introducing ParallelFor.WithTemporalBuffer(): common utility for the parallel buffer reusal trick

src/ImageSharp/Common/Helpers/ParallelFor.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Threading.Tasks;
+using SixLabors.ImageSharp.Memory;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Utility methods for Parallel.For() execution. Use this instead of raw <see cref="Parallel"/> calls!
+    /// </summary>
+    internal static class ParallelFor
+    {
+        /// <summary>
+        /// Helper method to execute Parallel.For using the settings in <see cref="Configuration.ParallelOptions"/>
+        /// </summary>
+        public static void WithConfiguration(int fromInclusive, int toExclusive, Configuration configuration, Action<int> body)
+        {
+            Parallel.For(fromInclusive, toExclusive, configuration.ParallelOptions, body);
+        }
+
+        /// <summary>
+        /// Helper method to execute Parallel.For with temporal worker buffer in an optimized way.
+        /// The buffer will be only instantiated for each worker Task, the contents are not cleaned automatically.
+        /// </summary>
+        /// <typeparam name="T">The value type of the buffer</typeparam>
+        /// <param name="fromInclusive">The start index, inclusive.</param>
+        /// <param name="toExclusive">The end index, exclusive.</param>
+        /// <param name="configuration">The <see cref="Configuration"/> used for getting the <see cref="MemoryManager"/> and <see cref="ParallelOptions"/></param>
+        /// <param name="bufferLength">The length of the requested parallel buffer</param>
+        /// <param name="body">The delegate that is invoked once per iteration.</param>
+        public static void WithTemporalBuffer<T>(
+            int fromInclusive,
+            int toExclusive,
+            Configuration configuration,
+            int bufferLength,
+            Action<int, IBuffer<T>> body)
+            where T : struct
+        {
+            MemoryManager memoryManager = configuration.MemoryManager;
+            ParallelOptions parallelOptions = configuration.ParallelOptions;
+
+            IBuffer<T> InitBuffer()
+            {
+                return memoryManager.Allocate<T>(bufferLength);
+            }
+
+            void CleanUpBuffer(IBuffer<T> buffer)
+            {
+                buffer.Dispose();
+            }
+
+            IBuffer<T> BodyFunc(int i, ParallelLoopState state, IBuffer<T> buffer)
+            {
+                body(i, buffer);
+                return buffer;
+            }
+
+            Parallel.For(fromInclusive, toExclusive, parallelOptions, InitBuffer, BodyFunc, CleanUpBuffer);
+        }
+    }
+}

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

@@ -128,12 +128,12 @@ namespace SixLabors.ImageSharp.Processing.Processors
             {
                 firstPassPixels.Buffer.Clear();
 
-                Parallel.For(
+                ParallelFor.WithTemporalBuffer(
                     0,
                     sourceRectangle.Bottom,
-                    configuration.ParallelOptions,
+                    configuration,
-                    () => this.MemoryManager.Allocate<Vector4>(source.Width),
+                    source.Width,
-                    (int y, ParallelLoopState sate, IBuffer<Vector4> tempRowBuffer) =>
+                    (int y, IBuffer<Vector4> tempRowBuffer) =>
                         {
                             Span<Vector4> firstPassRow = firstPassPixels.GetRowSpan(y);
                             Span<TPixel> sourceRow = source.GetPixelRowSpan(y);
@@ -157,10 +157,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                     firstPassRow[x] = window.ComputeWeightedRowSum(tempRowSpan, sourceX);
                                 }
                             }
-
+                        });
-                            return tempRowBuffer;
-                        },
-                    (IBuffer<Vector4> tmp) => tmp.Dispose());
 
                 // Now process the rows.
                 Parallel.For(