/// <summary>
/// Constructor.
/// </summary>
/// <param name="inputStream">
/// The stream from which the image data is to be read, starting with
/// the LZW minimum code size, and ending with a block terminator.
/// </param>
/// <param name="pixelCount">
/// Number of pixels in the image.
/// </param>
/// <remarks>
/// The input stream is read, first into the LZW minimum code size, then
/// into data blocks. Bytes are extracted from the data blocks into a
/// datum until the datum contains enough bits to form a code; this code
/// is then extracted from the datum and decoded into a pixel index.
/// Once all data has been read, or a block terminator,
/// end-of-information code or error condition is encountered, any
/// remaining pixel indices not already populated default to zero.
/// </remarks>
public TableBasedImageData(Stream inputStream, int pixelCount)
{
#region guard against silly image sizes
if (pixelCount < 1)
{
string message
= "The pixel count must be greater than zero. "
+ "Supplied value was " + pixelCount;
throw new ArgumentOutOfRangeException(nameof(pixelCount), message);
}
#endregion
#region declare / initialise local variables
_pixelIndexes = new byte[pixelCount];
int nextAvailableCode; // the next code to be added to the dictionary
int currentCodeSize;
int in_code;
int previousCode;
int code;
int datum = 0; // temporary storage for codes read from the input stream
int meaningfulBitsInDatum = 0; // number of bits of useful information held in the datum variable
int firstCode = 0; // first code read from the stream since last clear code
int indexInDataBlock = 0;
int pixelIndex;
// number of bytes still to be extracted from the current data block
int bytesToExtract = 0;
short[] prefix = new short[MaxStackSize];
byte[] suffix = new byte[MaxStackSize];
Stack <byte> pixelStack = new Stack <byte>();
#endregion
// Initialize GIF data stream decoder.
_lzwMinimumCodeSize = Read(inputStream); // number of bits initially used for LZW codes in image data
int clearCode = ClearCode;
int endOfInformation = EndOfInformation;
nextAvailableCode = clearCode + 2;
previousCode = NullCode;
currentCodeSize = InitialCodeSize;
#region guard against LZW code size being too large
if (clearCode >= MaxStackSize)
{
string message
= "LZW minimum code size: " + _lzwMinimumCodeSize
+ ". Clear code: " + clearCode
+ ". Max stack size: " + MaxStackSize;
SetStatus(ErrorState.LzwMinimumCodeSizeTooLarge, message);
return;
}
#endregion
// TODO: what are prefix and suffix and why are we initialising them like this?
for (code = 0; code < clearCode; code++)
{
prefix[code] = 0;
suffix[code] = (byte)code;
}
#region decode LZW image data
// Initialise block to an empty data block. This will be overwritten
// first time through the loop with a data block read from the input
// stream.
var block = new DataBlock(0, new byte[0]);
for (pixelIndex = 0; pixelIndex < pixelCount;)
{
if (pixelStack.Count == 0)
{
// There are no pixels in the stack at the moment, so...
#region get some pixels and put them on the stack
if (meaningfulBitsInDatum < currentCodeSize)
{
// Then we don't have enough bits in the datum to make
// a code; we need to get some more from the current
// data block, or we may need to read another data
// block from the input stream
#region get another byte from the current data block
if (bytesToExtract == 0)
{
// Then we've extracted all the bytes from the
// current data block, so...
#region read the next data block from the stream
block = ReadDataBlock(inputStream);
bytesToExtract = block.ActualBlockSize;
// Point to the first byte in the new data block
indexInDataBlock = 0;
if (block.TestState(ErrorState.DataBlockTooShort))
{
// then we've reached the end of the stream
// prematurely
break;
}
if (bytesToExtract == 0)
{
// then it's a block terminator, end of the
// image data (this is a data block other than
// the first one)
break;
}
#endregion
}
// Append the contents of the current byte in the data
// block to the beginning of the datum
int newDatum = block[indexInDataBlock] << meaningfulBitsInDatum;
datum += newDatum;
// so we've now got 8 more bits of information in the
// datum.
meaningfulBitsInDatum += 8;
// Point to the next byte in the data block
indexInDataBlock++;
// We've one less byte still to read from the data block
// now.
bytesToExtract--;
// and carry on reading through the data block
continue;
#endregion
}
#region get the next code from the datum
// Get the least significant bits from the read datum, up
// to the maximum allowed by the current code size.
code = datum & GetMaximumPossibleCode(currentCodeSize);
// Drop the bits we've just extracted from the datum.
