Add ARGBExtractAlpha function

source/planar_functions.cc

Index: source/planar_functions.cc
diff --git a/source/planar_functions.cc b/source/planar_functions.cc
index 73fa7d284af31478d1f4667e8c02b572f16ce6fe..b1b1f2c830b4ec931eedcdc8036c9774416c6b1c 100644
--- a/source/planar_functions.cc
+++ b/source/planar_functions.cc
@@ -2374,6 +2374,49 @@ int ARGBCopyAlpha(const uint8* src_argb, int src_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,