Merge pull request #720 from carbon/png

Improve PNG CQ2

src/ImageSharp/Common/Extensions/EncoderExtensions.cs

@@ -0,0 +1,34 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+#if !NETCOREAPP2_1
+using System;
+using System.Text;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Extension methods for the <see cref="Encoder"/> type.
+    /// </summary>
+    internal static unsafe class EncoderExtensions
+    {
+        /// <summary>
+        /// Gets a string from the provided buffer data.
+        /// </summary>
+        /// <param name="encoding">The encoding.</param>
+        /// <param name="buffer">The buffer.</param>
+        /// <returns>The string.</returns>
+        public static string GetString(this Encoding encoding, ReadOnlySpan<byte> buffer)
+        {
+#if NETSTANDARD1_1
+            return encoding.GetString(buffer.ToArray());
+#else
+            fixed (byte* bytes = buffer)
+            {
+                return encoding.GetString(bytes, buffer.Length);
+            }
+#endif
+        }
+    }
+}
+#endif

src/ImageSharp/Formats/Png/Chunks/PhysicalChunkData.cs

@@ -0,0 +1,107 @@
+using System;
+using System.Buffers.Binary;
+using SixLabors.ImageSharp.Common.Helpers;
+using SixLabors.ImageSharp.MetaData;
+
+namespace SixLabors.ImageSharp.Formats.Png.Chunks
+{
+    /// <summary>
+    /// The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image.
+    /// </summary>
+    internal readonly struct PhysicalChunkData
+    {
+        public const int Size = 9;
+
+        public PhysicalChunkData(uint x, uint y, byte unitSpecifier)
+        {
+            this.XAxisPixelsPerUnit = x;
+            this.YAxisPixelsPerUnit = y;
+            this.UnitSpecifier = unitSpecifier;
+        }
+
+        /// <summary>
+        /// Gets the number of pixels per unit on the X axis.
+        /// </summary>
+        public uint XAxisPixelsPerUnit { get; }
+
+        /// <summary>
+        /// Gets the number of pixels per unit on the Y axis.
+        /// </summary>
+        public uint YAxisPixelsPerUnit { get; }
+
+        /// <summary>
+        /// Gets the unit specifier.
+        /// 0: unit is unknown
+        /// 1: unit is the meter
+        /// When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
+        /// </summary>
+        public byte UnitSpecifier { get; }
+
+        /// <summary>
+        /// Parses the PhysicalChunkData from the given buffer.
+        /// </summary>
+        /// <param name="data">The data buffer.</param>
+        /// <returns>The parsed PhysicalChunkData.</returns>
+        public static PhysicalChunkData Parse(ReadOnlySpan<byte> data)
+        {
+            uint hResolution = BinaryPrimitives.ReadUInt32BigEndian(data.Slice(0, 4));
+            uint vResolution = BinaryPrimitives.ReadUInt32BigEndian(data.Slice(4, 4));
+            byte unit = data[8];
+
+            return new PhysicalChunkData(hResolution, vResolution, unit);
+        }
+
+        /// <summary>
+        /// Constructs the PngPhysicalChunkData from the provided metadata.
+        /// If the resolution units are not in meters, they are automatically convereted.
+        /// </summary>
+        /// <param name="meta">The metadata.</param>
+        /// <returns>The constructed PngPhysicalChunkData instance.</returns>
+        public static PhysicalChunkData FromMetadata(ImageMetaData meta)
+        {
+            byte unitSpecifier = 0;
+            uint x;
+            uint y;
+
+            switch (meta.ResolutionUnits)
+            {
+                case PixelResolutionUnit.AspectRatio:
+                    unitSpecifier = 0; // Unspecified
+                    x = (uint)Math.Round(meta.HorizontalResolution);
+                    y = (uint)Math.Round(meta.VerticalResolution);
+                    break;
+
+                case PixelResolutionUnit.PixelsPerInch:
+                    unitSpecifier = 1; // Per meter
+                    x = (uint)Math.Round(UnitConverter.InchToMeter(meta.HorizontalResolution));
+                    y = (uint)Math.Round(UnitConverter.InchToMeter(meta.VerticalResolution));
+                    break;
+
+                case PixelResolutionUnit.PixelsPerCentimeter:
+                    unitSpecifier = 1; // Per meter
+                    x = (uint)Math.Round(UnitConverter.CmToMeter(meta.HorizontalResolution));
+                    y = (uint)Math.Round(UnitConverter.CmToMeter(meta.VerticalResolution));
+                    break;
+
+                default:
+                    unitSpecifier = 1; // Per meter
+                    x = (uint)Math.Round(meta.HorizontalResolution);
+                    y = (uint)Math.Round(meta.VerticalResolution);
+                    break;
+            }
+
+            return new PhysicalChunkData(x, y, unitSpecifier);
+        }
+
+        /// <summary>
+        /// Writes the data to the given buffer.
+        /// </summary>
+        /// <param name="buffer">The buffer.</param>
+        public void WriteTo(Span<byte> buffer)
+        {
+            BinaryPrimitives.WriteUInt32BigEndian(buffer.Slice(0, 4), this.XAxisPixelsPerUnit);
+            BinaryPrimitives.WriteUInt32BigEndian(buffer.Slice(4, 4), this.YAxisPixelsPerUnit);
+            buffer[8] = this.UnitSpecifier;
+        }
+    }
+}

src/ImageSharp/Formats/Png/PngConstants.cs

@@ -41,5 +41,17 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// The header bytes as a big endian coded ulong.
         /// </summary>
         public const ulong HeaderValue = 0x89504E470D0A1A0AUL;
+
+        /// <summary>
+        /// The dictionary of available color types.
+        /// </summary>
+        public static readonly Dictionary<PngColorType, byte[]> ColorTypes = new Dictionary<PngColorType, byte[]>()
+        {
+            [PngColorType.Grayscale] = new byte[] { 1, 2, 4, 8, 16 },
+            [PngColorType.Rgb] = new byte[] { 8, 16 },
+            [PngColorType.Palette] = new byte[] { 1, 2, 4, 8 },
+            [PngColorType.GrayscaleWithAlpha] = new byte[] { 8, 16 },
+            [PngColorType.RgbWithAlpha] = new byte[] { 8, 16 }
+        };
     }
 }

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -3,13 +3,12 @@
 
 using System;
 using System.Buffers.Binary;
-using System.Collections.Generic;
 using System.IO;
-using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Formats.Png.Chunks;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
 using SixLabors.ImageSharp.Memory;
@@ -26,31 +25,9 @@ namespace SixLabors.ImageSharp.Formats.Png
     internal sealed class PngDecoderCore
     {
         /// <summary>
-        /// The dictionary of available color types.
+        /// Reusable buffer.
         /// </summary>
-        private static readonly Dictionary<PngColorType, byte[]> ColorTypes = new Dictionary<PngColorType, byte[]>()
+        private readonly byte[] buffer = new byte[4];
-        {
-            [PngColorType.Grayscale] = new byte[] { 1, 2, 4, 8, 16 },
-            [PngColorType.Rgb] = new byte[] { 8, 16 },
-            [PngColorType.Palette] = new byte[] { 1, 2, 4, 8 },
-            [PngColorType.GrayscaleWithAlpha] = new byte[] { 8, 16 },
-            [PngColorType.RgbWithAlpha] = new byte[] { 8, 16 }
-        };
-
-        /// <summary>
-        /// Reusable buffer for reading chunk types.
-        /// </summary>
-        private readonly byte[] chunkTypeBuffer = new byte[4];
-
-        /// <summary>
-        /// Reusable buffer for reading chunk lengths.
-        /// </summary>
-        private readonly byte[] chunkLengthBuffer = new byte[4];
-
-        /// <summary>
-        /// Reusable buffer for reading crc values.
-        /// </summary>
-        private readonly byte[] crcBuffer = new byte[4];
 
