Move filtering setup to the correct place.

src/ImageSharp/Formats/Png/PngEncoderOptions.cs

@@ -18,11 +18,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         {
             this.BitDepth = source.BitDepth;
             this.ColorType = source.ColorType;
-
-            // Specification recommends default filter method None for paletted images and Paeth for others.
-            this.FilterMethod = source.FilterMethod ?? (source.ColorType == PngColorType.Palette
-                ? PngFilterMethod.None
-                : PngFilterMethod.Paeth);
+            this.FilterMethod = source.FilterMethod;
             this.CompressionLevel = source.CompressionLevel;
             this.TextCompressionThreshold = source.TextCompressionThreshold;
             this.Gamma = source.Gamma;
@@ -41,7 +37,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         public PngColorType? ColorType { get; set; }
 
         /// <inheritdoc/>
-        public PngFilterMethod? FilterMethod { get; }
+        public PngFilterMethod? FilterMethod { get; set; }
 
         /// <inheritdoc/>
         public PngCompressionLevel CompressionLevel { get; } = PngCompressionLevel.DefaultCompression;

src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs

@@ -34,6 +34,18 @@ namespace SixLabors.ImageSharp.Formats.Png
             // a sensible default based upon the pixel format.
             options.ColorType ??= pngMetadata.ColorType ?? SuggestColorType<TPixel>();
             options.BitDepth ??= pngMetadata.BitDepth ?? SuggestBitDepth<TPixel>();
+            if (!options.FilterMethod.HasValue)
+            {
+                // Specification recommends default filter method None for paletted images and Paeth for others.
+                if (options.ColorType == PngColorType.Palette)
+                {
+                    options.FilterMethod = PngFilterMethod.None;
+                }
+                else
+                {
+                    options.FilterMethod = PngFilterMethod.Paeth;
+                }
+            }
 
             // Ensure bit depth and color type are a supported combination.
             // Bit8 is the only bit depth supported by all color types.