Skip some safety readonly struct copies

src/ImageSharp/Advanced/IRowIntervalAction.cs

@@ -95,7 +95,8 @@ namespace SixLabors.ImageSharp.Advanced
             int yMax = Math.Min(yMin + this.info.StepY, this.info.MaxY);
             var rows = new RowInterval(yMin, yMax);
 
-            this.action.Invoke(in rows);
+            // Skip the safety copy when invoking a potentially impure method on a readonly field
+            Unsafe.AsRef(this.action).Invoke(in rows);
         }
     }
 }

src/ImageSharp/Advanced/IRowIntervalAction{TBuffer}.cs

@@ -92,7 +92,8 @@ namespace SixLabors.ImageSharp.Advanced
             var rows = new RowInterval(yMin, yMax);
 
             using IMemoryOwner<TBuffer> buffer = this.allocator.Allocate<TBuffer>(this.info.MaxX);
-            this.action.Invoke(in rows, buffer.Memory);
+
+            Unsafe.AsRef(this.action).Invoke(in rows, buffer.Memory);
         }
     }
 }

src/ImageSharp/Advanced/ParallelRowIterator.cs

@@ -60,7 +60,7 @@ namespace SixLabors.ImageSharp.Advanced
             if (numOfSteps == 1)
             {
                 var rows = new RowInterval(top, bottom);
-                body.Invoke(in rows);
+                Unsafe.AsRef(body).Invoke(in rows);
                 return;
             }
 
@@ -121,7 +121,7 @@ namespace SixLabors.ImageSharp.Advanced
                 var rows = new RowInterval(top, bottom);
                 using (IMemoryOwner<TBuffer> buffer = allocator.Allocate<TBuffer>(width))
                 {
-                    body.Invoke(rows, buffer.Memory);
+                    Unsafe.AsRef(body).Invoke(rows, buffer.Memory);
                 }
 
                 return;

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -554,8 +554,14 @@ namespace SixLabors.ImageSharp.Formats.Png
                 return;
             }
 
-            // Grab the palette and write it to the stream.
+            /* Grab the palette and write it to the stream.
-            ReadOnlySpan<TPixel> palette = quantized.Palette.Span;
+             * Here the palette is reinterpreted as a mutable Memory<TPixel> value,
+             * which is possible because the two memory types have the same layout.
+             * This is done so that the Span<TPixel> we're working on is mutable,
+             * so that we can skip the safety copies done by the compiler when we
+             * invoke the IPixel.ToRgba32 method below, which is not marked as readonly. */
+            ReadOnlyMemory<TPixel> paletteMemory = quantized.Palette;
+            Span<TPixel> palette = Unsafe.As<ReadOnlyMemory<TPixel>, Memory<TPixel>>(ref paletteMemory).Span;
             int paletteLength = Math.Min(palette.Length, 256);
             int colorTableLength = paletteLength * 3;
             bool anyAlpha = false;

src/ImageSharp/PixelFormats/PixelImplementations/Rgba64.cs

@@ -218,7 +218,7 @@ namespace SixLabors.ImageSharp.PixelFormats
 
         /// <inheritdoc />
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Vector4 ToVector4() => new Vector4(this.R, this.G, this.B, this.A) / Max;
+        public readonly Vector4 ToVector4() => new Vector4(this.R, this.G, this.B, this.A) / Max;
 
         /// <inheritdoc />
         [MethodImpl(InliningOptions.ShortMethod)]
@@ -343,7 +343,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// </summary>
         /// <returns>The <see cref="Rgba32"/>.</returns>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Rgba32 ToRgba32()
+        public readonly Rgba32 ToRgba32()
         {
             byte r = ImageMaths.DownScaleFrom16BitTo8Bit(this.R);
             byte g = ImageMaths.DownScaleFrom16BitTo8Bit(this.G);
@@ -357,7 +357,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// </summary>
         /// <returns>The <see cref="Bgra32"/>.</returns>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Bgra32 ToBgra32()
+        public readonly Bgra32 ToBgra32()
         {
             byte r = ImageMaths.DownScaleFrom16BitTo8Bit(this.R);
             byte g = ImageMaths.DownScaleFrom16BitTo8Bit(this.G);
@@ -371,7 +371,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// </summary>
         /// <returns>The <see cref="Argb32"/>.</returns>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Argb32 ToArgb32()
+        public readonly Argb32 ToArgb32()
         {
             byte r = ImageMaths.DownScaleFrom16BitTo8Bit(this.R);
             byte g = ImageMaths.DownScaleFrom16BitTo8Bit(this.G);
@@ -385,7 +385,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// </summary>
         /// <returns>The <see cref="Rgb24"/>.</returns>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Rgb24 ToRgb24()
+        public readonly Rgb24 ToRgb24()
         {
             byte r = ImageMaths.DownScaleFrom16BitTo8Bit(this.R);
             byte g = ImageMaths.DownScaleFrom16BitTo8Bit(this.G);
@@ -398,7 +398,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// </summary>
         /// <returns>The <see cref="Bgr24"/>.</returns>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public Bgr24 ToBgr24()
+        public readonly Bgr24 ToBgr24()
         {
             byte r = ImageMaths.DownScaleFrom16BitTo8Bit(this.R);
             byte g = ImageMaths.DownScaleFrom16BitTo8Bit(this.G);

src/ImageSharp/PixelFormats/Utils/Vector4Converters.RgbaCompatible.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -66,7 +66,8 @@ namespace SixLabors.ImageSharp.PixelFormats.Utils
                     MemoryMarshal.Cast<Rgba32, byte>(lastQuarterOfDestBuffer),
                     MemoryMarshal.Cast<Vector4, float>(destVectors.Slice(0, countWithoutLastItem)));
 
-                destVectors[countWithoutLastItem] = sourcePixels[countWithoutLastItem].ToVector4();
+                // Reinterpret as a mutable reference to skip the safety copy of the readonly value
+                destVectors[countWithoutLastItem] = Unsafe.AsRef(sourcePixels[countWithoutLastItem]).ToVector4();
 
                 // TODO: Investigate optimized 1-pass approach!
                 ApplyForwardConversionModifiers(destVectors, modifiers);
@@ -126,4 +127,4 @@ namespace SixLabors.ImageSharp.PixelFormats.Utils
             }
         }
     }
-}
+}

src/ImageSharp/Processing/Processors/Effects/PixelRowDelegateProcessor{TPixel,TDelegate}.cs

@@ -95,7 +95,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Effects
                     PixelOperations<TPixel>.Instance.ToVector4(this.configuration, rowSpan, vectorSpan, this.modifiers);
 
                     // Run the user defined pixel shader to the current row of pixels
-                    this.rowProcessor.Invoke(vectorSpan, new Point(this.startX, y));
+                    Unsafe.AsRef(this.rowProcessor).Invoke(vectorSpan, new Point(this.startX, y));
 
                     PixelOperations<TPixel>.Instance.FromVector4Destructive(this.configuration, vectorSpan, rowSpan, this.modifiers);
                 }

src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeKernelMap.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -81,7 +81,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
         /// </summary>
         public void Dispose()
         {
-            this.pinHandle.Dispose();
+            Unsafe.AsRef(this.pinHandle).Dispose();
             this.data.Dispose();
         }
 
@@ -248,4 +248,4 @@ namespace SixLabors.ImageSharp.Processing.Processors.Transforms
             return new ResizeKernel(left, rowPtr, length);
         }
     }
-}
+}