Added API for SKBitmap.ExtractAlpha

binding/Binding/SKBitmap.cs

@@ -179,6 +179,31 @@ namespace SkiaSharp
 			return SkiaApi.sk_bitmap_extract_subset (Handle, destination.Handle, ref subset);
 		}
 
+		public bool ExtractAlpha(SKBitmap destination)
+		{
+			SKPointI offset;
+			return ExtractAlpha (destination, null, out offset);
+		}
+
+		public bool ExtractAlpha(SKBitmap destination, out SKPointI offset)
+		{
+			return ExtractAlpha (destination, null, out offset);
+		}
+
+		public bool ExtractAlpha(SKBitmap destination, SKPaint paint)
+		{
+			SKPointI offset;
+			return ExtractAlpha (destination, paint, out offset);
+		}
+
+		public bool ExtractAlpha(SKBitmap destination, SKPaint paint, out SKPointI offset)
+		{
+			if (destination == null) {
+				throw new ArgumentNullException (nameof (destination));
+			}
+			return SkiaApi.sk_bitmap_extract_alpha (Handle, destination.Handle, paint == null ? IntPtr.Zero : paint.Handle, out offset);
+		}
+
 		public bool ReadyToDraw => SkiaApi.sk_bitmap_ready_to_draw (Handle); 
 
 		public SKImageInfo Info {

binding/Binding/SkiaApi.cs

@@ -1221,6 +1221,9 @@ namespace SkiaSharp
 		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
 		[return: MarshalAs(UnmanagedType.I1)]
 		public extern static bool sk_bitmap_extract_subset(sk_bitmap_t cbitmap, sk_bitmap_t cdst, ref SKRectI subset);
+		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
+		[return: MarshalAs(UnmanagedType.I1)]
+		public extern static bool sk_bitmap_extract_alpha(sk_bitmap_t cbitmap, sk_bitmap_t dst, sk_paint_t paint, out SKPointI offset);
 
 		// SkPixmap
 

externals/skia

@@ -1 +1 @@
-Subproject commit 21d14eb3d5b3389859f1648fbd9e1bac80553bbf
+Subproject commit d44a5051d62783ef9440658f987bb6a1eca59b2b

tests/Tests/SKBitmapTest.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Runtime.InteropServices;
 using Xunit;
 
@@ -39,6 +40,24 @@ namespace SkiaSharp.Tests
 			Marshal.FreeCoTaskMem(pixels);
 		}
 
+		[Fact]
+		public void TestExtractAlpha()
+		{
+			var path = Path.Combine(PathToImages, "color-wheel.png");
+			var bitmap = SKBitmap.Decode(path);
+
+			var alpha = new SKBitmap();
+
+			SKPointI offset;
+			SKPaint paint = new SKPaint {
+				MaskFilter = SKMaskFilter.CreateBlur(SKBlurStyle.Normal, 5.0f)
+			};
+
+			Assert.True(bitmap.ExtractAlpha(alpha, paint, out offset));
+
+			Assert.Equal(new SKPointI(-7, -7), offset);
+		}
+
 		[Fact]
 		public void BitmapAndPixmapAreValid()
 		{