Add optimised implementation for 8-bit RGB images

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/Rgb888TiffColor.cs

@@ -0,0 +1,50 @@
+// <copyright file="Rgb888TiffColor.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Formats.Tiff
+{
+    using System;
+    using System.Numerics;
+    using System.Runtime.CompilerServices;
+    using ImageSharp;
+    using ImageSharp.PixelFormats;
+
+    /// <summary>
+    /// Implements the 'RGB' photometric interpretation (optimised for 8-bit full color images).
+    /// </summary>
+    internal static class Rgb888TiffColor
+    {
+        /// <summary>
+        /// Decodes pixel data using the current photometric interpretation.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="data">The buffer to read image data from.</param>
+        /// <param name="pixels">The image buffer to write pixels to.</param>
+        /// <param name="left">The x-coordinate of the left-hand side of the image block.</param>
+        /// <param name="top">The y-coordinate of the  top of the image block.</param>
+        /// <param name="width">The width of the image block.</param>
+        /// <param name="height">The height of the image block.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static void Decode<TPixel>(byte[] data, PixelAccessor<TPixel> pixels, int left, int top, int width, int height)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            TPixel color = default(TPixel);
+
+            uint offset = 0;
+
+            for (int y = top; y < top + height; y++)
+            {
+                for (int x = left; x < left + width; x++)
+                {
+                    byte r = data[offset++];
+                    byte g = data[offset++];
+                    byte b = data[offset++];
+                    color.PackFromBytes(r, g, b, 255);
+                    pixels[x, y] = color;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/TiffColorType.cs

@@ -59,5 +59,10 @@ namespace ImageSharp.Formats.Tiff
         /// RGB Full Color.
         /// </summary>
         Rgb,
+
+        /// <summary>
+        /// RGB Full Color. Optimised implementation for 8-bit images.
+        /// </summary>
+        Rgb888
     }
 }

src/ImageSharp/Formats/Tiff/TiffDecoderCore.cs

@@ -395,7 +395,7 @@ namespace ImageSharp.Formats
                         {
                             if (this.BitsPerSample[0] == 8 && this.BitsPerSample[1] == 8 && this.BitsPerSample[2] == 8)
                             {
-                                this.ColorType = TiffColorType.Rgb;
+                                this.ColorType = TiffColorType.Rgb888;
                             }
                             else
                             {
@@ -529,6 +529,9 @@ namespace ImageSharp.Formats
                 case TiffColorType.Rgb:
                     RgbTiffColor.Decode(data, this.BitsPerSample, pixels, left, top, width, height);
                     break;
+                case TiffColorType.Rgb888:
+                    Rgb888TiffColor.Decode(data, pixels, left, top, width, height);
+                    break;
                 case TiffColorType.PaletteColor:
                     PaletteTiffColor.Decode(data, this.BitsPerSample, this.ColorMap, pixels, left, top, width, height);
                     break;

tests/ImageSharp.Formats.Tiff.Tests/Formats/Tiff/PhotometricInterpretation/RgbTiffColorTests.cs

@@ -147,5 +147,15 @@ namespace ImageSharp.Tests
                     RgbTiffColor.Decode(inputData, bitsPerSample, pixels, left, top, width, height);
                 });
         }
+
+        [Theory]
+        [MemberData(nameof(Rgb8_Data))]
+        public void Decode_WritesPixelData_8Bit(byte[] inputData, uint[] bitsPerSample, int left, int top, int width, int height, Rgba32[][] expectedResult)
+        {
+            AssertDecode(expectedResult, pixels =>
+                {
+                    Rgb888TiffColor.Decode(inputData, pixels, left, top, width, height);
+                });
+        }
     }
 }

tests/ImageSharp.Formats.Tiff.Tests/Formats/Tiff/TiffDecoderImageTests.cs

@@ -202,8 +202,8 @@ namespace ImageSharp.Tests
         [InlineData(true, TiffPhotometricInterpretation.PaletteColor, new[] { 1 }, TiffColorType.PaletteColor)]
         [InlineData(false, TiffPhotometricInterpretation.Rgb, new[] { 4, 4, 4 }, TiffColorType.Rgb)]
         [InlineData(true, TiffPhotometricInterpretation.Rgb, new[] { 4, 4, 4 }, TiffColorType.Rgb)]
-        [InlineData(false, TiffPhotometricInterpretation.Rgb, new[] { 8, 8, 8 }, TiffColorType.Rgb)]
-        [InlineData(true, TiffPhotometricInterpretation.Rgb, new[] { 8, 8, 8 }, TiffColorType.Rgb)]
+        [InlineData(false, TiffPhotometricInterpretation.Rgb, new[] { 8, 8, 8 }, TiffColorType.Rgb888)]
+        [InlineData(true, TiffPhotometricInterpretation.Rgb, new[] { 8, 8, 8 }, TiffColorType.Rgb888)]
         public void ReadImageFormat_DeterminesCorrectColorImplementation(bool isLittleEndian, ushort photometricInterpretation, int[] bitsPerSample, int colorType)
         {
             Stream stream = CreateTiffGenIfd()