/// <exception cref="PngProcessingException"/>
        /// <exception cref="System.IO.IOException"/>
        private static IEnumerable<Directory> ProcessChunk([NotNull] PngChunk chunk)
        {
            // For more guidance:
            // http://www.w3.org/TR/PNG-Decoders.html#D.Text-chunk-processing
            // http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.iCCP
            // by spec, PNG is generally supposed to use this encoding
            const string defaultEncodingName = "ISO-8859-1";
            var defaultEncoding = Encoding.GetEncoding(defaultEncodingName);

            var chunkType = chunk.ChunkType;
            var bytes = chunk.Bytes;

            if (chunkType == PngChunkType.IHDR)
            {
                var header = new PngHeader(bytes);
                var directory = new PngDirectory(PngChunkType.IHDR);
                directory.Set(PngDirectory.TagImageWidth, header.ImageWidth);
                directory.Set(PngDirectory.TagImageHeight, header.ImageHeight);
                directory.Set(PngDirectory.TagBitsPerSample, header.BitsPerSample);
                directory.Set(PngDirectory.TagColorType, header.ColorType.NumericValue);
                directory.Set(PngDirectory.TagCompressionType, header.CompressionType);
                directory.Set(PngDirectory.TagFilterMethod, header.FilterMethod);
                directory.Set(PngDirectory.TagInterlaceMethod, header.InterlaceMethod);
                yield return directory;
            }
            else if (chunkType == PngChunkType.PLTE)
            {
                var directory = new PngDirectory(PngChunkType.PLTE);
                directory.Set(PngDirectory.TagPaletteSize, bytes.Length / 3);
                yield return directory;
            }
            else if (chunkType == PngChunkType.tRNS)
            {
                var directory = new PngDirectory(PngChunkType.tRNS);
                directory.Set(PngDirectory.TagPaletteHasTransparency, 1);
                yield return directory;
            }
            else if (chunkType == PngChunkType.sRGB)
            {
                int srgbRenderingIntent = unchecked((sbyte)bytes[0]);
                var directory = new PngDirectory(PngChunkType.sRGB);
                directory.Set(PngDirectory.TagSrgbRenderingIntent, srgbRenderingIntent);
                yield return directory;
            }
            else if (chunkType == PngChunkType.cHRM)
            {
                var chromaticities = new PngChromaticities(bytes);
                var directory = new PngChromaticitiesDirectory();
                directory.Set(PngChromaticitiesDirectory.TagWhitePointX, chromaticities.WhitePointX);
                directory.Set(PngChromaticitiesDirectory.TagWhitePointY, chromaticities.WhitePointY);
                directory.Set(PngChromaticitiesDirectory.TagRedX, chromaticities.RedX);
                directory.Set(PngChromaticitiesDirectory.TagRedY, chromaticities.RedY);
                directory.Set(PngChromaticitiesDirectory.TagGreenX, chromaticities.GreenX);
                directory.Set(PngChromaticitiesDirectory.TagGreenY, chromaticities.GreenY);
                directory.Set(PngChromaticitiesDirectory.TagBlueX, chromaticities.BlueX);
                directory.Set(PngChromaticitiesDirectory.TagBlueY, chromaticities.BlueY);
                yield return directory;
            }
            else if (chunkType == PngChunkType.gAMA)
            {
                var gammaInt = ByteConvert.ToInt32BigEndian(bytes);
                var directory = new PngDirectory(PngChunkType.gAMA);
                directory.Set(PngDirectory.TagGamma, gammaInt / 100000.0);
                yield return directory;
            }
            else if (chunkType == PngChunkType.iCCP)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var profileName = reader.GetNullTerminatedStringValue(maxLengthBytes: 79);
                var directory = new PngDirectory(PngChunkType.iCCP);
                directory.Set(PngDirectory.TagIccProfileName, profileName);
                var compressionMethod = reader.GetSByte();
                if (compressionMethod == 0)
                {
                    // Only compression method allowed by the spec is zero: deflate
                    // This assumes 1-byte-per-char, which it is by spec.
                    var bytesLeft = bytes.Length - profileName.Bytes.Length - 2;
                    var compressedProfile = reader.GetBytes(bytesLeft);
                    using (var inflaterStream = new InflaterInputStream(new MemoryStream(compressedProfile)))
                    {
                        var iccDirectory = new IccReader().Extract(new IndexedCapturingReader(inflaterStream));
                        iccDirectory.Parent = directory;
                        yield return iccDirectory;
                    }
                }
                else
                {
                    directory.AddError("Invalid compression method value");
                }
                yield return directory;
            }
            else if (chunkType == PngChunkType.bKGD)
            {
                var directory = new PngDirectory(PngChunkType.bKGD);
                directory.Set(PngDirectory.TagBackgroundColor, bytes);
                yield return directory;
            }
            else if (chunkType == PngChunkType.tEXt)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var keyword = reader.GetNullTerminatedStringValue(maxLengthBytes: 79).ToString(defaultEncoding);
                var bytesLeft = bytes.Length - keyword.Length - 1;
                var value = reader.GetNullTerminatedStringValue(bytesLeft, defaultEncoding);

