Reduced intermediate allocations

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -453,6 +453,7 @@ private void ReadRle24<TPixel>(BufferedReadStream stream, Buffer2D<TPixel> pixel
     /// <param name="rowsWithUndefinedPixels">Keeps track of rows, which have undefined pixels.</param>
     private void UncompressRle4(BufferedReadStream stream, int w, Span<byte> buffer, Span<bool> undefinedPixels, Span<bool> rowsWithUndefinedPixels)
     {
+        Span<byte> scratchBuffer = stackalloc byte[128];
         Span<byte> cmd = stackalloc byte[2];
         int count = 0;
 
@@ -491,9 +492,9 @@ private void UncompressRle4(BufferedReadStream stream, int w, Span<byte> buffer,
                         int max = cmd[1];
                         int bytesToRead = (int)(((uint)max + 1) / 2);
 
-                        byte[] run = new byte[bytesToRead];
+                        Span<byte> run = bytesToRead <= 128 ? scratchBuffer.Slice(0, bytesToRead) : new byte[bytesToRead];
 
-                        stream.Read(run, 0, run.Length);
+                        stream.Read(run);
 
                         int idx = 0;
                         for (int i = 0; i < max; i++)
@@ -559,6 +560,7 @@ private void UncompressRle4(BufferedReadStream stream, int w, Span<byte> buffer,
     /// <param name="rowsWithUndefinedPixels">Keeps track of rows, which have undefined pixels.</param>
     private void UncompressRle8(BufferedReadStream stream, int w, Span<byte> buffer, Span<bool> undefinedPixels, Span<bool> rowsWithUndefinedPixels)
     {
+        Span<byte> scratchBuffer = stackalloc byte[128];
         Span<byte> cmd = stackalloc byte[2];
         int count = 0;
 
@@ -596,13 +598,13 @@ private void UncompressRle8(BufferedReadStream stream, int w, Span<byte> buffer,
                         // Take this number of bytes from the stream as uncompressed data.
                         int length = cmd[1];
 
-                        byte[] run = new byte[length];
+                        Span<byte> run = length <= 128 ? scratchBuffer.Slice(0, length) : new byte[length];
 
-                        stream.Read(run, 0, run.Length);
+                        stream.Read(run);
 
-                        run.AsSpan().CopyTo(buffer[count..]);
+                        run.CopyTo(buffer[count..]);
 
-                        count += run.Length;
+                        count += length;
 
                         // Absolute mode data is aligned to two-byte word-boundary.
                         int padding = length & 1;
@@ -639,6 +641,7 @@ private void UncompressRle8(BufferedReadStream stream, int w, Span<byte> buffer,
     /// <param name="rowsWithUndefinedPixels">Keeps track of rows, which have undefined pixels.</param>
     private void UncompressRle24(BufferedReadStream stream, int w, Span<byte> buffer, Span<bool> undefinedPixels, Span<bool> rowsWithUndefinedPixels)
     {
+        Span<byte> scratchBuffer = stackalloc byte[128];
         Span<byte> cmd = stackalloc byte[2];
         int uncompressedPixels = 0;
 
@@ -675,17 +678,18 @@ private void UncompressRle24(BufferedReadStream stream, int w, Span<byte> buffer
                         // If the second byte > 2, we are in 'absolute mode'.
                         // Take this number of bytes from the stream as uncompressed data.
                         int length = cmd[1];
+                        int length3 = length * 3;
 
-                        byte[] run = new byte[length * 3];
+                        Span<byte> run = length3 <= 128 ? scratchBuffer.Slice(0, length3) : new byte[length3];
 
-                        stream.Read(run, 0, run.Length);
+                        stream.Read(run);
 
-                        run.AsSpan().CopyTo(buffer[(uncompressedPixels * 3)..]);
+                        run.CopyTo(buffer[(uncompressedPixels * 3)..]);
 
                         uncompressedPixels += length;
 
                         // Absolute mode data is aligned to two-byte word-boundary.
-                        int padding = run.Length & 1;
+                        int padding = length3 & 1;
 
                         stream.Skip(padding);
 
