CieXyz color space: add transform CieXyzToColor

Former-commit-id: e596ceb45d6865499a639cf69302a6e493a68d43
Former-commit-id: 489871ee81e15cb14b8a9abf4a2de3edbf479aa4
Former-commit-id: 3f99cab909be147d8de52b98eddb016ccdbc08ff

src/ImageProcessor/Colors/ColorTransforms.cs

@@ -67,6 +67,31 @@ namespace ImageProcessor
             return new Color(r, g, b);
         }
 
+        public static implicit operator Color(CieXyz color)
+        {
+            var x = color.X;
+            var y = color.Y;
+            var z = color.Z;
+
+            // assume sRGB
+            // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
+            float r = (x * 3.240969941904521F) + (y * -1.537383177570093F) + (z * -0.498610760293F);
+            float g = (x * -0.96924363628087F) + (y * 1.87596750150772F) + (z * 0.041555057407175F);
+            float b = (x * 0.055630079696993F) + (y * -0.20397695888897F) + (z * 1.056971514242878F);
+
+            r = r > 0.0031308 ? (float)((1.055 * Math.Pow(r, 1.0 / 2.4)) - 0.055) : r = (r * 12.92F);
+
+            g = g > 0.0031308 ? (float)((1.055 * Math.Pow(g, 1.0 / 2.4)) - 0.055) : g = (g * 12.92F);
+
+            b = b > 0.0031308 ? (float)((1.055 * Math.Pow(b, 1.0 / 2.4)) - 0.055) : b = (b * 12.92F);
+
+            r = Math.Min(Math.Max(0, r), 1);
+            g = Math.Min(Math.Max(0, g), 1);
+            b = Math.Min(Math.Max(0, b), 1);
+
+            return new Color(r, g, b);
+        }
+
         /// <summary>
         /// Allows the implicit conversion of an instance of <see cref="Hsv"/> to a
         /// <see cref="Color"/>.

tests/ImageProcessor.Tests/Colors/ColorConversionTests.cs

@@ -103,6 +103,22 @@ namespace ImageProcessor.Tests
             
         }
 
+        [Fact]
+        public void CieXyzToColor()
+        {
+            CieXyz cieXyz = new CieXyz(0.25F, 0.40F, 0.10F);
+            Color color = cieXyz;
+
+            Assert.Equal(0.4174F, color.R, 3);
+            Assert.Equal(0.7434F, color.G, 3);
+            Assert.Equal(0.2162F, color.B, 2);
+
+            //xyz2rgb([0.25 0.40 0.10])
+            //ans =     0.4174    0.7434    0.2152
+
+
+        }
+
         /// <summary>
         /// Tests the implicit conversion from <see cref="Color"/> to <see cref="Hsv"/>.
         /// </summary>