Enhance Blend to use point and Size Fix #14

Settings.StyleCop

@@ -35,6 +35,7 @@
       <Value>Paeth</Value>
       <Value>th</Value>
       <Value>desensitivity</Value>
+      <Value>premultiplied</Value>
     </CollectionProperty>
   </GlobalSettings>
   <Analyzers>

src/ImageSharp/Colors/Vector4BlendTransforms.cs

@@ -185,6 +185,28 @@ namespace ImageSharp
             return new Vector4(BlendExclusion(backdrop.X, source.X), BlendExclusion(backdrop.Y, source.Y), BlendExclusion(backdrop.Z, source.Z), backdrop.W);
         }
 
+        /// <summary>
+        /// Linearly interpolates from one vector to another based on the given weighting. 
+        /// The two vectors are premultiplied by their W component before operating.
+        /// </summary>
+        /// <param name="backdrop">The backdrop vector.</param>
+        /// <param name="source">The source vector.</param>
+        /// <param name="amount">
+        /// A value between 0 and 1 indicating the weight of the second source vector.
+        /// At amount = 0, "from" is returned, at amount = 1, "to" is returned.
+        /// </param>
+        /// <returns>
+        /// The <see cref="Vector4"/>
+        /// </returns>
+        public static Vector4 PremultipliedLerp(Vector4 backdrop, Vector4 source, float amount)
+        {
+            amount = amount.Clamp(0, 1);
+            backdrop = backdrop * new Vector4(backdrop.X, backdrop.Y, backdrop.Z, 1) * backdrop.W;
+            source = source * new Vector4(source.X, source.Y, source.Z, 1) * source.W;
+
+            return Vector4.Lerp(backdrop, source, amount) / new Vector4(source.W, source.W, source.W, 1);
+        }
+
         /// <summary>
         /// Multiplies or screens the color component, depending on the component value.
         /// </summary>

src/ImageSharp/Filters/Overlays/Blend.cs

@@ -21,11 +21,11 @@ namespace ImageSharp
         /// <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{TColor, TPacked}"/>.</returns>
-        public static Image<TColor, TPacked> Blend<TColor, TPacked>(this Image<TColor, TPacked> source, ImageBase<TColor, TPacked> image, int percent = 50)
+        public static Image<TColor, TPacked> Blend<TColor, TPacked>(this Image<TColor, TPacked> source, Image<TColor, TPacked> image, int percent = 50)
             where TColor : struct, IPackedPixel<TPacked>
             where TPacked : struct
         {
-            return Blend(source, image, percent, source.Bounds);
+            return Blend(source, image, percent, default(Size), default(Point));
         }
 
         /// <summary>
@@ -36,15 +36,24 @@ namespace ImageSharp
         /// <typeparam name="TColor">The pixel format.</typeparam>
         /// <typeparam name="TPacked">The packed format. <example>uint, long, float.</example></typeparam>
         /// <param name="percent">The opacity of the image image to blend. Must be between 0 and 100.</param>
-        /// <param name="rectangle">
-        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
-        /// </param>
+        /// <param name="size">The size to draw the blended image.</param>
+        /// <param name="location">The location to draw the blended image.</param>
         /// <returns>The <see cref="Image{TColor, TPacked}"/>.</returns>
-        public static Image<TColor, TPacked> Blend<TColor, TPacked>(this Image<TColor, TPacked> source, ImageBase<TColor, TPacked> image, int percent, Rectangle rectangle)
+        public static Image<TColor, TPacked> Blend<TColor, TPacked>(this Image<TColor, TPacked> source, Image<TColor, TPacked> image, int percent, Size size, Point location)
             where TColor : struct, IPackedPixel<TPacked>
             where TPacked : struct
         {
-            return source.Process(rectangle, new BlendProcessor<TColor, TPacked>(image, percent));
+            if (size == default(Size))
+            {
+                size = new Size(image.Width, image.Height);
+            }
+
+            if (location == default(Point))
+            {
+                location = Point.Empty;
+            }
+
+            return source.Process(source.Bounds, new BlendProcessor<TColor, TPacked>(image, size, location, percent));
         }
     }
 }

src/ImageSharp/Filters/Processors/Overlays/BlendProcessor.cs

@@ -18,58 +18,60 @@ namespace ImageSharp.Processors
         where TColor : struct, IPackedPixel<TPacked>
         where TPacked : struct
     {
-        /// <summary>
-        /// The image to blend.
-        /// </summary>
-        private readonly ImageBase<TColor, TPacked> blend;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="BlendProcessor{TColor,TPacked}"/> class.
         /// </summary>
-        /// <param name="image">
-        /// The image to blend with the currently processing image.
-        /// Disposal of this image is the responsibility of the developer.
-        /// </param>
+        /// <param name="image">The image to blend with the currently processing image.</param>
+        /// <param name="size">The size to draw the blended image.</param>
+        /// <param name="location">The location to draw the blended image.</param>
         /// <param name="alpha">The opacity of the image to blend. Between 0 and 100.</param>
-        public BlendProcessor(ImageBase<TColor, TPacked> image, int alpha = 100)
+        public BlendProcessor(Image<TColor, TPacked> image, Size size, Point location, int alpha = 100)
         {
             Guard.MustBeBetweenOrEqualTo(alpha, 0, 100, nameof(alpha));
-            this.blend = image;
-            this.Value = alpha;
+            this.Image = image;
+            this.Size = size;
+            this.Alpha = alpha;
+            this.Location = location;
         }
 