         /// <summary>
         /// Reusable crc for validating chunks.
@@ -220,7 +197,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                         {
                             case PngChunkType.Header:
                                 this.ReadHeaderChunk(pngMetaData, chunk.Data.Array);
-                                this.ValidateHeader();
                                 break;
                             case PngChunkType.Physical:
                                 this.ReadPhysicalChunk(metaData, chunk.Data.GetSpan());
@@ -253,7 +229,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                                 this.AssignTransparentMarkers(alpha);
                                 break;
                             case PngChunkType.Text:
-                                this.ReadTextChunk(metaData, chunk.Data.Array, chunk.Length);
+                                this.ReadTextChunk(metaData, chunk.Data.Array.AsSpan(0, chunk.Length));
                                 break;
                             case PngChunkType.Exif:
                                 if (!this.ignoreMetadata)
@@ -309,7 +285,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                         {
                             case PngChunkType.Header:
                                 this.ReadHeaderChunk(pngMetaData, chunk.Data.Array);
-                                this.ValidateHeader();
                                 break;
                             case PngChunkType.Physical:
                                 this.ReadPhysicalChunk(metaData, chunk.Data.GetSpan());
@@ -321,7 +296,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                                 this.SkipChunkDataAndCrc(chunk);
                                 break;
                             case PngChunkType.Text:
-                                this.ReadTextChunk(metaData, chunk.Data.Array, chunk.Length);
+                                this.ReadTextChunk(metaData, chunk.Data.Array.AsSpan(0, chunk.Length));
                                 break;
                             case PngChunkType.End:
                                 this.isEndChunkReached = true;
@@ -402,26 +377,14 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="data">The data containing physical data.</param>
         private void ReadPhysicalChunk(ImageMetaData metadata, ReadOnlySpan<byte> data)
         {
-            // The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:
+            var physicalChunk = PhysicalChunkData.Parse(data);
-            // Pixels per unit, X axis: 4 bytes (unsigned integer)
+
-            // Pixels per unit, Y axis: 4 bytes (unsigned integer)
+            metadata.ResolutionUnits = physicalChunk.UnitSpecifier == byte.MinValue
-            // Unit specifier:          1 byte
-            //
-            // The following values are legal for the unit specifier:
-            //   0: unit is unknown
-            //   1: unit is the meter
-            //
-            // When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
-            int hResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(0, 4));
-            int vResolution = BinaryPrimitives.ReadInt32BigEndian(data.Slice(4, 4));
-            byte unit = data[8];
-
-            metadata.ResolutionUnits = unit == byte.MinValue
                 ? PixelResolutionUnit.AspectRatio
                 : PixelResolutionUnit.PixelsPerMeter;
 
-            metadata.HorizontalResolution = hResolution;
+            metadata.HorizontalResolution = physicalChunk.XAxisPixelsPerUnit;
-            metadata.VerticalResolution = vResolution;
+            metadata.VerticalResolution = physicalChunk.YAxisPixelsPerUnit;
         }
 
         /// <summary>
@@ -573,22 +536,18 @@ namespace SixLabors.ImageSharp.Formats.Png
                         break;
 
                     case FilterType.Sub:
-
                         SubFilter.Decode(scanlineSpan, this.bytesPerPixel);
                         break;
 
                     case FilterType.Up:
-
                         UpFilter.Decode(scanlineSpan, this.previousScanline.GetSpan());
                         break;
 
                     case FilterType.Average:
-
                         AverageFilter.Decode(scanlineSpan, this.previousScanline.GetSpan(), this.bytesPerPixel);
                         break;
 
                     case FilterType.Paeth:
-
                         PaethFilter.Decode(scanlineSpan, this.previousScanline.GetSpan(), this.bytesPerPixel);
                         break;
 
@@ -647,22 +606,18 @@ namespace SixLabors.ImageSharp.Formats.Png
                             break;
 
                         case FilterType.Sub:
-
                             SubFilter.Decode(scanSpan, this.bytesPerPixel);
                             break;
 
                         case FilterType.Up:
-
                             UpFilter.Decode(scanSpan, prevSpan);
                             break;
 
                         case FilterType.Average:
-
                             AverageFilter.Decode(scanSpan, prevSpan, this.bytesPerPixel);
                             break;
 
                         case FilterType.Paeth:
-
                             PaethFilter.Decode(scanSpan, prevSpan, this.bytesPerPixel);
                             break;
 