                var textPairs = new List<KeyValuePair> { new KeyValuePair(keyword, value) };
                var directory = new PngDirectory(PngChunkType.iTXt);
                directory.Set(PngDirectory.TagTextualData, textPairs);
                yield return directory;
            }
            else if (chunkType == PngChunkType.iTXt)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var keyword = reader.GetNullTerminatedStringValue(maxLengthBytes: 79).ToString(defaultEncoding);
                var compressionFlag = reader.GetSByte();
                var compressionMethod = reader.GetSByte();
                var languageTag = reader.GetNullTerminatedStringValue(bytes.Length, defaultEncoding);

                var translatedKeyword = reader.GetNullTerminatedStringValue(bytes.Length, defaultEncoding);

                var bytesLeft = bytes.Length - keyword.Length - 1 - 1 - 1 - languageTag.Bytes.Length - 1 - translatedKeyword.Bytes.Length - 1;
                byte[] textBytes = null;
                if (compressionFlag == 0)
                {
                    textBytes = reader.GetNullTerminatedBytes(bytesLeft);
                }
                else if (compressionFlag == 1)
                {
                    if (compressionMethod == 0)
                    {
                        using (var inflaterStream = new DeflateStream(new MemoryStream(bytes, bytes.Length - bytesLeft, bytesLeft), CompressionMode.Decompress))
                        using (var decompStream = new MemoryStream())
                        {
#if !NET35
                            inflaterStream.CopyTo(decompStream);
#else
                            byte[] buffer = new byte[256];
                            int count;
                            int totalBytes = 0;
                            while ((count = inflaterStream.Read(buffer, 0, 256)) > 0)
                            {
                                decompStream.Write(buffer, 0, count);
                                totalBytes += count;
                            }
#endif
                            textBytes = decompStream.ToArray();
                        }
                    }
                    else
                    {
                        var directory = new PngDirectory(PngChunkType.iTXt);
                        directory.AddError("Invalid compression method value");
                        yield return directory;
                    }
                }
                else
                {
                    var directory = new PngDirectory(PngChunkType.iTXt);
                    directory.AddError("Invalid compression flag value");
                    yield return directory;
                }

                if (textBytes != null)
                {
                    if (keyword == "XML:com.adobe.xmp")
                    {
                        // NOTE in testing images, the XMP has parsed successfully, but we are not extracting tags from it as necessary
                        yield return new XmpReader().Extract(textBytes);
                    }
                    else
                    {
                        var textPairs = new List<KeyValuePair> { new KeyValuePair(keyword, new StringValue(textBytes, defaultEncoding)) };
                        var directory = new PngDirectory(PngChunkType.iTXt);
                        directory.Set(PngDirectory.TagTextualData, textPairs);
                        yield return directory;
                    }
                }
            }
            else if (chunkType == PngChunkType.tIME)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var year = reader.GetUInt16();
                var month = reader.GetByte();
                int day = reader.GetByte();
                int hour = reader.GetByte();
                int minute = reader.GetByte();
                int second = reader.GetByte();
                var directory = new PngDirectory(PngChunkType.tIME);
                try
                {
                    var time = new DateTime(year, month, day, hour, minute, second, DateTimeKind.Unspecified);
                    directory.Set(PngDirectory.TagLastModificationTime, time);
                }
                catch (ArgumentOutOfRangeException e)
                {
                    directory.AddError("Error constructing DateTime: " + e.Message);
                }
                yield return directory;
            }
            else if (chunkType == PngChunkType.pHYs)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var pixelsPerUnitX = reader.GetInt32();
                var pixelsPerUnitY = reader.GetInt32();
                var unitSpecifier = reader.GetSByte();
                var directory = new PngDirectory(PngChunkType.pHYs);
                directory.Set(PngDirectory.TagPixelsPerUnitX, pixelsPerUnitX);
                directory.Set(PngDirectory.TagPixelsPerUnitY, pixelsPerUnitY);
                directory.Set(PngDirectory.TagUnitSpecifier, unitSpecifier);
                yield return directory;
            }
            else if (chunkType.Equals(PngChunkType.sBIT))
            {
                var directory = new PngDirectory(PngChunkType.sBIT);
                directory.Set(PngDirectory.TagSignificantBits, bytes);
                yield return directory;
            }
        }
        /// <exception cref="PngProcessingException"/>
        /// <exception cref="System.IO.IOException"/>
        private static IEnumerable<Directory> ProcessChunk([NotNull] PngChunk chunk)
        {
            var chunkType = chunk.ChunkType;
            var bytes = chunk.Bytes;

            if (chunkType == PngChunkType.IHDR)
            {
                var header = new PngHeader(bytes);
                var directory = new PngDirectory(PngChunkType.IHDR);
                directory.Set(PngDirectory.TagImageWidth, header.ImageWidth);
                directory.Set(PngDirectory.TagImageHeight, header.ImageHeight);
                directory.Set(PngDirectory.TagBitsPerSample, header.BitsPerSample);
                directory.Set(PngDirectory.TagColorType, header.ColorType.NumericValue);
                directory.Set(PngDirectory.TagCompressionType, header.CompressionType);
                directory.Set(PngDirectory.TagFilterMethod, header.FilterMethod);
                directory.Set(PngDirectory.TagInterlaceMethod, header.InterlaceMethod);
                yield return directory;
            }
            else if (chunkType == PngChunkType.PLTE)
            {
                var directory = new PngDirectory(PngChunkType.PLTE);
                directory.Set(PngDirectory.TagPaletteSize, bytes.Length / 3);
                yield return directory;
            }
            else if (chunkType == PngChunkType.tRNS)
            {
                var directory = new PngDirectory(PngChunkType.tRNS);
                directory.Set(PngDirectory.TagPaletteHasTransparency, 1);
                yield return directory;
            }
            else if (chunkType == PngChunkType.sRGB)
            {
                int srgbRenderingIntent = unchecked((sbyte)bytes[0]);
                var directory = new PngDirectory(PngChunkType.sRGB);
                directory.Set(PngDirectory.TagSrgbRenderingIntent, srgbRenderingIntent);
                yield return directory;
            }
            else if (chunkType == PngChunkType.cHRM)
            {
                var chromaticities = new PngChromaticities(bytes);
                var directory = new PngChromaticitiesDirectory();
                directory.Set(PngChromaticitiesDirectory.TagWhitePointX, chromaticities.WhitePointX);
                directory.Set(PngChromaticitiesDirectory.TagWhitePointY, chromaticities.WhitePointY);
                directory.Set(PngChromaticitiesDirectory.TagRedX, chromaticities.RedX);
                directory.Set(PngChromaticitiesDirectory.TagRedY, chromaticities.RedY);
                directory.Set(PngChromaticitiesDirectory.TagGreenX, chromaticities.GreenX);
                directory.Set(PngChromaticitiesDirectory.TagGreenY, chromaticities.GreenY);
                directory.Set(PngChromaticitiesDirectory.TagBlueX, chromaticities.BlueX);
                directory.Set(PngChromaticitiesDirectory.TagBlueY, chromaticities.BlueY);
                yield return directory;
            }
            else if (chunkType == PngChunkType.gAMA)
            {
                var gammaInt = ByteConvert.ToInt32BigEndian(bytes);
                var directory = new PngDirectory(PngChunkType.gAMA);
                directory.Set(PngDirectory.TagGamma, gammaInt / 100000.0);
                yield return directory;
            }
            else if (chunkType == PngChunkType.iCCP)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var profileName = reader.GetNullTerminatedString(maxLengthBytes: 79);
                var directory = new PngDirectory(PngChunkType.iCCP);
                directory.Set(PngDirectory.TagIccProfileName, profileName);
                var compressionMethod = reader.GetSByte();
                if (compressionMethod == 0)
                {
                    // Only compression method allowed by the spec is zero: deflate
                    // This assumes 1-byte-per-char, which it is by spec.
                    var bytesLeft = bytes.Length - profileName.Length - 2;
                    var compressedProfile = reader.GetBytes(bytesLeft);
                    using (var inflaterStream = new InflaterInputStream(new MemoryStream(compressedProfile)))
                        yield return new IccReader().Extract(new IndexedCapturingReader(inflaterStream));
                }
                else
                {
                    directory.AddError("Invalid compression method value");
                }
                yield return directory;
            }
            else if (chunkType == PngChunkType.bKGD)
            {
                var directory = new PngDirectory(PngChunkType.bKGD);
                directory.Set(PngDirectory.TagBackgroundColor, bytes);
                yield return directory;
            }
            else if (chunkType == PngChunkType.tEXt)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var keyword = reader.GetNullTerminatedString(maxLengthBytes: 79);
                var bytesLeft = bytes.Length - keyword.Length - 1;
                var value = reader.GetNullTerminatedString(bytesLeft);
                var textPairs = new List<KeyValuePair> { new KeyValuePair(keyword, value) };
                var directory = new PngDirectory(PngChunkType.iTXt);
                directory.Set(PngDirectory.TagTextualData, textPairs);
                yield return directory;
            }
            else if (chunkType == PngChunkType.iTXt)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var keyword = reader.GetNullTerminatedString(maxLengthBytes: 79);
                var compressionFlag = reader.GetSByte();
                var compressionMethod = reader.GetSByte();
                var languageTag = reader.GetNullTerminatedString(bytes.Length);
                var translatedKeyword = reader.GetNullTerminatedString(bytes.Length);
                var bytesLeft = bytes.Length - keyword.Length - 1 - 1 - 1 - languageTag.Length - 1 - translatedKeyword.Length - 1;
                string text = null;
                if (compressionFlag == 0)
                {
                    text = reader.GetNullTerminatedString(bytesLeft);
                }
                else if (compressionFlag == 1)
                {
                    if (compressionMethod == 0)
                    {
                        using (var inflaterStream = new DeflateStream(new MemoryStream(bytes, bytes.Length - bytesLeft, bytesLeft), CompressionMode.Decompress))
                            text = new StreamReader(inflaterStream).ReadToEnd();
                    }
                    else
                    {
                        var directory = new PngDirectory(PngChunkType.iTXt);
                        directory.AddError("Invalid compression method value");
                        yield return directory;
                    }
                }
                else
                {
                    var directory = new PngDirectory(PngChunkType.iTXt);
                    directory.AddError("Invalid compression flag value");
                    yield return directory;
                }

