public static bool GetWatermarkDctForBlock(MatrixBase<Color> block, int bitIndex, ZigZagAccessorFactory<double> zigzagAccessorFactory, Double[,] quantizationTable,
ChannelType channelType, bool log = false)
{
var yCbCrColorBlock = block.ToYCbCr();
var yChannel = yCbCrColorBlock.GetChannel(channelType);
var shiftetBlock = yChannel - 128;
var dctBlockTransform = (DiscreteCosineTransform.ForwardDct8Block(shiftetBlock).CrossDivide(quantizationTable)).Round();
if (log)
{
_debugDecodeStringBuilder.AppendLine(dctBlockTransform.ToString());
}
var accessor = zigzagAccessorFactory.Access(dctBlockTransform);
var coefficient = accessor[bitIndex];
var curentElelement = (int) Math.Round(coefficient);
bool isEven = curentElelement%2 == 0;
return isEven;
}
private static MatrixBase<Color> WatermarkDctBlock(MatrixBase<Color> block, ZigZagAccessorFactory<Double> accessorFactory, double[,] quantizationTable, bool canEncrypt,
byte[] secretMessage,
ref int embeddingSecretMessageBitIndex, int byteIndexForEncryption, ChannelType channelType)
{
if (!canEncrypt)
{
return block;
}
var byteIndex = embeddingSecretMessageBitIndex / 8;
if (embeddingSecretMessageBitIndex % 8 == 0)
{
byteIndex = embeddingSecretMessageBitIndex / 8;
_debugEncodeStringBuilder.AppendLine("Appending byte:" + (byteIndex + 1));
}
var correction = 1;
bool embeddCorrectly;
MatrixBase<Color> ret;
//#region Convert block
var yCbCrColorBlock = block.ToYCbCr();
var channel = yCbCrColorBlock.GetChannel(channelType);
channel = channel - 128;
DoubleMatrix dctBlockTransform = null;
//#endregion
do
{
// For luminance channel yMax = 127, ymin = -128
DoubleMatrix stegoBlock;
bool redo = false;
var nextSecretBit = MathUtilities.GetBit(secretMessage, embeddingSecretMessageBitIndex);
bool changed = false;
dctBlockTransform = DiscreteCosineTransform.ForwardDct8Block(channel);
dctBlockTransform = (dctBlockTransform.CrossDivide(quantizationTable));
dctBlockTransform = dctBlockTransform.Round();
var accessor = accessorFactory.Access(dctBlockTransform);
var curentElelement = accessor[byteIndexForEncryption];
bool isEven = ((int) Math.Round(curentElelement))%2 == 0;
double possibleValue = curentElelement;
DoubleMatrix oldStegoBlock = null;
do
{
if ((isEven && nextSecretBit) || (!isEven && !nextSecretBit))
{
if (!redo)
{
possibleValue = isEven ? curentElelement - correction : curentElelement + correction;
}
else
{
possibleValue = isEven ? curentElelement + correction : curentElelement - correction;
}
changed = true;
}
accessor[byteIndexForEncryption] = possibleValue;
stegoBlock = dctBlockTransform.CrossProduct(quantizationTable);
stegoBlock = DiscreteCosineTransform.InverseDct8Block(stegoBlock);
if (!redo && changed)
{
for (var i = 0; i < 8; i++)
{
for (var j = 0; j < 8; j++)
{
var value = Math.Round(stegoBlock[i, j]);
if (value < -128 || value > 127)
{
redo = true;
oldStegoBlock = stegoBlock;
break;
}
}
if (redo)
{
break;
}
}
}
else
{
if (changed)
{
var maximumValue = oldStegoBlock.MaximumValue;
var minimumValue = oldStegoBlock.MinimumValue;
var currentMaximumValue = stegoBlock.MaximumValue;
var currentMinimumValue = stegoBlock.MinimumValue;
if (currentMinimumValue < -128 || currentMaximumValue > 127)
{
if ((currentMinimumValue < -128 && minimumValue > currentMinimumValue) || (currentMaximumValue > 127 && maximumValue < currentMaximumValue))
{
stegoBlock = oldStegoBlock;
}
}
}
redo = false;
}
} while (redo);
stegoBlock = stegoBlock + 128; //
var tmpyCbCrColorBlock = yCbCrColorBlock.Copy;
tmpyCbCrColorBlock.UpdateChannel(channelType, stegoBlock);
ret = tmpyCbCrColorBlock.ToRgb();
#region Verification (check if embeddding is correctly (rounding problems might occurs))
if (changed)
{
embeddCorrectly = !GetWatermarkDctForBlock(ret, byteIndexForEncryption, accessorFactory, quantizationTable, channelType) == nextSecretBit;
if (!embeddCorrectly)
{
correction = correction + 2;
}
else
{
correction = 1;
}
}
else
{
embeddCorrectly = true;
correction = 1;
}
#endregion
} while (!embeddCorrectly);
_debugEncodeStringBuilder.AppendLine(dctBlockTransform.ToString());
embeddingSecretMessageBitIndex++;
return ret;
}