separate PreMultiply from Convonution for rows

src/ImageSharp/Common/Helpers/ImageMaths.cs

@@ -298,5 +298,24 @@ namespace SixLabors.ImageSharp
                 v *= s;
             }
         }
+
+        /// <summary>
+        /// Revers <see cref="Premultiply"/>
+        /// </summary>
+        /// <seealso cref="Vector4Extensions.UnPremultiply"/>
+        /// <param name="vectors">The span of vectors</param>
+        public static void UnPremultiply(Span<Vector4> vectors)
+        {
+            // TODO: This method can be AVX2 optimized using Vector<float>
+            ref Vector4 baseRef = ref MemoryMarshal.GetReference(vectors);
+
+            for (int i = 0; i < vectors.Length; i++)
+            {
+                ref Vector4 v = ref Unsafe.Add(ref baseRef, i);
+                var s = new Vector4(1 / v.W);
+                s.W = 1;
+                v *= s;
+            }
+        }
     }
 }

src/ImageSharp/Processing/Processors/Transforms/ResizeKernel.cs

@@ -78,7 +78,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
         /// <param name="sourceX">The source row position.</param>
         /// <returns>The weighted sum</returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public Vector4 ConvolvePremultipliedRows(Span<Vector4> rowSpan, int sourceX)
+        public Vector4 ConvolveRows(Span<Vector4> rowSpan, int sourceX)
         {
             ref float horizontalValues = ref this.GetStartReference();
             int left = this.Left;
@@ -91,7 +91,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
             {
                 float weight = Unsafe.Add(ref horizontalValues, i);
                 Vector4 v = Unsafe.Add(ref vecPtr, i);
-                result += v.Premultiply() * weight;
+                result += v * weight;
             }
 
             return result;
@@ -105,7 +105,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
         /// <param name="sourceX">The source row position.</param>
         /// <returns>The weighted sum</returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public Vector4 ConvolvePremultipliedExpandedRows(Span<Vector4> rowSpan, int sourceX)
+        public Vector4 ConvolveExpandRows(Span<Vector4> rowSpan, int sourceX)
         {
             ref float horizontalValues = ref this.GetStartReference();
             int left = this.Left;
@@ -118,10 +118,10 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
             {
                 float weight = Unsafe.Add(ref horizontalValues, i);
                 Vector4 v = Unsafe.Add(ref vecPtr, i);
-                result += v.Premultiply().Expand() * weight;
+                result += v.Expand() * weight;
             }
 
-            return result.UnPremultiply();
+            return result;
         }
 
         /// <summary>

src/ImageSharp/Processing/Processors/Transforms/ResizeProcessor.cs

@@ -142,7 +142,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
         /// </summary>
         public bool Compand { get; }
 
-
         /// <inheritdoc/>
         protected override Image<TPixel> CreateDestination(Image<TPixel> source, Rectangle sourceRectangle)
         {
@@ -254,6 +253,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
                                 Span<Vector4> tempRowSpan = tempRowBuffer.Span;
 
                                 PixelOperations<TPixel>.Instance.ToVector4(sourceRow, tempRowSpan, sourceRow.Length);
+                                ImageMaths.Premultiply(tempRowSpan);
 
                                 if (this.Compand)
                                 {
@@ -261,7 +261,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
                                     {
                                         ResizeKernel window = this.horizontalKernelMap.Kernels[x - startX];
                                         Unsafe.Add(ref firstPassRow, x) =
-                                            window.ConvolvePremultipliedExpandedRows(tempRowSpan, sourceX);
+                                            window.ConvolveExpandRows(tempRowSpan, sourceX).UnPremultiply();
                                     }
                                 }
                                 else
@@ -270,7 +270,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
                                     {
                                         ResizeKernel window = this.horizontalKernelMap.Kernels[x - startX];
                                         Unsafe.Add(ref firstPassRow, x) =
-                                            window.ConvolvePremultipliedRows(tempRowSpan, sourceX);
+                                            window.ConvolveRows(tempRowSpan, sourceX);
                                     }
                                 }
                             }

tests/ImageSharp.Tests/Helpers/ImageMathsTests.cs

@@ -54,6 +54,21 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             Assert.Equal(expected, source, new ApproximateFloatComparer(1e-6f));
         }
 
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(30)]
+        public void UnPremultiply_VectorSpan(int length)
+        {
+            var rnd = new Random(42);
+            Vector4[] source = rnd.GenerateRandomVectorArray(length, 0, 1);
+            Vector4[] expected = source.Select(v => v.UnPremultiply()).ToArray();
+
+            ImageMaths.UnPremultiply(source);
+
+            Assert.Equal(expected, source, new ApproximateFloatComparer(1e-6f));
+        }
+
         // TODO: We need to test all ImageMaths methods!
     }
 }