EXIF Support for PNG's

src/ImageSharp/Formats/Png/PngChunkType.cs

@@ -4,7 +4,7 @@
 namespace SixLabors.ImageSharp.Formats.Png
 {
     /// <summary>
-    /// Contains a list of of chunk types.
+    /// Contains a list of chunk types.
     /// </summary>
     internal enum PngChunkType : uint
     {
@@ -55,6 +55,11 @@ internal enum PngChunkType : uint
         /// <summary>
         /// The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image.
         /// </summary>
-        Physical = 0x70485973U // pHYs
+        Physical = 0x70485973U, // pHYs
+
+        /// <summary>
+        /// The data chunk which contains the Exif profile.
+        /// </summary>
+        Exif = 0x65584966U // eXIf
     }
 }

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -15,6 +15,7 @@
 using SixLabors.ImageSharp.Formats.Png.Zlib;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.MetaData;
+using SixLabors.ImageSharp.MetaData.Profiles.Exif;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Png
@@ -249,6 +250,15 @@ public Image<TPixel> Decode<TPixel>(Stream stream)
                                     break;
                                 case PngChunkType.Text:
                                     this.ReadTextChunk(metadata, chunk.Data.Array, chunk.Length);
+                                    break;
+                                case PngChunkType.Exif:
+                                    if (!this.ignoreMetadata)
+                                    {
+                                        byte[] exifData = new byte[chunk.Length];
+                                        Buffer.BlockCopy(chunk.Data.Array, 0, exifData, 0, chunk.Length);
+                                        metadata.ExifProfile = new ExifProfile(exifData);
+                                    }
+
                                     break;
                                 case PngChunkType.End:
                                     this.isEndChunkReached = true;

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -221,6 +221,7 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
 
             this.WritePhysicalChunk(stream, image);
             this.WriteGammaChunk(stream);
+            this.WriteExifChunk(stream, image);
             this.WriteDataChunks(image.Frames.RootFrame, stream);
             this.WriteEndChunk(stream);
             stream.Flush();
@@ -397,7 +398,7 @@ private IManagedByteBuffer GetOptimalFilteredScanline()
         /// <summary>
         /// Calculates the correct number of bytes per pixel for the given color type.
         /// </summary>
-        /// <returns>The <see cref="int"/></returns>
+        /// <returns>Bytes per pixel</returns>
         private int CalculateBytesPerPixel()
         {
             switch (this.pngColorType)
@@ -523,6 +524,21 @@ private void WritePhysicalChunk<TPixel>(Stream stream, Image<TPixel> image)
             }
         }
 
+        /// <summary>
+        /// Writes the eXIf chunk to the stream, if any EXIF Profile values are present in the meta data.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
+        /// <param name="image">The image.</param>
+        private void WriteExifChunk<TPixel>(Stream stream, Image<TPixel> image)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            if (image.MetaData.ExifProfile?.Values.Count > 0)
+            {
+                this.WriteChunk(stream, PngChunkType.Exif, image.MetaData.ExifProfile.ToByteArray(includeExifIdCode: false));
+            }
+        }
+
         /// <summary>
         /// Writes the gamma information to the stream.
         /// </summary>

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

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.IO;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Primitives;
@@ -233,8 +232,10 @@ public void SetValue(ExifTag tag, object value)
         /// <summary>
         /// Converts this instance to a byte array.
         /// </summary>
+        /// <param name="includeExifIdCode">Indicates, if the Exif ID code should be included.
+        /// This Exif ID code should not be included in case of PNG's. Defaults to true.</param>
         /// <returns>The <see cref="T:byte[]"/></returns>
-        public byte[] ToByteArray()
+        public byte[] ToByteArray(bool includeExifIdCode = true)
         {
             if (this.values == null)
             {
@@ -247,7 +248,7 @@ public byte[] ToByteArray()
             }
 
             var writer = new ExifWriter(this.values, this.Parts);
-            return writer.GetData();
+            return writer.GetData(includeExifIdCode);
         }
 
         /// <summary>

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

@@ -78,6 +78,7 @@ public List<ExifValue> ReadValues()
 
             if (this.ReadString(4) == "Exif")
             {
+                // two zeros are expected to follow the Exif Id code
                 if (this.ReadUInt16() != 0)
                 {
                     return values;

