Use Lerp silly!

Former-commit-id: eaf0f42e5396ade62ae183cebf6e5184497d4080
Former-commit-id: 549126e52b96fa9964cd784d4b9d77d49a144406
Former-commit-id: ab690f14924de2ae267ffc189dd741c6281b10bc

src/ImageProcessor/Filters/Blend.cs

@@ -59,13 +59,9 @@ namespace ImageProcessor.Filters
                             {
                                 Color blendedColor = this.toBlend[x, y];
 
-                                // Combining colors is dependent on the alpha of the blended color
+                                // Lerping 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);
+                                color = Color.Lerp(color, blendedColor, alphaFactor);
                             }
 
                             target[x, y] = color;

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

@@ -16,7 +16,7 @@ namespace ImageProcessor.Tests
             //{ "Brightness--50", new Brightness(-50) },
             //{ "Contrast-50", new Contrast(50) },
             //{ "Contrast--50", new Contrast(-50) },
-            { "Blend", new Blend(new Image(File.OpenRead("../../TestImages/Formats/Bmp/Car.bmp")),50)},
+            { "Blend", new Blend(new Image(File.OpenRead("../../TestImages/Formats/Bmp/Car.bmp")),75)},
             //{ "Saturation-50", new Saturation(50) },
             //{ "Saturation--50", new Saturation(-50) },
             //{ "Alpha--50", new Alpha(50) },