@@ -1286,18 +1290,18 @@ private void ReadInfoHeader(BufferedReadStream stream)
             // color masks for each color channel follow the info header.
             if (this.infoHeader.Compression == BmpCompression.BitFields)
             {
-                byte[] bitfieldsBuffer = new byte[12];
-                stream.Read(bitfieldsBuffer, 0, 12);
-                Span<byte> data = bitfieldsBuffer.AsSpan();
+                Span<byte> bitfieldsBuffer = stackalloc byte[12];
+                stream.Read(bitfieldsBuffer);
+                Span<byte> data = bitfieldsBuffer;
                 this.infoHeader.RedMask = BinaryPrimitives.ReadInt32LittleEndian(data[..4]);
                 this.infoHeader.GreenMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4));
                 this.infoHeader.BlueMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4));
             }
             else if (this.infoHeader.Compression == BmpCompression.BI_ALPHABITFIELDS)
             {
-                byte[] bitfieldsBuffer = new byte[16];
-                stream.Read(bitfieldsBuffer, 0, 16);
-                Span<byte> data = bitfieldsBuffer.AsSpan();
+                Span<byte> bitfieldsBuffer = stackalloc byte[16];
+                stream.Read(bitfieldsBuffer);
+                Span<byte> data = bitfieldsBuffer;
                 this.infoHeader.RedMask = BinaryPrimitives.ReadInt32LittleEndian(data[..4]);
                 this.infoHeader.GreenMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4));
                 this.infoHeader.BlueMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4));
@@ -1470,7 +1474,7 @@ private int ReadImageHeaders(BufferedReadStream stream, out bool inverted, out b
         {
             // Usually the color palette is 1024 byte (256 colors * 4), but the documentation does not mention a size limit.
             // Make sure, that we will not read pass the bitmap offset (starting position of image data).
-            if ((stream.Position + colorMapSizeBytes) > this.fileHeader.Offset)
+            if (stream.Position > this.fileHeader.Offset - colorMapSizeBytes)
             {
                 BmpThrowHelper.ThrowInvalidImageContentException(
                     $"Reading the color map would read beyond the bitmap offset. Either the color map size of '{colorMapSizeBytes}' is invalid or the bitmap offset.");

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -22,7 +22,7 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
     /// <summary>
     /// The temp buffer used to reduce allocations.
     /// </summary>
-    private readonly byte[] buffer = new byte[16];
+    private ScratchBuffer buffer;   // mutable struct, don't make readonly
 
     /// <summary>
     /// The global color table.
@@ -249,13 +249,13 @@ public ImageInfo Identify(BufferedReadStream stream, CancellationToken cancellat
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadGraphicalControlExtension(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 6);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 6);
         if (bytesRead != 6)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the graphic control extension");
         }
 
-        this.graphicsControlExtension = GifGraphicControlExtension.Parse(this.buffer);
+        this.graphicsControlExtension = GifGraphicControlExtension.Parse(this.buffer.Span);
     }
 
     /// <summary>
@@ -264,13 +264,13 @@ private void ReadGraphicalControlExtension(BufferedReadStream stream)
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadImageDescriptor(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 9);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 9);
         if (bytesRead != 9)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the image descriptor");
         }
 
-        this.imageDescriptor = GifImageDescriptor.Parse(this.buffer);
+        this.imageDescriptor = GifImageDescriptor.Parse(this.buffer.Span);
         if (this.imageDescriptor.Height == 0 || this.imageDescriptor.Width == 0)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Width or height should not be 0");
@@ -283,13 +283,13 @@ private void ReadImageDescriptor(BufferedReadStream stream)
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadLogicalScreenDescriptor(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 7);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 7);
         if (bytesRead != 7)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the logical screen descriptor");
         }
 
-        this.logicalScreenDescriptor = GifLogicalScreenDescriptor.Parse(this.buffer);
+        this.logicalScreenDescriptor = GifLogicalScreenDescriptor.Parse(this.buffer.Span);
     }
 
     /// <summary>
@@ -306,8 +306,8 @@ private void ReadApplicationExtension(BufferedReadStream stream)
         long position = stream.Position;
         if (appLength == GifConstants.ApplicationBlockSize)
         {
-            stream.Read(this.buffer, 0, GifConstants.ApplicationBlockSize);
-            bool isXmp = this.buffer.AsSpan().StartsWith(GifConstants.XmpApplicationIdentificationBytes);
+            stream.Read(this.buffer.Span, 0, GifConstants.ApplicationBlockSize);
+            bool isXmp = this.buffer.Span.StartsWith(GifConstants.XmpApplicationIdentificationBytes);
             if (isXmp && !this.skipMetadata)
             {
                 GifXmpApplicationExtension extension = GifXmpApplicationExtension.Read(stream, this.memoryAllocator);
@@ -331,8 +331,8 @@ private void ReadApplicationExtension(BufferedReadStream stream)
             // http://www.vurdalakov.net/misc/gif/netscape-buffering-application-extension
             if (subBlockSize == GifConstants.NetscapeLoopingSubBlockSize)
             {
-                stream.Read(this.buffer, 0, GifConstants.NetscapeLoopingSubBlockSize);
-                this.gifMetadata!.RepeatCount = GifNetscapeLoopingApplicationExtension.Parse(this.buffer.AsSpan(1)).RepeatCount;
+                stream.Read(this.buffer.Span, 0, GifConstants.NetscapeLoopingSubBlockSize);
+                this.gifMetadata!.RepeatCount = GifNetscapeLoopingApplicationExtension.Parse(this.buffer.Span.Slice(1)).RepeatCount;
                 stream.Skip(1); // Skip the terminator.
                 return;
             }
@@ -762,4 +762,12 @@ private void ReadLogicalScreenDescriptorAndGlobalColorTable(BufferedReadStream s
             }
         }
     }
+
+    private unsafe struct ScratchBuffer
+    {
+        private const int Size = 16;
+        private fixed byte scratch[Size];
+
+        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], Size);
+    }
 }