                if (text != null)
                {
                    if (keyword == "XML:com.adobe.xmp")
                    {
                        // NOTE in testing images, the XMP has parsed successfully, but we are not extracting tags from it as necessary
                        yield return new XmpReader().Extract(text);
                    }
                    else
                    {
                        var textPairs = new List<KeyValuePair> { new KeyValuePair(keyword, text) };
                        var directory = new PngDirectory(PngChunkType.iTXt);
                        directory.Set(PngDirectory.TagTextualData, textPairs);
                        yield return directory;
                    }
                }
            }
            else if (chunkType == PngChunkType.tIME)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var year = reader.GetUInt16();
                var month = reader.GetByte();
                int day = reader.GetByte();
                int hour = reader.GetByte();
                int minute = reader.GetByte();
                int second = reader.GetByte();
                var directory = new PngDirectory(PngChunkType.tIME);
                try
                {
                    var time = new DateTime(year, month, day, hour, minute, second, DateTimeKind.Utc);
                    directory.Set(PngDirectory.TagLastModificationTime, time);
                }
                catch (ArgumentOutOfRangeException e)
                {
                    directory.AddError("Error constructing DateTime: " + e.Message);
                }
                yield return directory;
            }
            else if (chunkType == PngChunkType.pHYs)
            {
                var reader = new SequentialByteArrayReader(bytes);
                var pixelsPerUnitX = reader.GetInt32();
                var pixelsPerUnitY = reader.GetInt32();
                var unitSpecifier = reader.GetSByte();
                var directory = new PngDirectory(PngChunkType.pHYs);
                directory.Set(PngDirectory.TagPixelsPerUnitX, pixelsPerUnitX);
                directory.Set(PngDirectory.TagPixelsPerUnitY, pixelsPerUnitY);
                directory.Set(PngDirectory.TagUnitSpecifier, unitSpecifier);
                yield return directory;
            }
            else if (chunkType.Equals(PngChunkType.sBIT))
            {
                var directory = new PngDirectory(PngChunkType.sBIT);
                directory.Set(PngDirectory.TagSignificantBits, bytes);
                yield return directory;
            }
        }
Ejemplo n.º 3
0
        /// <exception cref="PngProcessingException"/>
        /// <exception cref="IOException"/>
        private static IEnumerable <Directory> ProcessChunk(PngChunk chunk)
        {
            var chunkType = chunk.ChunkType;
            var bytes     = chunk.Bytes;

