Add blending functionlity.

Former-commit-id: 35f80976d4212395f217df999689b6be437746d4
Former-commit-id: 450afda3ccbfa96b2fd788569aee363711819971
Former-commit-id: 448dcb8c150762d54d053f1598b5d0a231acced6

src/ImageProcessor/Filters/Blend.cs

@@ -0,0 +1,77 @@
+// <copyright file="Blend.cs" company="James South">
+// Copyright (c) James South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageProcessor.Filters
+{
+    using System.Threading.Tasks;
+
+    /// <summary>
+    /// Combines two images together by blending the pixels.
+    /// </summary>
+    public class Blend : ParallelImageProcessor
+    {
+        /// <summary>
+        /// The image to blend.
+        /// </summary>
+        private readonly ImageBase toBlend;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Blend"/> class.
+        /// </summary>
+        /// <param name="image">The image to blend.</param>
+        /// <param name="alpha">The opacity of the image to blend. Between 0 and 100.</param>
+        public Blend(ImageBase image, int alpha = 100)
+        {
+            Guard.MustBeBetweenOrEqualTo(alpha, 0, 100, nameof(alpha));
+            this.toBlend = image;
+            this.Value = alpha;
+        }
+
+        /// <summary>
+        /// Gets the alpha percentage value.
+        /// </summary>
+        public int Value { get; }
+
+        /// <inheritdoc/>
+        protected override void Apply(ImageBase target, ImageBase source, Rectangle targetRectangle, Rectangle sourceRectangle, int startY, int endY)
+        {
+            int sourceY = sourceRectangle.Y;
+            int sourceBottom = sourceRectangle.Bottom;
+            int startX = sourceRectangle.X;
+            int endX = sourceRectangle.Right;
+            Rectangle bounds = this.toBlend.Bounds;
+            float alpha = this.Value / 100f;
+
+            Parallel.For(
+                startY,
+                endY,
+                y =>
+                {
+                    if (y >= sourceY && y < sourceBottom)
+                    {
+                        for (int x = startX; x < endX; x++)
+                        {
+                            Color color = source[x, y];
+
+                            if (bounds.Contains(x, y))
+                            {
+                                Color blendedColor = this.toBlend[x, y];
+
+                                // Combining colors is dependent on the alpha of the blended color
+                                float alphaFactor = alpha > 0 ? alpha : blendedColor.A;
+                                float invertedAlphaFactor = 1 - alphaFactor;
+
+                                color.R = (color.R * invertedAlphaFactor) + (blendedColor.R * alphaFactor);
+                                color.G = (color.G * invertedAlphaFactor) + (blendedColor.G * alphaFactor);
+                                color.B = (color.B * invertedAlphaFactor) + (blendedColor.B * alphaFactor);
+                            }
+
+                            target[x, y] = color;
+                        }
+                    }
+                });
+        }
+    }
+}

src/ImageProcessor/Filters/ImageFilterExtensions.cs

@@ -35,6 +35,18 @@ namespace ImageProcessor.Filters
             return source.Process(rectangle, new Alpha(percent));
         }
 
+        /// <summary>
+        /// Combines the given image together with the current one by blending their pixels.
+        /// </summary>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="image">The image to blend with the currently processing image.</param>
+        /// <param name="percent">The opacity of the image image to blend. Must be between 0 and 100.</param>
+        /// <returns>The <see cref="Image"/>.</returns>
+        public static Image Blend(this Image source, ImageBase image, int percent = 100)
+        {
+            return source.Process(source.Bounds, new Blend(image, percent));
+        }
+
         /// <summary>
         /// Alters the contrast component of the image.
         /// </summary>

src/ImageProcessor/ImageProcessor.csproj

@@ -48,9 +48,10 @@
     <Compile Include="Common\Extensions\EnumerableExtensions.cs" />
     <Compile Include="Common\Helpers\ImageMaths.cs" />
     <Compile Include="Common\Helpers\PixelOperations.cs" />
+    <Compile Include="Filters\Blend.cs" />
     <Compile Include="Filters\ColorMatrix\ColorMatrixFilter.cs" />
     <Compile Include="Filters\ColorMatrix\Kodachrome.cs" />
-    <Compile Include="Filters\Saturation.cs" />
+    <Compile Include="Filters\ColorMatrix\Saturation.cs" />
     <Compile Include="Filters\ColorMatrix\GreyscaleMode.cs" />
     <Compile Include="Filters\Brightness.cs" />
     <Compile Include="Filters\Invert.cs" />

src/ImageProcessor/Numerics/Rectangle.cs

@@ -140,6 +140,21 @@ namespace ImageProcessor
             return !left.Equals(right);
         }
 
+        /// <summary>
+        /// Determines if the specfied point is contained within the rectangular region defined by 
+        /// this <see cref="Rectangle"/>.
+        /// </summary>
+        /// <param name="x">The x-coordinate of the given point.</param>
+        /// <param name="y">The y-coordinate of the given point.</param>
+        /// <returns>The <see cref="bool"/></returns>
+        public bool Contains(int x, int y)
+        {
+            return this.X <= x
+                   && x < this.X + this.Width
+                   && this.Y <= y
+                   && y < this.Y + this.Height;
+        }
+
         /// <summary>
         /// Indicates whether this instance and a specified object are equal.
         /// </summary>

tests/ImageProcessor.Tests/Processors/Filters/FilterTests.cs

@@ -16,8 +16,9 @@ namespace ImageProcessor.Tests
             //{ "Brightness--50", new Brightness(-50) },
             //{ "Contrast-50", new Contrast(50) },
             //{ "Contrast--50", new Contrast(-50) },
-            { "Saturation-50", new Saturation(50) },
-            { "Saturation--50", new Saturation(-50) },
+            { "Blend", new Blend(new Image(File.OpenRead("../../TestImages/Formats/Bmp/Car.bmp")),50)},
+            //{ "Saturation-50", new Saturation(50) },
+            //{ "Saturation--50", new Saturation(-50) },
             //{ "Alpha--50", new Alpha(50) },
             //{ "Invert", new Invert() },
             //{ "Sepia", new Sepia() },