Add CMYK support for image mask array

Author: BobLd

UglyToad.PdfPig.Rendering.Skia/Helpers/SkiaImageExtensions.cs

@@ -182,7 +182,9 @@ private static bool TryGenerate(this IPdfImage pdfImage, out SKImage skImage)
                             throw new ArgumentException($"The size of the transformed mask array does not match number of components of 4: got {range.Length} but expected 8.");
                         }
 
-                        throw new NotImplementedException("Mask CMYK");
+                        // TODO - Add tests
+                        CmykToRgb(in range[0], in range[1], in range[2], in range[3], out rMin, out gMin, out bMin);
+                        CmykToRgb(in range[4], in range[5], in range[6], in range[7], out rMax, out gMax, out bMax);
                     }
                     else if (numberOfComponents == 3)
                     {
@@ -219,20 +221,8 @@ private static bool TryGenerate(this IPdfImage pdfImage, out SKImage skImage)
                     {
                         for (int col = 0; col < width; ++col)
                         {
-                            /*
-                             * Where CMYK in 0..1
-                             * R = 255 × (1-C) × (1-K)
-                             * G = 255 × (1-M) × (1-K)
-                             * B = 255 × (1-Y) × (1-K)
-                             */
-
-                            double c = (bytesPure[i++] / 255d);
-                            double m = (bytesPure[i++] / 255d);
-                            double y = (bytesPure[i++] / 255d);
-                            double k = (bytesPure[i++] / 255d);
-                            var r = (byte)(255 * (1 - c) * (1 - k));
-                            var g = (byte)(255 * (1 - m) * (1 - k));
-                            var b = (byte)(255 * (1 - y) * (1 - k));
+                            CmykToRgb(in bytesPure[i++], in bytesPure[i++], in bytesPure[i++], in bytesPure[i++],
+                                out byte r, out byte g, out byte b);
 
                             var start = (row * (width * bytesPerPixel)) + (col * bytesPerPixel);
                             raster[start++] = r;
@@ -283,6 +273,24 @@ private static bool TryGenerate(this IPdfImage pdfImage, out SKImage skImage)
             return false;
         }
 
+        private static void CmykToRgb(in byte c, in byte m, in byte y, in byte k, out byte r, out byte g, out byte b)
+        {
+            /*
+             * Where CMYK in 0..1
+             * R = 255 × (1-C) × (1-K)
+             * G = 255 × (1-M) × (1-K)
+             * B = 255 × (1-Y) × (1-K)
+             */
+
+            double cD = c / 255d;
+            double mD = m / 255d;
+            double yD = y / 255d;
+            double kD = k / 255d;
+            r = (byte)(255 * (1 - cD) * (1 - kD));
+            g = (byte)(255 * (1 - mD) * (1 - kD));
+            b = (byte)(255 * (1 - yD) * (1 - kD));
+        }
+
         private static bool TryGetGray8Bitmap(int width, int height, ReadOnlySpan<byte> bytesPure, out SKImage? bitmap)
         {
             bitmap = null;