            if (chunkType == PngChunkType.IHDR)
            {
                var header    = new PngHeader(bytes);
                var directory = new PngDirectory(PngChunkType.IHDR);
                directory.Set(PngDirectory.TagImageWidth, header.ImageWidth);
                directory.Set(PngDirectory.TagImageHeight, header.ImageHeight);
                directory.Set(PngDirectory.TagBitsPerSample, header.BitsPerSample);
                directory.Set(PngDirectory.TagColorType, header.ColorType.NumericValue);
                directory.Set(PngDirectory.TagCompressionType, header.CompressionType);
                directory.Set(PngDirectory.TagFilterMethod, header.FilterMethod);
                directory.Set(PngDirectory.TagInterlaceMethod, header.InterlaceMethod);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.PLTE)
            {
                var directory = new PngDirectory(PngChunkType.PLTE);
                directory.Set(PngDirectory.TagPaletteSize, bytes.Length / 3);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.tRNS)
            {
                var directory = new PngDirectory(PngChunkType.tRNS);
                directory.Set(PngDirectory.TagPaletteHasTransparency, 1);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.sRGB)
            {
                int srgbRenderingIntent = unchecked ((sbyte)bytes[0]);
                var directory           = new PngDirectory(PngChunkType.sRGB);
                directory.Set(PngDirectory.TagSrgbRenderingIntent, srgbRenderingIntent);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.cHRM)
            {
                var chromaticities = new PngChromaticities(bytes);
                var directory      = new PngChromaticitiesDirectory();
                directory.Set(PngChromaticitiesDirectory.TagWhitePointX, chromaticities.WhitePointX);
                directory.Set(PngChromaticitiesDirectory.TagWhitePointY, chromaticities.WhitePointY);
                directory.Set(PngChromaticitiesDirectory.TagRedX, chromaticities.RedX);
                directory.Set(PngChromaticitiesDirectory.TagRedY, chromaticities.RedY);
                directory.Set(PngChromaticitiesDirectory.TagGreenX, chromaticities.GreenX);
                directory.Set(PngChromaticitiesDirectory.TagGreenY, chromaticities.GreenY);
                directory.Set(PngChromaticitiesDirectory.TagBlueX, chromaticities.BlueX);
                directory.Set(PngChromaticitiesDirectory.TagBlueY, chromaticities.BlueY);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.gAMA)
            {
                var gammaInt  = ByteConvert.ToInt32BigEndian(bytes);
                var directory = new PngDirectory(PngChunkType.gAMA);
                directory.Set(PngDirectory.TagGamma, gammaInt / 100000.0);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.iCCP)
            {
                var reader      = new SequentialByteArrayReader(bytes);
                var profileName = reader.GetNullTerminatedStringValue(maxLengthBytes: 79);
                var directory   = new PngDirectory(PngChunkType.iCCP);
                directory.Set(PngDirectory.TagIccProfileName, profileName);
                var compressionMethod = reader.GetSByte();
                if (compressionMethod == 0)
                {
                    // Only compression method allowed by the spec is zero: deflate
                    // This assumes 1-byte-per-char, which it is by spec.
                    var bytesLeft = bytes.Length - profileName.Bytes.Length - 2;

                    // http://george.chiramattel.com/blog/2007/09/deflatestream-block-length-does-not-match.html
                    // First two bytes are part of the zlib specification (RFC 1950), not the deflate specification (RFC 1951).
                    reader.Skip(2);
                    bytesLeft -= 2;

                    var compressedProfile = reader.GetBytes(bytesLeft);

