Add 16 bit png support

src/ImageSharp/Common/Helpers/ImageMaths.cs

@@ -38,7 +38,7 @@ public static int FastAbs(int x)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int GetBitsNeededForColorDepth(int colors)
         {
-            return (int)Math.Ceiling(Math.Log(colors, 2));
+            return Math.Max(1, (int)Math.Ceiling(Math.Log(colors, 2)));
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/IPngEncoderOptions.cs

@@ -11,14 +11,20 @@ namespace SixLabors.ImageSharp.Formats.Png
     internal interface IPngEncoderOptions
     {
         /// <summary>
-        /// Gets the png color type
+        /// Gets the number of bits per sample or per palette index (not per pixel).
+        /// Not all values are allowed for all <see cref="ColorType"/> values.
         /// </summary>
-        PngColorType PngColorType { get; }
+        PngBitDepth BitDepth { get; }
 
         /// <summary>
-        /// Gets the png filter method.
+        /// Gets the color type
         /// </summary>
-        PngFilterMethod PngFilterMethod { get; }
+        PngColorType ColorType { get; }
+
+        /// <summary>
+        /// Gets the filter method.
+        /// </summary>
+        PngFilterMethod FilterMethod { get; }
 
         /// <summary>
         /// Gets the compression level 1-9.

src/ImageSharp/Formats/Png/PngBitDepth.cs

@@ -0,0 +1,22 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+// Note the value assignment, This will allow us to add 1, 2, and 4 bit encoding when we support it.
+namespace SixLabors.ImageSharp.Formats.Png
+{
+    /// <summary>
+    /// Provides enumeration for the available PNG bit depths.
+    /// </summary>
+    public enum PngBitDepth
+    {
+        /// <summary>
+        /// 8 bits per sample or per palette index (not per pixel).
+        /// </summary>
+        Bit8 = 8,
+
+        /// <summary>
+        /// 16 bits per sample or per palette index (not per pixel).
+        /// </summary>
+        Bit16 = 16
+    }
+}

src/ImageSharp/Formats/Png/PngConfigurationModule.cs

@@ -9,11 +9,11 @@ namespace SixLabors.ImageSharp.Formats.Png
     public sealed class PngConfigurationModule : IConfigurationModule
     {
         /// <inheritdoc/>
-        public void Configure(Configuration config)
+        public void Configure(Configuration configuration)
         {
-            config.ImageFormatsManager.SetEncoder(ImageFormats.Png, new PngEncoder());
-            config.ImageFormatsManager.SetDecoder(ImageFormats.Png, new PngDecoder());
-            config.ImageFormatsManager.AddImageFormatDetector(new PngImageFormatDetector());
+            configuration.ImageFormatsManager.SetEncoder(ImageFormats.Png, new PngEncoder());
+            configuration.ImageFormatsManager.SetDecoder(ImageFormats.Png, new PngDecoder());
+            configuration.ImageFormatsManager.AddImageFormatDetector(new PngImageFormatDetector());
         }
     }
 }