@@ -715,7 +670,6 @@ namespace SixLabors.ImageSharp.Formats.Png
             switch (this.pngColorType)
             {
                 case PngColorType.Grayscale:
-
                     PngScanlineProcessor.ProcessGrayscaleScanline(
                         this.header,
                         scanlineSpan,
@@ -727,7 +681,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.GrayscaleWithAlpha:
-
                     PngScanlineProcessor.ProcessGrayscaleWithAlphaScanline(
                         this.header,
                         scanlineSpan,
@@ -738,7 +691,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.Palette:
-
                     PngScanlineProcessor.ProcessPaletteScanline(
                         this.header,
                         scanlineSpan,
@@ -749,7 +701,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.Rgb:
-
                     PngScanlineProcessor.ProcessRgbScanline(
                         this.header,
                         scanlineSpan,
@@ -763,7 +714,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.RgbWithAlpha:
-
                     PngScanlineProcessor.ProcessRgbaScanline(
                         this.header,
                         scanlineSpan,
@@ -799,7 +749,6 @@ namespace SixLabors.ImageSharp.Formats.Png
             switch (this.pngColorType)
             {
                 case PngColorType.Grayscale:
-
                     PngScanlineProcessor.ProcessInterlacedGrayscaleScanline(
                         this.header,
                         scanlineSpan,
@@ -813,7 +762,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.GrayscaleWithAlpha:
-
                     PngScanlineProcessor.ProcessInterlacedGrayscaleWithAlphaScanline(
                         this.header,
                         scanlineSpan,
@@ -826,7 +774,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.Palette:
-
                     PngScanlineProcessor.ProcessInterlacedPaletteScanline(
                         this.header,
                         scanlineSpan,
@@ -839,7 +786,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.Rgb:
-
                     PngScanlineProcessor.ProcessInterlacedRgbScanline(
                         this.header,
                         scanlineSpan,
@@ -855,7 +801,6 @@ namespace SixLabors.ImageSharp.Formats.Png
                     break;
 
                 case PngColorType.RgbWithAlpha:
-
                     PngScanlineProcessor.ProcessInterlacedRgbaScanline(
                         this.header,
                         scanlineSpan,
@@ -886,6 +831,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                         ushort rc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(0, 2));
                         ushort gc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(2, 2));
                         ushort bc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(4, 2));
+
                         this.rgb48Trans = new Rgb48(rc, gc, bc);
                         this.hasTrans = true;
                         return;
@@ -925,37 +871,10 @@ namespace SixLabors.ImageSharp.Formats.Png
         {
             this.header = PngHeader.Parse(data);
 
+            this.header.Validate();
+
             pngMetaData.BitDepth = (PngBitDepth)this.header.BitDepth;
             pngMetaData.ColorType = this.header.ColorType;
-        }
-
-        /// <summary>
-        /// Validates the png header.
-        /// </summary>
-        /// <exception cref="NotSupportedException">
-        /// Thrown if the image does pass validation.
-        /// </exception>
-        private void ValidateHeader()
-        {
-            if (!ColorTypes.ContainsKey(this.header.ColorType))
-            {
-                throw new NotSupportedException("Color type is not supported or not valid.");
-            }
-
-            if (!ColorTypes[this.header.ColorType].Contains(this.header.BitDepth))
-            {
-                throw new NotSupportedException("Bit depth is not supported or not valid.");
-            }
-
-            if (this.header.FilterMethod != 0)
-            {
-                throw new NotSupportedException("The png specification only defines 0 as filter method.");
-            }
-
-            if (this.header.InterlaceMethod != PngInterlaceMode.None && this.header.InterlaceMethod != PngInterlaceMode.Adam7)
-            {
-                throw new NotSupportedException("The png specification only defines 'None' and 'Adam7' as interlaced methods.");
-            }
 
             this.pngColorType = this.header.ColorType;
         }
@@ -964,28 +883,18 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Reads a text chunk containing image properties from the data.
         /// </summary>
         /// <param name="metadata">The metadata to decode to.</param>
-        /// <param name="data">The <see cref="T:byte[]"/> containing  data.</param>
+        /// <param name="data">The <see cref="T:Span"/> containing the data.</param>
-        /// <param name="length">The maximum length to read.</param>
+        private void ReadTextChunk(ImageMetaData metadata, ReadOnlySpan<byte> data)
-        private void ReadTextChunk(ImageMetaData metadata, byte[] data, int length)
         {
             if (this.ignoreMetadata)
             {
                 return;
             }
 
-            int zeroIndex = 0;
+            int zeroIndex = data.IndexOf((byte)0);
-
-            for (int i = 0; i < length; i++)
-            {
-                if (data[i] == 0)
-                {
-                    zeroIndex = i;
-                    break;
-                }
-            }
 
