/// <summary>
/// Reads an tEXt Chunk from file. The current position must be set
/// to the start of the Chunk Data. Such a Chunk contains plain
/// keywords.
/// </summary>
/// <param name="data_length">
/// A <see cref="System.Int32"/> with the length of the Chunk Data.
/// </param>
private void ReadtEXtChunk(int data_length)
{
long position = Tell;
// tEXt Chunk
//
// N Bytes Keyword
// 1 Byte Null Separator
// N Bytes Txt
//
// Followed by 4 Bytes CRC data
ByteVector data = ReadChunkData(data_length);
CheckCRC(tEXt_CHUNK_TYPE, data, ReadCRC());
int keyword_terminator;
string keyword = ReadKeyword(data, 0, out keyword_terminator);
string value = data.Mid(keyword_terminator + 1).ToString();
PngTag png_tag = GetTag(TagTypes.Png, true) as PngTag;
if (png_tag.GetKeyword(keyword) == null)
{
png_tag.SetKeyword(keyword, value);
}
AddMetadataBlock(position - 8, data_length + 8 + 4);
}
/// <summary>
/// Reads an zTXt Chunk from file. The current position must be set
/// to the start of the Chunk Data. Such a Chunk contains compressed
/// keywords.
/// </summary>
/// <param name="data_length">
/// A <see cref="System.Int32"/> with the length of the Chunk Data.
/// </param>
/// <remarks>
/// The Chunk may also contain compressed Exif data which is written
/// by other tools. But, since the PNG specification does not support
/// Exif data, we ignore it here.
/// </remarks>
private void ReadzTXtChunk(int data_length)
{
long position = Tell;
// zTXt Chunk
//
// N Bytes Keyword
// 1 Byte Null Separator
// 1 Byte Compression Method
// N Bytes Txt
//
// Followed by 4 Bytes CRC data
ByteVector data = ReadChunkData(data_length);
CheckCRC(zTXt_CHUNK_TYPE, data, ReadCRC());
int terminator_index;
string keyword = ReadKeyword(data, 0, out terminator_index);
if (terminator_index + 1 >= data_length)
{
throw new CorruptFileException("Compression Method byte expected");
}
byte compression_method = data[terminator_index + 1];
ByteVector plain_data = Decompress(compression_method, data.Mid(terminator_index + 2));
// ignore unknown compression methods
if (plain_data == null)
{
return;
}
string value = plain_data.ToString();
PngTag png_tag = GetTag(TagTypes.Png, true) as PngTag;
if (png_tag.GetKeyword(keyword) == null)
{
png_tag.SetKeyword(keyword, value);
}
AddMetadataBlock(position - 8, data_length + 8 + 4);
}
/// <summary>
/// Reads an iTXt Chunk from file. The current position must be set
/// to the start of the Chunk Data. Such a Chunk may contain XMP data
/// or translated keywords.
/// </summary>
/// <param name="data_length">
/// A <see cref="System.Int32"/> with the length of the Chunk Data.
/// </param>
private void ReadiTXtChunk(int data_length)
{
long position = Tell;
// iTXt Chunk
//
// N Bytes Keyword
// 1 Byte Null Separator
// 1 Byte Compression Flag (0 for uncompressed data)
// 1 Byte Compression Method
// N Bytes Language Tag
// 1 Byte Null Separator
// N Bytes Translated Keyword
// 1 Byte Null Terminator
// N Bytes Txt
//
// Followed by 4 Bytes CRC data
ByteVector data = ReadChunkData(data_length);
CheckCRC(iTXt_CHUNK_TYPE, data, ReadCRC());
// handle XMP, which has a fixed header
if (data.StartsWith(XMP_CHUNK_HEADER))
{
ImageTag.AddTag(new XmpTag(data.Mid(XMP_CHUNK_HEADER.Length).ToString(StringType.UTF8), this));
AddMetadataBlock(position - 8, data_length + 8 + 4);
return;
}
int terminator_index;
string keyword = ReadKeyword(data, 0, out terminator_index);
if (terminator_index + 2 >= data_length)
{
throw new CorruptFileException("Compression Flag and Compression Method byte expected");
}
byte compression_flag = data[terminator_index + 1];
byte compression_method = data[terminator_index + 2];
//string language = ReadTerminatedString (data, terminator_index + 3, out terminator_index);
//string translated_keyword = ReadTerminatedString (data, terminator_index + 1, out terminator_index);
ByteVector txt_data = data.Mid(terminator_index + 1);
if (compression_flag != 0x00)
{
txt_data = Decompress(compression_method, txt_data);
// ignore unknown compression methods
if (txt_data == null)
{
return;
}
}
string value = txt_data.ToString();
PngTag png_tag = GetTag(TagTypes.Png, true) as PngTag;
if (png_tag.GetKeyword(keyword) == null)
{
png_tag.SetKeyword(keyword, value);
}
AddMetadataBlock(position - 8, data_length + 8 + 4);
}