Add ARGBExtractAlpha function

source/planar_functions.cc

 /*
  *  Copyright 2011 The LibYuv Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS. All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 2356 matching lines @@
 #endif
 
   for (y = 0; y < height; ++y) {
     ARGBCopyAlphaRow(src_argb, dst_argb, width);
     src_argb += src_stride_argb;
     dst_argb += dst_stride_argb;
   }
   return 0;
 }
 
+// Extract just the alpha channel from ARGB.
+LIBYUV_API
+int ARGBExtractAlpha(const uint8* src_argb, int src_stride,
+                     uint8* dst_a, int dst_stride,
+                     int width, int height) {
+  if (!src_argb || !dst_a || width <= 0 || height == 0) {
+    return -1;
+  }
+  // Negative height means invert the image.
+  if (height < 0) {
+    height = -height;
+    src_argb += (height - 1) * src_stride;
+    src_stride = -src_stride;
+  }
+  // Coalesce rows.
+  if (src_stride == width * 4 && dst_stride == width) {
+    width *= height;
+    height = 1;
+    src_stride = dst_stride = 0;
+  }
+  void (*ARGBExtractAlphaRow)(const uint8 *src_argb, uint8 *dst_a, int width) =
+      ARGBExtractAlphaRow_C;
+#if defined(HAS_ARGBEXTRACTALPHAROW_SSE2)
+  if (TestCpuFlag(kCpuHasSSE2)) {
+    ARGBExtractAlphaRow = IS_ALIGNED(width, 8) ? ARGBExtractAlphaRow_SSE2
+                                               : ARGBExtractAlphaRow_Any_SSE2;
+  }
+#endif
+#if defined(HAS_ARGBEXTRACTALPHAROW_NEON)
+  if (TestCpuFlag(kCpuHasNEON)) {
+    ARGBExtractAlphaRow = IS_ALIGNED(width, 8) ? ARGBExtractAlphaRow_NEON
+                                               : ARGBExtractAlphaRow_Any_NEON;
+  }
+#endif
+
+  for (int y = 0; y < height; ++y) {
+    ARGBExtractAlphaRow(src_argb, dst_a, width);
+    src_argb += src_stride;
+    dst_a += dst_stride;
+  }
+  return 0;
+}
+
 // Copy a planar Y channel to the alpha channel of a destination ARGB image.
 LIBYUV_API
 int ARGBCopyYToAlpha(const uint8* src_y, int src_stride_y,
                      uint8* dst_argb, int dst_stride_argb,
                      int width, int height) {
   int y;
   void (*ARGBCopyYToAlphaRow)(const uint8* src_y, uint8* dst_argb, int width) =
       ARGBCopyYToAlphaRow_C;
   if (!src_y || !dst_argb || width <= 0 || height == 0) {
     return -1;
@@ skipping 232 matching lines @@
     }
     free_aligned_buffer_64(rows);
   }
   return 0;
 }
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif