/// <summary>
/// Compute the dct of a given vector
/// </summary>
/// <param name="featureVector">vector of input series</param>
/// <returns>the dct of R</returns>
internal static Digest ComputeDct(float[] featureVector)
{
int N = featureVector.Length;
float[] R = featureVector;
float[] coefficients = new float[Digest.LENGTH];
float max = 0f;
float min = 0f;
float div2n = MathF.PI / (2 * N);
float divSq = 1 / MathF.Sqrt(N);
for (int k = 0; k < Digest.LENGTH; k++)
{
float sum = 0f;
for (int n = 0; n < N; n++)
{
sum += R[n] * MathF.Cos((2 * n + 1) * k * div2n);
}
float v = coefficients[k] = sum * divSq * (k == 0 ? 1 : SQRT_TWO);
max = Math.Max(max, v);
min = Math.Min(min, v);
}
return(NormalizeDigest(coefficients, max, min));
}
/// <summary>
/// return dct matrix, C Return DCT matrix of square size, <paramref name="size" />
/// </summary>
/// <param name="size">int denoting the size of the square matrix to create.</param>
/// <returns>size <paramref name="size" />x<paramref name="size" /> containing the dct matrix</returns>
internal static FloatImage CreateDctMatrix(int size)
{
FloatImage ret = new FloatImage(size, size, 1 / (float)Math.Sqrt(size));
float c1 = MathF.Sqrt(2f / size);
float rad = MathF.PI / 2 / size;
for (int x = 0; x < size; x++)
{
for (int y = 1; y < size; y++)
{
ret[x, y] = c1 * MathF.Cos(rad * y * (2 * x + 1));
}
}
return(ret);
}