                    IccDirectory?iccDirectory = null;
                    Exception?   ex           = null;
                    try
                    {
                        using var inflaterStream = new DeflateStream(new MemoryStream(compressedProfile), CompressionMode.Decompress);
                        iccDirectory             = new IccReader().Extract(new IndexedCapturingReader(inflaterStream));
                        iccDirectory.Parent      = directory;
                    }
                    catch (Exception e)
                    {
                        ex = e;
                    }

                    if (iccDirectory != null)
                    {
                        yield return(iccDirectory);
                    }
                    else if (ex != null)
                    {
                        directory.AddError($"Exception decompressing {nameof(PngChunkType.iCCP)} chunk: {ex.Message}");
                    }
                }
                else
                {
                    directory.AddError("Invalid compression method value");
                }
                yield return(directory);
            }
            else if (chunkType == PngChunkType.bKGD)
            {
                var directory = new PngDirectory(PngChunkType.bKGD);
                directory.Set(PngDirectory.TagBackgroundColor, bytes);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.tEXt)
            {
                var reader    = new SequentialByteArrayReader(bytes);
                var keyword   = reader.GetNullTerminatedStringValue(maxLengthBytes: 79).ToString(_latin1Encoding);
                var bytesLeft = bytes.Length - keyword.Length - 1;
                var value     = reader.GetNullTerminatedStringValue(bytesLeft, _latin1Encoding);

                var textPairs = new List <KeyValuePair> {
                    new KeyValuePair(keyword, value)
                };
                var directory = new PngDirectory(PngChunkType.tEXt);
                directory.Set(PngDirectory.TagTextualData, textPairs);
                yield return(directory);
            }
            else if (chunkType == PngChunkType.zTXt)
            {
                var reader            = new SequentialByteArrayReader(bytes);
                var keyword           = reader.GetNullTerminatedStringValue(maxLengthBytes: 79).ToString(_latin1Encoding);
                var compressionMethod = reader.GetSByte();

                var bytesLeft = bytes.Length - keyword.Length - 1 - 1 - 1 - 1;
                byte[]? textBytes = null;
                if (compressionMethod == 0)
                {
                    if (!TryDeflate(bytes, bytesLeft, out textBytes, out string?errorMessage))
                    {
                        var directory = new PngDirectory(PngChunkType.zTXt);
                        directory.AddError($"Exception decompressing {nameof(PngChunkType.zTXt)} chunk with keyword \"{keyword}\": {errorMessage}");
                        yield return(directory);
                    }
                }
                else
                {
                    var directory = new PngDirectory(PngChunkType.zTXt);
                    directory.AddError("Invalid compression method value");
                    yield return(directory);
                }

                if (textBytes != null)
                {
                    foreach (var directory in ProcessTextChunk(keyword, textBytes))
                    {
                        yield return(directory);
                    }
                }
            }
            else if (chunkType == PngChunkType.iTXt)
            {
                var reader            = new SequentialByteArrayReader(bytes);
                var keyword           = reader.GetNullTerminatedStringValue(maxLengthBytes: 79).ToString(_latin1Encoding);
                var compressionFlag   = reader.GetSByte();
                var compressionMethod = reader.GetSByte();

                // TODO we currently ignore languageTagBytes and translatedKeywordBytes
                var languageTagBytes       = reader.GetNullTerminatedBytes(bytes.Length);
                var translatedKeywordBytes = reader.GetNullTerminatedBytes(bytes.Length);

                var bytesLeft = bytes.Length - keyword.Length - 1 - 1 - 1 - languageTagBytes.Length - 1 - translatedKeywordBytes.Length - 1;
                byte[]? textBytes = null;
                if (compressionFlag == 0)
                {
                    textBytes = reader.GetNullTerminatedBytes(bytesLeft);
                }
                else if (compressionFlag == 1)
                {
                    if (compressionMethod == 0)
                    {
                        if (!TryDeflate(bytes, bytesLeft, out textBytes, out string?errorMessage))
                        {
                            var directory = new PngDirectory(PngChunkType.iTXt);
                            directory.AddError($"Exception decompressing {nameof(PngChunkType.iTXt)} chunk with keyword \"{keyword}\": {errorMessage}");
                            yield return(directory);
                        }
                    }
                    else
                    {
                        var directory = new PngDirectory(PngChunkType.iTXt);
                        directory.AddError("Invalid compression method value");
                        yield return(directory);
                    }
                }
                else
                {
                    var directory = new PngDirectory(PngChunkType.iTXt);
                    directory.AddError("Invalid compression flag value");
                    yield return(directory);
                }