-            string name = this.textEncoding.GetString(data, 0, zeroIndex);
+            string name = this.textEncoding.GetString(data.Slice(0, zeroIndex));
-            string value = this.textEncoding.GetString(data, zeroIndex + 1, length - zeroIndex - 1);
+            string value = this.textEncoding.GetString(data.Slice(zeroIndex + 1));
 
             metadata.Properties.Add(new ImageProperty(name, value));
         }
@@ -1001,7 +910,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                 return 0;
             }
 
-            this.currentStream.Read(this.crcBuffer, 0, 4);
+            this.currentStream.Read(this.buffer, 0, 4);
 
             if (this.TryReadChunk(out PngChunk chunk))
             {
@@ -1086,13 +995,17 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="chunk">The <see cref="PngChunk"/>.</param>
         private void ValidateChunk(in PngChunk chunk)
         {
+            Span<byte> chunkType = stackalloc byte[4];
+
+            BinaryPrimitives.WriteUInt32BigEndian(chunkType, (uint)chunk.Type);
+
             this.crc.Reset();
-            this.crc.Update(this.chunkTypeBuffer);
+            this.crc.Update(chunkType);
             this.crc.Update(chunk.Data.GetSpan());
 
             if (this.crc.Value != chunk.Crc)
             {
-                string chunkTypeName = Encoding.UTF8.GetString(this.chunkTypeBuffer, 0, 4);
+                string chunkTypeName = Encoding.UTF8.GetString(chunkType);
 
                 throw new ImageFormatException($"CRC Error. PNG {chunkTypeName} chunk is corrupt!");
             }
@@ -1106,14 +1019,9 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// </exception>
         private uint ReadChunkCrc()
         {
-            int numBytes = this.currentStream.Read(this.crcBuffer, 0, 4);
+            return this.currentStream.Read(this.buffer, 0, 4) == 4
-
+                ? BinaryPrimitives.ReadUInt32BigEndian(this.buffer)
-            if (numBytes >= 1 && numBytes <= 3)
+                : throw new ImageFormatException("Image stream is not valid!");
-            {
-                throw new ImageFormatException("Image stream is not valid!");
-            }
-
-            return BinaryPrimitives.ReadUInt32BigEndian(this.crcBuffer);
         }
 
         /// <summary>
@@ -1148,22 +1056,22 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// </exception>
         private PngChunkType ReadChunkType()
         {
-            return this.currentStream.Read(this.chunkTypeBuffer, 0, 4) == 4
+            return this.currentStream.Read(this.buffer, 0, 4) == 4
-                ? (PngChunkType)BinaryPrimitives.ReadUInt32BigEndian(this.chunkTypeBuffer.AsSpan())
+                ? (PngChunkType)BinaryPrimitives.ReadUInt32BigEndian(this.buffer)
                 : throw new ImageFormatException("Invalid PNG data.");
         }
 
         /// <summary>
-        /// Calculates the length of the given chunk.
+        /// Attempts to read the length of the next chunk.
         /// </summary>
-        /// <exception cref="ImageFormatException">
+        /// <returns>
-        /// Thrown if the input stream is not valid.
+        /// Whether the the length was read.
-        /// </exception>
+        /// </returns>
         private bool TryReadChunkLength(out int result)
         {
-            if (this.currentStream.Read(this.chunkLengthBuffer, 0, 4) == 4)
+            if (this.currentStream.Read(this.buffer, 0, 4) == 4)
             {
-                result = BinaryPrimitives.ReadInt32BigEndian(this.chunkLengthBuffer);
+                result = BinaryPrimitives.ReadInt32BigEndian(this.buffer);
 