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

@@ -14,11 +14,6 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
     /// </summary>
     internal sealed class ExifWriter
     {
-        /// <summary>
-        /// The start index.
-        /// </summary>
-        private const int StartIndex = 6;
-
         /// <summary>
         /// Which parts will be written.
         /// </summary>
@@ -46,11 +41,14 @@ public ExifWriter(IList<ExifValue> values, ExifParts allowedParts)
         /// <summary>
         /// Returns the EXIF data.
         /// </summary>
+        /// <param name="includeExifIdCode">Indicates, if the Exif ID code should be included.
+        /// This Exif ID code should not be included in case of PNG's. Defaults to true.</param>
         /// <returns>
         /// The <see cref="T:byte[]"/>.
         /// </returns>
-        public byte[] GetData()
+        public byte[] GetData(bool includeExifIdCode = true)
         {
+            uint startIndex = 6;
             uint length;
             int exifIndex = -1;
             int gpsIndex = -1;
@@ -86,23 +84,51 @@ public byte[] GetData()
                 return null;
             }
 
-            length += 10 + 4 + 2;
+            if (includeExifIdCode)
+            {
+                // Exif Code (6 bytes) + byte order marker (4 bytes)
+                length += 10;
+            }
+            else
+            {
+                // special case for PNG eXIf Chunk:
+                // two bytes for the byte Order marker 'II', followed by the number 42 (0x2A) and a 0, making 4 bytes total
+                length += 4;
+
+                // if the Exif Code ("Exif00") is not included, the start index is 0 instead of 6
+                startIndex = 0;
+            }
+
+            length += 4 + 2;
 
             byte[] result = new byte[length];
 
-            result[0] = (byte)'E';
-            result[1] = (byte)'x';
-            result[2] = (byte)'i';
-            result[3] = (byte)'f';
-            result[4] = 0x00;
-            result[5] = 0x00;
-            result[6] = (byte)'I';
-            result[7] = (byte)'I';
-            result[8] = 0x2A;
-            result[9] = 0x00;
-
-            int i = 10;
-            uint ifdOffset = ((uint)i - StartIndex) + 4;
+            int i = 0;
+            if (includeExifIdCode)
+            {
+                result[0] = (byte)'E';
+                result[1] = (byte)'x';
+                result[2] = (byte)'i';
+                result[3] = (byte)'f';
+                result[4] = 0x00;
+                result[5] = 0x00;
+                result[6] = (byte)'I';
+                result[7] = (byte)'I';
+                result[8] = 0x2A;
+                result[9] = 0x00;
+                i = 10;
+            }
+            else
+            {
+                // the byte order marker followed by the number 42 and a 0
+                result[0] = (byte)'I';
+                result[1] = (byte)'I';
+                result[2] = 0x2A;
+                result[3] = 0x00;
+                i = 4;
+            }
+
+            uint ifdOffset = ((uint)i - startIndex) + 4;
             uint thumbnailOffset = ifdOffset + ifdLength + exifLength + gpsLength;
 
             if (exifLength > 0)
@@ -118,18 +144,18 @@ public byte[] GetData()
             i = WriteUInt32(ifdOffset, result, i);
             i = this.WriteHeaders(this.ifdIndexes, result, i);
             i = WriteUInt32(thumbnailOffset, result, i);
-            i = this.WriteData(this.ifdIndexes, result, i);
+            i = this.WriteData(startIndex, this.ifdIndexes, result, i);
 
             if (exifLength > 0)
             {
                 i = this.WriteHeaders(this.exifIndexes, result, i);
-                i = this.WriteData(this.exifIndexes, result, i);
+                i = this.WriteData(startIndex, this.exifIndexes, result, i);
             }
 
             if (gpsLength > 0)
             {
                 i = this.WriteHeaders(this.gpsIndexes, result, i);
-                i = this.WriteData(this.gpsIndexes, result, i);
+                i = this.WriteData(startIndex, this.gpsIndexes, result, i);
             }
 
             WriteUInt16((ushort)0, result, i);
@@ -266,7 +292,7 @@ private int WriteArray(ExifValue value, byte[] destination, int offset)
             return newOffset;
         }
 
-        private int WriteData(List<int> indexes, byte[] destination, int offset)
+        private int WriteData(uint startIndex, List<int> indexes, byte[] destination, int offset)
         {
             if (this.dataOffsets.Count == 0)
             {
@@ -281,7 +307,7 @@ private int WriteData(List<int> indexes, byte[] destination, int offset)
                 ExifValue value = this.values[index];
                 if (value.Length > 4)
                 {
-                    WriteUInt32((uint)(newOffset - StartIndex), destination, this.dataOffsets[i++]);
+                    WriteUInt32((uint)(newOffset - startIndex), destination, this.dataOffsets[i++]);
                     newOffset = this.WriteValue(value, destination, newOffset);
                 }
             }