bandpass butterworth filter implementation in C++

Question

I am implementing an image analysis algorithm using openCV and c++, but I found out openCV doesnt have any function for Butterworth Bandpass filter officially. in my project I

Answer 1

I know this is a post on an old thread, and I would usually leave this as a comment, but I'm apparently not able to do that.

In any case, for people searching for similar code, I thought I would post the link from where this code originates (it also has C code for other types of Butterworth filter coefficients and some other cool signal processing code).

The code is located here: http://www.exstrom.com/journal/sigproc/

Additionally, I think there is a piece of code which calculates said scaling factor for you already.

/**********************************************************************
sf_bwbp - calculates the scaling factor for a butterworth bandpass filter.
The scaling factor is what the c coefficients must be multiplied by so
that the filter response has a maximum value of 1.
*/

double sf_bwbp( int n, double f1f, double f2f )
{
    int k;            // loop variables
    double ctt;       // cotangent of theta
    double sfr, sfi;  // real and imaginary parts of the scaling factor
    double parg;      // pole angle
    double sparg;     // sine of pole angle
    double cparg;     // cosine of pole angle
    double a, b, c;   // workspace variables

    ctt = 1.0 / tan(M_PI * (f2f - f1f) / 2.0);
    sfr = 1.0;
    sfi = 0.0;

    for( k = 0; k < n; ++k )
    {
        parg = M_PI * (double)(2*k+1)/(double)(2*n);
        sparg = ctt + sin(parg);
        cparg = cos(parg);
        a = (sfr + sfi)*(sparg - cparg);
        b = sfr * sparg;
        c = -sfi * cparg;
        sfr = b - c;
        sfi = a - b - c;
    }

    return( 1.0 / sfr );
}