+// dimension 'height'x'width', as that of the image.
 //
 // Returns 1 on successfully decoding the compressed alpha and
 //         0 if either:
@@ -41,16 +31,16 @@ static void CopyPlane(const uint8_t* src, int src_stride,
 //           error returned by appropriate compression method.
 
 static int DecodeAlpha(const uint8_t* data, size_t data_size,
-                       int width, int height, int stride, uint8_t* output) {
-  uint8_t* decoded_data = NULL;
-  const size_t decoded_size = height * width;
+                       int width, int height, uint8_t* output) {
   WEBP_FILTER_TYPE filter;
   int pre_processing;
   int rsrv;
   int ok = 0;
   int method;
+  const uint8_t* const alpha_data = data + ALPHA_HEADER_LEN;
+  const size_t alpha_data_size = data_size - ALPHA_HEADER_LEN;
 
-  assert(width > 0 && height > 0 && stride >= width);
+  assert(width > 0 && height > 0);
   assert(data != NULL && output != NULL);
 
   if (data_size <= ALPHA_HEADER_LEN) {
@@ -70,15 +60,12 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
   }
 
   if (method == ALPHA_NO_COMPRESSION) {
-    ok = (data_size >= decoded_size);
-    decoded_data = (uint8_t*)data + ALPHA_HEADER_LEN;
+    const size_t alpha_decoded_size = height * width;
+    ok = (alpha_data_size >= alpha_decoded_size);
+    if (ok) memcpy(output, alpha_data, alpha_decoded_size);
   } else {
-    decoded_data = (uint8_t*)malloc(decoded_size);
-    if (decoded_data == NULL) return 0;
-    ok = VP8LDecodeAlphaImageStream(width, height,
-                                    data + ALPHA_HEADER_LEN,
-                                    data_size - ALPHA_HEADER_LEN,
-                                    decoded_data);
+    ok = VP8LDecodeAlphaImageStream(width, height, alpha_data, alpha_data_size,
+                                    output);
   }
 
   if (ok) {
@@ -86,18 +73,13 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
     if (unfilter_func != NULL) {
       // TODO(vikas): Implement on-the-fly decoding & filter mechanism to decode
       // and apply filter per image-row.
-      unfilter_func(width, height, width, decoded_data);
+      unfilter_func(width, height, width, output);
     }
-    // Construct raw_data (height x stride) from alpha data (height x width).
-    CopyPlane(decoded_data, width, output, stride, width, height);
     if (pre_processing == ALPHA_PREPROCESSED_LEVELS) {
-      ok = DequantizeLevels(decoded_data, width, height);
+      ok = DequantizeLevels(output, width, height);
     }
   }
 
-  if (method != ALPHA_NO_COMPRESSION) {
-    free(decoded_data);
-  }
   return ok;
 }
 
@@ -105,9 +87,10 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
 
 const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
                                       int row, int num_rows) {
-  const int stride = dec->pic_hdr_.width_;
+  const int width = dec->pic_hdr_.width_;
+  const int height = dec->pic_hdr_.height_;
 
-  if (row < 0 || num_rows < 0 || row + num_rows > dec->pic_hdr_.height_) {
+  if (row < 0 || num_rows < 0 || row + num_rows > height) {
     return NULL;    // sanity check.
   }
 
@@ -115,15 +98,14 @@ const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
     // Decode everything during the first call.
     assert(!dec->is_alpha_decoded_);
     if (!DecodeAlpha(dec->alpha_data_, (size_t)dec->alpha_data_size_,
-                     dec->pic_hdr_.width_, dec->pic_hdr_.height_, stride,
-                     dec->alpha_plane_)) {
+                     width, height, dec->alpha_plane_)) {
       return NULL;  // Error.
     }
     dec->is_alpha_decoded_ = 1;
   }
 
   // Return a pointer to the current decoded row.
-  return dec->alpha_plane_ + row * stride;
+  return dec->alpha_plane_ + row * width;
 }
 
 #if defined(__cplusplus) || defined(c_plusplus)