                 return true;
             }

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -9,7 +9,7 @@ using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
-using SixLabors.ImageSharp.Common.Helpers;
+using SixLabors.ImageSharp.Formats.Png.Chunks;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
 using SixLabors.ImageSharp.Memory;
@@ -674,47 +674,9 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="meta">The image meta data.</param>
         private void WritePhysicalChunk(Stream stream, ImageMetaData meta)
         {
-            // The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:
+            PhysicalChunkData.FromMetadata(meta).WriteTo(this.chunkDataBuffer);
-            // Pixels per unit, X axis: 4 bytes (unsigned integer)
-            // Pixels per unit, Y axis: 4 bytes (unsigned integer)
-            // Unit specifier:          1 byte
-            //
-            // The following values are legal for the unit specifier:
-            //   0: unit is unknown
-            //   1: unit is the meter
-            //
-            // When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
-            Span<byte> hResolution = this.chunkDataBuffer.AsSpan(0, 4);
-            Span<byte> vResolution = this.chunkDataBuffer.AsSpan(4, 4);
-
-            switch (meta.ResolutionUnits)
-            {
-                case PixelResolutionUnit.AspectRatio:
-                    this.chunkDataBuffer[8] = 0;
-                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution));
-                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.VerticalResolution));
-                    break;
-
-                case PixelResolutionUnit.PixelsPerInch:
-                    this.chunkDataBuffer[8] = 1; // Per meter
-                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(UnitConverter.InchToMeter(meta.HorizontalResolution)));
-                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(UnitConverter.InchToMeter(meta.VerticalResolution)));
-                    break;
-
-                case PixelResolutionUnit.PixelsPerCentimeter:
-                    this.chunkDataBuffer[8] = 1; // Per meter
-                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(UnitConverter.CmToMeter(meta.HorizontalResolution)));
-                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(UnitConverter.CmToMeter(meta.VerticalResolution)));
-                    break;
-
-                default:
-                    this.chunkDataBuffer[8] = 1; // Per meter
-                    BinaryPrimitives.WriteInt32BigEndian(hResolution, (int)Math.Round(meta.HorizontalResolution));
-                    BinaryPrimitives.WriteInt32BigEndian(vResolution, (int)Math.Round(meta.VerticalResolution));
-                    break;
-            }
 
-            this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, 9);
+            this.WriteChunk(stream, PngChunkType.Physical, this.chunkDataBuffer, 0, PhysicalChunkData.Size);
         }
 
         /// <summary>

src/ImageSharp/Formats/Png/PngHeader.cs

@@ -80,6 +80,36 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// </summary>
         public PngInterlaceMode InterlaceMethod { get; }
 
+        /// <summary>
+        /// Validates the png header.
+        /// </summary>
+        /// <exception cref="NotSupportedException">
+        /// Thrown if the image does pass validation.
+        /// </exception>
+        public void Validate()
+        {
+            if (!PngConstants.ColorTypes.TryGetValue(this.ColorType, out byte[] supportedBitDepths))
+            {
+                throw new NotSupportedException($"Invalid or unsupported color type. Was '{this.ColorType}'.");
+            }
+
+            if (supportedBitDepths.AsSpan().IndexOf(this.BitDepth) == -1)
+            {
+                throw new NotSupportedException($"Invalid or unsupported bit depth. Was '{this.BitDepth}'.");
+            }
+
+            if (this.FilterMethod != 0)
+            {
+                throw new NotSupportedException($"Invalid filter method. Expected 0. Was '{this.FilterMethod}'.");
+            }
+
+            // The png specification only defines 'None' and 'Adam7' as interlaced methods.
+            if (this.InterlaceMethod != PngInterlaceMode.None && this.InterlaceMethod != PngInterlaceMode.Adam7)
+            {
+                throw new NotSupportedException($"Invalid interlace method. Expected 'None' or 'Adam7'. Was '{this.InterlaceMethod}'.");
+            }
+        }
+
         /// <summary>
         /// Writes the header to the given buffer.
         /// </summary>

src/ImageSharp/MetaData/Profiles/Exif/ExifReader.cs

@@ -127,25 +127,14 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
 
         private unsafe string ConvertToString(ReadOnlySpan<byte> buffer)
         {
-            Span<byte> nullChar = stackalloc byte[1] { 0 };
+            int nullCharIndex = buffer.IndexOf((byte)0);
-
-            int nullCharIndex = buffer.IndexOf(nullChar);
 
             if (nullCharIndex > -1)
             {
                 buffer = buffer.Slice(0, nullCharIndex);
             }
 
-#if NETSTANDARD1_1
-            return Encoding.UTF8.GetString(buffer.ToArray(), 0, buffer.Length);
-#elif NETCOREAPP2_1
             return Encoding.UTF8.GetString(buffer);
-#else
-            fixed (byte* pointer = &MemoryMarshal.GetReference(buffer))
-            {
-                return Encoding.UTF8.GetString(pointer, buffer.Length);
-            }
-#endif
         }
 
         /// <summary>