From abe1263b17e2c347d0d436d1176fe2d89defb0e2 Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Tue, 20 Oct 2020 18:42:30 +0100
Subject: [PATCH] Auto repair Png options to use Bit8. Fixes #935

---
 .../Formats/Png/PngEncoderOptionsHelpers.cs   | 22 +++++++++----------
 .../Formats/Png/PngEncoderTests.cs            | 11 ++++++++++
 tests/ImageSharp.Tests/TestImages.cs          |  3 +++
 tests/Images/Input/Png/issues/Issue_935.png   |  3 +++
 4 files changed, 27 insertions(+), 12 deletions(-)
 create mode 100644 tests/Images/Input/Png/issues/Issue_935.png

diff --git a/src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs b/src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs
index b0311f088..9342e09df 100644
--- a/src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs
+++ b/src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs
@@ -35,6 +35,15 @@ namespace SixLabors.ImageSharp.Formats.Png
             options.ColorType ??= pngMetadata.ColorType ?? SuggestColorType<TPixel>();
             options.BitDepth ??= pngMetadata.BitDepth ?? SuggestBitDepth<TPixel>();
 
+            // Ensure bit depth and color type are a supported combination.
+            // Bit8 is the only bit depth supported by all color types.
+            byte bits = (byte)options.BitDepth;
+            byte[] validBitDepths = PngConstants.ColorTypes[options.ColorType.Value];
+            if (Array.IndexOf(validBitDepths, bits) == -1)
+            {
+                options.BitDepth = PngBitDepth.Bit8;
+            }
+
             options.InterlaceMethod ??= pngMetadata.InterlaceMethod;
 
             use16Bit = options.BitDepth == PngBitDepth.Bit16;
@@ -44,12 +53,6 @@ namespace SixLabors.ImageSharp.Formats.Png
             {
                 options.ChunkFilter = PngChunkFilter.ExcludeAll;
             }
-
-            // Ensure we are not allowing impossible combinations.
-            if (!PngConstants.ColorTypes.ContainsKey(options.ColorType.Value))
-            {
-                throw new NotSupportedException("Color type is not supported or not valid.");
-            }
         }
 
         /// <summary>
@@ -68,15 +71,10 @@ namespace SixLabors.ImageSharp.Formats.Png
                 return null;
             }
 
-            byte bits = (byte)options.BitDepth;
-            if (Array.IndexOf(PngConstants.ColorTypes[options.ColorType.Value], bits) == -1)
-            {
-                throw new NotSupportedException("Bit depth is not supported or not valid.");
-            }
-
             // Use the metadata to determine what quantization depth to use if no quantizer has been set.
             if (options.Quantizer is null)
             {
+                byte bits = (byte)options.BitDepth;
                 var maxColors = ImageMaths.GetColorCountForBitDepth(bits);
                 options.Quantizer = new WuQuantizer(new QuantizerOptions { MaxColors = maxColors });
             }
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
index 9ba956d72..465bed8a1 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
@@ -556,6 +556,17 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
                 provider);
         }
 
+        [Fact]
+        public void EncodeFixesInvalidOptions()
+        {
+            // https://github.com/SixLabors/ImageSharp/issues/935
+            using var ms = new MemoryStream();
+            var testFile = TestFile.Create(TestImages.Png.Issue935);
+            using Image<Rgba32> image = testFile.CreateRgba32Image(new PngDecoder());
+
+            image.Save(ms, new PngEncoder { ColorType = PngColorType.RgbWithAlpha });
+        }
+
         private static void TestPngEncoderCore<TPixel>(
             TestImageProvider<TPixel> provider,
             PngColorType pngColorType,
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index fd5296c37..dce36bb0f 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -107,6 +107,9 @@ namespace SixLabors.ImageSharp.Tests
             public const string Issue1177_1 = "Png/issues/Issue_1177_1.png";
             public const string Issue1177_2 = "Png/issues/Issue_1177_2.png";
 
+            // Issue 935: https://github.com/SixLabors/ImageSharp/issues/935
+            public const string Issue935 = "Png/issues/Issue_935.png";
+
             public static class Bad
             {
                 public const string MissingDataChunk = "Png/xdtn0g01.png";
diff --git a/tests/Images/Input/Png/issues/Issue_935.png b/tests/Images/Input/Png/issues/Issue_935.png
new file mode 100644
index 000000000..9f9e84dc3
--- /dev/null
+++ b/tests/Images/Input/Png/issues/Issue_935.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:6a9c5cdacc9bedf481c883828de5bfb7902e2bec038fff08830171cf7075e4f9
+size 870