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -28,11 +28,6 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
     /// </summary>
     private readonly Configuration configuration;
 
-    /// <summary>
-    /// A reusable buffer used to reduce allocations.
-    /// </summary>
-    private readonly byte[] buffer = new byte[20];
-
     /// <summary>
     /// Whether to skip metadata during encode.
     /// </summary>
@@ -324,9 +319,10 @@ private void WriteLogicalScreenDescriptor(
             backgroundColorIndex: unchecked((byte)transparencyIndex),
             ratio);
 
-        descriptor.WriteTo(this.buffer);
+        Span<byte> buffer = stackalloc byte[20];
+        descriptor.WriteTo(buffer);
 
-        stream.Write(this.buffer, 0, GifLogicalScreenDescriptor.Size);
+        stream.Write(buffer, 0, GifLogicalScreenDescriptor.Size);
     }
 
     /// <summary>
@@ -365,12 +361,14 @@ private void WriteComments(GifMetadata metadata, Stream stream)
             return;
         }
 
+        Span<byte> buffer = stackalloc byte[2];
+
         for (int i = 0; i < metadata.Comments.Count; i++)
         {
             string comment = metadata.Comments[i];
-            this.buffer[0] = GifConstants.ExtensionIntroducer;
-            this.buffer[1] = GifConstants.CommentLabel;
-            stream.Write(this.buffer, 0, 2);
+            buffer[1] = GifConstants.CommentLabel;
+            buffer[0] = GifConstants.ExtensionIntroducer;
+            stream.Write(buffer);
 
             // Comment will be stored in chunks of 255 bytes, if it exceeds this size.
             ReadOnlySpan<char> commentSpan = comment.AsSpan();
@@ -437,22 +435,23 @@ private void WriteGraphicalControlExtension(GifFrameMetadata metadata, int trans
     private void WriteExtension<TGifExtension>(TGifExtension extension, Stream stream)
         where TGifExtension : struct, IGifExtension
     {
-        IMemoryOwner<byte>? owner = null;
-        Span<byte> extensionBuffer;
         int extensionSize = extension.ContentLength;
 
         if (extensionSize == 0)
         {
             return;
         }
-        else if (extensionSize > this.buffer.Length - 3)
+
+        IMemoryOwner<byte>? owner = null;
+        Span<byte> extensionBuffer = stackalloc byte[0];    // workaround compiler limitation
+        if (extensionSize > 128)
         {
             owner = this.memoryAllocator.Allocate<byte>(extensionSize + 3);
             extensionBuffer = owner.GetSpan();
         }
         else
         {
-            extensionBuffer = this.buffer;
+            extensionBuffer = stackalloc byte[extensionSize + 3];
         }
 
         extensionBuffer[0] = GifConstants.ExtensionIntroducer;
@@ -489,9 +488,10 @@ private void WriteImageDescriptor<TPixel>(ImageFrame<TPixel> image, bool hasColo
             height: (ushort)image.Height,
             packed: packedValue);
 
-        descriptor.WriteTo(this.buffer);
+        Span<byte> buffer = stackalloc byte[20];
+        descriptor.WriteTo(buffer);
 
-        stream.Write(this.buffer, 0, GifImageDescriptor.Size);
+        stream.Write(buffer, 0, GifImageDescriptor.Size);
     }
 
     /// <summary>

src/ImageSharp/Formats/Jpeg/Components/Decoder/AdobeMarker.cs

@@ -62,7 +62,7 @@ private AdobeMarker(short dctEncodeVersion, short app14Flags0, short app14Flags1
     /// </summary>
     /// <param name="bytes">The byte array containing metadata to parse.</param>
     /// <param name="marker">The marker to return.</param>
-    public static bool TryParse(byte[] bytes, out AdobeMarker marker)
+    public static bool TryParse(ReadOnlySpan<byte> bytes, out AdobeMarker marker)
     {
         if (ProfileResolver.IsProfile(bytes, ProfileResolver.AdobeMarker))
         {

src/ImageSharp/Formats/Jpeg/Components/Decoder/ArithmeticScanDecoder.cs

@@ -53,6 +53,7 @@ internal class ArithmeticScanDecoder : IJpegScanDecoder
 
     private ArithmeticDecodingTable[] acDecodingTables;
 
+    // Don't make this a ReadOnlySpan<byte>, as the values need to get updated.
     private readonly byte[] fixedBin = { 113, 0, 0, 0 };
 
     private readonly CancellationToken cancellationToken;

src/ImageSharp/Formats/Jpeg/Components/Decoder/JFifMarker.cs

@@ -69,7 +69,7 @@ private JFifMarker(byte majorVersion, byte minorVersion, byte densityUnits, shor
     /// </summary>
     /// <param name="bytes">The byte array containing metadata to parse.</param>
     /// <param name="marker">The marker to return.</param>
-    public static bool TryParse(byte[] bytes, out JFifMarker marker)
+    public static bool TryParse(ReadOnlySpan<byte> bytes, out JFifMarker marker)
     {
         if (ProfileResolver.IsProfile(bytes, ProfileResolver.JFifMarker))
         {