+        /// <summary>
+        /// Gets the image to blend.
+        /// </summary>
+        public Image<TColor, TPacked> Image { get; private set; }
+
         /// <summary>
         /// Gets the alpha percentage value.
         /// </summary>
-        public int Value { get; }
+        public int Alpha { get; }
+
+        /// <summary>
+        /// Gets the size to draw the blended image.
+        /// </summary>
+        public Size Size { get; }
+
+        /// <summary>
+        /// Gets the location to draw the blended image.
+        /// </summary>
+        public Point Location { get; }
 
         /// <inheritdoc/>
         protected override void Apply(ImageBase<TColor, TPacked> source, Rectangle sourceRectangle, int startY, int endY)
         {
-            int startX = sourceRectangle.X;
-            int endX = sourceRectangle.Right;
-            Rectangle bounds = this.blend.Bounds;
-
-            // Align start/end positions.
-            int minX = Math.Max(0, startX);
-            int maxX = Math.Min(source.Width, endX);
-            int minY = Math.Max(0, startY);
-            int maxY = Math.Min(source.Height, endY);
-
-            // Reset offset if necessary.
-            if (minX > 0)
+            if (this.Image.Bounds.Size != this.Size)
             {
-                startX = 0;
+                this.Image = this.Image.Resize(this.Size.Width, this.Size.Height);
             }
 
-            if (minY > 0)
-            {
-                startY = 0;
-            }
+            // Align start/end positions.
+            Rectangle bounds = this.Image.Bounds;
+            int minX = Math.Max(this.Location.X, sourceRectangle.X);
+            int maxX = Math.Min(this.Location.X + bounds.Width, sourceRectangle.Width);
+            int minY = Math.Max(this.Location.Y, startY);
+            int maxY = Math.Min(this.Location.Y + bounds.Height, endY);
 
-            float alpha = this.Value / 100F;
+            float alpha = this.Alpha / 100F;
 
-            using (PixelAccessor<TColor, TPacked> toBlendPixels = this.blend.Lock())
+            using (PixelAccessor<TColor, TPacked> toBlendPixels = this.Image.Lock())
             using (PixelAccessor<TColor, TPacked> sourcePixels = source.Lock())
             {
                 Parallel.For(
@@ -78,26 +80,20 @@ namespace ImageSharp.Processors
                     this.ParallelOptions,
                     y =>
                         {
-                            int offsetY = y - startY;
                             for (int x = minX; x < maxX; x++)
                             {
-                                int offsetX = x - startX;
-                                Vector4 color = sourcePixels[offsetX, offsetY].ToVector4();
+                                Vector4 color = sourcePixels[x, y].ToVector4();
+                                Vector4 blendedColor = toBlendPixels[x - minX, y - minY].ToVector4();
 
-                                if (bounds.Contains(offsetX, offsetY))
+                                if (blendedColor.W > 0)
                                 {
-                                    Vector4 blendedColor = toBlendPixels[offsetX, offsetY].ToVector4();
-
-                                    if (blendedColor.W > 0)
-                                    {
-                                        // Lerping colors is dependent on the alpha of the blended color
-                                        color = Vector4.Lerp(color, blendedColor, alpha > 0 ? alpha : blendedColor.W);
-                                    }
+                                    // Lerping colors is dependent on the alpha of the blended color
+                                    color = Vector4BlendTransforms.PremultipliedLerp(color, blendedColor, alpha);
                                 }
 
                                 TColor packed = default(TColor);
                                 packed.PackFromVector4(color);
-                                sourcePixels[offsetX, offsetY] = packed;
+                                sourcePixels[x, y] = packed;
                             }
                         });
             }

src/ImageSharp/Numerics/Rectangle.cs

@@ -127,6 +127,11 @@ namespace ImageSharp
             }
         }
 
+        /// <summary>
+        /// Gets the size of this <see cref="Rectangle"/>.
+        /// </summary>
+        public Size Size => new Size(this.Width, this.Height);
+
         /// <summary>
         /// Gets a value indicating whether this <see cref="Rectangle"/> is empty.
         /// </summary>

tests/ImageSharp.Tests/Processors/Filters/BlendTest.cs

@@ -6,6 +6,7 @@
 namespace ImageSharp.Tests
 {
     using System.IO;
+    using System.Linq;
 
     using Xunit;
 
@@ -16,7 +17,9 @@ namespace ImageSharp.Tests
         {
             string path = CreateOutputDirectory("Blend");
 
-            Image blend;
+            Image blend;// = new Image(400, 400);
+                        // blend.BackgroundColor(Color.RebeccaPurple);
+
             using (FileStream stream = File.OpenRead(TestImages.Bmp.Car))
             {
                 blend = new Image(stream);
@@ -28,8 +31,8 @@ namespace ImageSharp.Tests
 
                 using (FileStream output = File.OpenWrite($"{path}/{file.FileName}"))
                 {
-                    image.Blend(blend)
-                          .Save(output);
+                    image.Blend(blend, 75, new Size(image.Width / 2, image.Height / 2), new Point(image.Width / 4, image.Height / 4))
+                         .Save(output);
                 }
             }
         }