Add support for bt601 and bt709 grayscale methods

Author: synercoder

src/Synercoding.FileFormats.Pdf/GrayScaleMethod.cs

@@ -24,5 +24,13 @@ public enum GrayScaleMethod
     /// <summary>
     /// Use the average of the Red, Green and Blue channels.
     /// </summary>
-    AverageOfRGBChannels
+    AverageOfRGBChannels,
+    /// <summary>
+    /// The constants defined by ITU-R BT.601 are 0.299 red + 0.587 green + 0.114 blue.
+    /// </summary>
+    BT601,
+    /// <summary>
+    /// The constants defined by ITU-R BT.709 are 0.2126 red + 0.7152 green + 0.0722 blue.
+    /// </summary>
+    BT709,
 }

src/Synercoding.FileFormats.Pdf/Image.cs

@@ -118,6 +118,8 @@ internal static Stream AsImageByteStream(Image<Rgba32> image, GrayScaleMethod gr
                             GrayScaleMethod.GreenChannel => pixel.G,
                             GrayScaleMethod.BlueChannel => pixel.B,
                             GrayScaleMethod.AverageOfRGBChannels => (byte)( ( pixel.R + pixel.G + pixel.B ) / 3 ),
+                            GrayScaleMethod.BT601 => (byte)( ( pixel.R * 0.299 ) + ( pixel.G * 0.587 ) + ( pixel.B * 0.114 ) ),
+                            GrayScaleMethod.BT709 => (byte)( ( pixel.R * 0.2126 ) + ( pixel.G * 0.7152 ) + ( pixel.B * 0.0722 ) ),
                             _ => throw new NotImplementedException()
                         };