datum >>= currentCodeSize;
// Reduce the count of meaningful bits held in the datum
meaningfulBitsInDatum -= currentCodeSize;
#endregion
#region interpret the code
#region end of information?
if (code == endOfInformation)
{
// We've reached an explicit marker for the end of the
// image data.
break;
}
#endregion
#region code not in dictionary?
if (code > nextAvailableCode)
{
// We expect the code to be either one which is already
// in the dictionary, or the next available one to be
// added. If it's neither of these then abandon
// processing of the image data.
string message
= "Next available code: " + nextAvailableCode
+ ". Last code read from input stream: " + code;
SetStatus(ErrorState.CodeNotInDictionary, message);
break;
}
#endregion
#region clear code?
if (code == clearCode)
{
// We can get a clear code at any point in the image
// data, this is an instruction to reset the decoder
// and empty the dictionary of codes.
currentCodeSize = InitialCodeSize;
nextAvailableCode = ClearCode + 2;
previousCode = NullCode;
// Carry on reading from the input stream.
continue;
}
#endregion
#region first code since last clear code?
if (previousCode == NullCode)
{
// This is the first code read since the start of the
// image data or the most recent clear code.
// There's no previously read code in memory yet, so
// get the pixel index for the current code and add it
// to the stack.
pixelStack.Push(suffix[code]);
previousCode = code;
firstCode = code;
// and carry on to the next pixel
continue;
}
#endregion
in_code = code;
if (code == nextAvailableCode)
{
pixelStack.Push((byte)firstCode);
code = previousCode;
}
while (code > clearCode)
{
pixelStack.Push(suffix[code]);
code = prefix[code];
}
#endregion
firstCode = (suffix[code]) & 0xff;
pixelStack.Push((byte)firstCode);
#region add a new string to the string table
// This fix is based off of ImageSharp's LzwDecoder.cs:
// https://github.com/SixLabors/ImageSharp/blob/8899f23c1ddf8044d4dea7d5055386f684120761/src/ImageSharp/Formats/Gif/LzwDecoder.cs
// Fix for Gifs that have "deferred clear code" as per here :
// https://bugzilla.mozilla.org/show_bug.cgi?id=55918
if (nextAvailableCode < MaxStackSize)
{
// TESTME: constructor - next available code >- _maxStackSize
prefix[nextAvailableCode] = (short)previousCode;
suffix[nextAvailableCode] = (byte)firstCode;
nextAvailableCode++;
#endregion
#region do we need to increase the code size?
if ((nextAvailableCode & GetMaximumPossibleCode(currentCodeSize)) == 0)
{
// We've reached the largest code possible for this size
if (nextAvailableCode < MaxStackSize)
{
// so increase the code size by 1
currentCodeSize++;
}
}
#endregion
}
previousCode = in_code;
#endregion
}
// Pop all the pixels currently on the stack off, and add them
// to the return value.
_pixelIndexes[pixelIndex] = pixelStack.Pop();
pixelIndex++;
}
#endregion
#region check input stream contains enough data to fill the image
if (pixelIndex < pixelCount)
{
string message
= "Expected pixel count: " + pixelCount
+ ". Actual pixel count: " + pixelIndex;
SetStatus(ErrorState.TooFewPixelsInImageData, message);
}
#endregion
}
private DataBlock ReadDataBlock(Stream inputStream)
{
DataBlock block = new DataBlock(inputStream);
return(block);
}
/// <summary>
/// Skips variable length blocks up to and including next zero length
/// block (block terminator).
/// </summary>
/// <param name="inputStream">
/// The input stream to read.
/// </param>
protected static void SkipBlocksStatic(Stream inputStream)
{
while (DataBlock.SkipStream(inputStream) > 0)
{
}
}
/// <summary>
/// Constructor.
/// </summary>
/// <param name="identificationBlock">
/// Sets the <see cref="IdentificationBlock"/>
/// </param>
/// <param name="applicationData">
/// Sets the <see cref="ApplicationData"/>
/// </param>
public ApplicationExtension(DataBlock identificationBlock,
Collection <DataBlock> applicationData)
{
SaveData(identificationBlock, applicationData);
}
/// <summary>
/// Skips variable length blocks up to and including next zero length
/// block (block terminator).
/// </summary>
/// <param name="inputStream">
/// The input stream to read.
/// </param>
protected void SkipBlocks(Stream inputStream)
{
while (DataBlock.SkipStream(inputStream) > 0)
{
}
}
