Auto repair Png options to use Bit8.

src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs

@@ -35,6 +35,15 @@ public static void AdjustOptions<TPixel>(
             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 @@ public static void AdjustOptions<TPixel>(
             {
                 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 @@ public static IndexedImageFrame<TPixel> CreateQuantizedFrame<TPixel>(
                 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 });
             }

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -556,6 +556,17 @@ static void RunTest(string serialized)
                 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,

tests/ImageSharp.Tests/TestImages.cs

@@ -107,6 +107,9 @@ public static class Png
             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";