                if (textBytes != null)
                {
                    foreach (var directory in ProcessTextChunk(keyword, textBytes))
                    {
                        yield return(directory);
                    }
                }
            }
            else if (chunkType == PngChunkType.tIME)
            {
                var reader    = new SequentialByteArrayReader(bytes);
                var year      = reader.GetUInt16();
                var month     = reader.GetByte();
                int day       = reader.GetByte();
                int hour      = reader.GetByte();
                int minute    = reader.GetByte();
                int second    = reader.GetByte();
                var directory = new PngDirectory(PngChunkType.tIME);
                if (DateUtil.IsValidDate(year, month, day) && DateUtil.IsValidTime(hour, minute, second))
                {
                    var time = new DateTime(year, month, day, hour, minute, second, DateTimeKind.Unspecified);
                    directory.Set(PngDirectory.TagLastModificationTime, time);
                }
                else
                {
                    directory.AddError($"PNG tIME data describes an invalid date/time: year={year} month={month} day={day} hour={hour} minute={minute} second={second}");
                }
                yield return(directory);
            }
            else if (chunkType == PngChunkType.pHYs)
            {
                var reader         = new SequentialByteArrayReader(bytes);
                var pixelsPerUnitX = reader.GetInt32();
                var pixelsPerUnitY = reader.GetInt32();
                var unitSpecifier  = reader.GetSByte();
                var directory      = new PngDirectory(PngChunkType.pHYs);
                directory.Set(PngDirectory.TagPixelsPerUnitX, pixelsPerUnitX);
                directory.Set(PngDirectory.TagPixelsPerUnitY, pixelsPerUnitY);
                directory.Set(PngDirectory.TagUnitSpecifier, unitSpecifier);
                yield return(directory);
            }
            else if (chunkType.Equals(PngChunkType.sBIT))
            {
                var directory = new PngDirectory(PngChunkType.sBIT);
                directory.Set(PngDirectory.TagSignificantBits, bytes);
                yield return(directory);
            }
            else if (chunkType.Equals(PngChunkType.eXIf))
            {
                var directories = new List <Directory>();
                try
                {
                    TiffReader.ProcessTiff(
                        new ByteArrayReader(bytes),
                        new ExifTiffHandler(directories));
                }
                catch (Exception ex)
                {
                    var directory = new PngDirectory(PngChunkType.eXIf);
                    directory.AddError(ex.Message);
                    directories.Add(directory);
                }

                foreach (var directory in directories)
                {
                    yield return(directory);
                }
            }

            yield break;

            IEnumerable <Directory> ProcessTextChunk(string keyword, byte[] textBytes)
            {
                if (keyword == "XML:com.adobe.xmp")
                {
                    yield return(new XmpReader().Extract(textBytes));
                }
                else if (keyword == "Raw profile type xmp")
                {
                    if (TryProcessRawProfile(out int byteCount))
                    {
                        yield return(new XmpReader().Extract(textBytes, 0, byteCount));
                    }
                    else
                    {
                        yield return(ReadTextDirectory(keyword, textBytes, chunkType));
                    }
                }
                else if (keyword == "Raw profile type exif" || keyword == "Raw profile type APP1")
                {
                    if (TryProcessRawProfile(out _))
                    {
                        foreach (var exifDirectory in new ExifReader().Extract(new ByteArrayReader(textBytes)))
                        {
                            yield return(exifDirectory);
                        }
                    }
                    else
                    {
                        yield return(ReadTextDirectory(keyword, textBytes, chunkType));
                    }
                }
                else if (keyword == "Raw profile type icc" || keyword == "Raw profile type icm")
                {
                    if (TryProcessRawProfile(out _))
                    {
                        yield return(new IccReader().Extract(new ByteArrayReader(textBytes)));
                    }
                    else
                    {
                        yield return(ReadTextDirectory(keyword, textBytes, chunkType));
                    }
                }
                else if (keyword == "Raw profile type iptc")
                {
                    if (TryProcessRawProfile(out int byteCount))
                    {
                        yield return(new IptcReader().Extract(new SequentialByteArrayReader(textBytes), byteCount));
                    }
                    else
                    {
                        yield return(ReadTextDirectory(keyword, textBytes, chunkType));
                    }
                }
                else
                {
                    yield return(ReadTextDirectory(keyword, textBytes, chunkType));
                }