Fix Separation color space and add tests

Author: BobLd

src/UglyToad.PdfPig.Tests/Integration/ColorSpaceTests.cs

@@ -120,7 +120,7 @@ public void SeparationColorSpaceImages()
             {
                 var page1 = document.GetPage(1);
                 var images = page1.GetImages().ToArray();
-                var image1page1 = images.ElementAt(0);
+                var image1page1 = images[0];
                 var separationCs = image1page1.ColorSpaceDetails as SeparationColorSpaceDetails;
                 Assert.NotNull(separationCs);
                 Assert.True(separationCs.AlternateColorSpace is DeviceCmykColorSpaceDetails);
@@ -135,6 +135,39 @@ public void SeparationColorSpaceImages()
             }
         }
 
+        [Fact]
+        public void SeparationColorSpace()
+        {
+            var path = IntegrationHelpers.GetDocumentPath("MOZILLA-3136-0.pdf");
+
+            using (var document = PdfDocument.Open(path))
+            {
+                var page = document.GetPage(4);
+                var images = page.GetImages().ToArray();
+
+                var image4 = images[4];
+
+                var separation = image4.ColorSpaceDetails as SeparationColorSpaceDetails;
+                Assert.NotNull(separation);
+
+                Assert.True(image4.TryGetPng(out var png4));
+
+                File.WriteAllBytes(Path.Combine(OutputFolder, "MOZILLA-3136-0_4_separation.png"), png4);
+
+                // Green dolphin image 
+                // "Colorized TIFF (should appear only in GWG Green separation)"
+                var image9 = images[9];
+
+                var indexedCs = image9.ColorSpaceDetails as IndexedColorSpaceDetails;
+                Assert.NotNull(indexedCs);
+                Assert.Equal(ColorSpace.Separation, indexedCs.BaseColorSpace.Type);
+
+                Assert.True(image9.TryGetPng(out var png9));
+
+                File.WriteAllBytes(Path.Combine(OutputFolder, "MOZILLA-3136-0_9_separation.png"), png9);
+            }
+        }
+
         [Fact]
         public void IndexedCalRgbColorSpaceImages()
         {
@@ -236,7 +269,7 @@ public void StencilIndexedIccColorSpaceImages()
                 File.WriteAllBytes(Path.Combine(OutputFolder, "MOZILLA-10225-0_341_0.png"), bytes341_0);
             }
         }
-
+        
         [Fact]
         public void SeparationLabColorSpace()
         {

src/UglyToad.PdfPig/Graphics/Colors/ColorSpaceDetails.cs

@@ -728,18 +728,19 @@ public override IColor GetColor(params double[] values)
         /// <inheritdoc/>
         internal override ReadOnlySpan<byte> Transform(ReadOnlySpan<byte> values)
         {
-            var cache = new Dictionary<int, double[]>(values.Length * 3);
-            var transformed = new List<byte>(values.Length * 3);
-            for (var i = 0; i < values.Length; i += 3)
+            var colorCache = new Dictionary<int, double[]>(values.Length);
+            var transformed = new List<byte>(values.Length);
+
+            for (var i = 0; i < values.Length; ++i)
             {
-                byte b = values[i++];
-                if (!cache.TryGetValue(b, out double[]? colors))
+                byte b = values[i];
+                if (!colorCache.TryGetValue(b, out double[]? colors))
                 {
                     colors = Process(b / 255.0);
-                    cache[b] = colors;
+                    colorCache[b] = colors;
                 }
 
-                for (int c = 0; c < colors.Length; c++)
+                for (int c = 0; c < colors.Length; ++c)
                 {
                     transformed.Add(ConvertToByte(colors[c]));
                 }