superbuild/modules/openal-soft/Alc/filters/nfc.h

#ifndef FILTER_NFC_H
#define FILTER_NFC_H

struct NfcFilter1 {
    float base_gain, gain;
    float b1, a1;
    float z[1];
};
struct NfcFilter2 {
    float base_gain, gain;
    float b1, b2, a1, a2;
    float z[2];
};
struct NfcFilter3 {
    float base_gain, gain;
    float b1, b2, b3, a1, a2, a3;
    float z[3];
};
struct NfcFilter4 {
    float base_gain, gain;
    float b1, b2, b3, b4, a1, a2, a3, a4;
    float z[4];
};

class NfcFilter {
    NfcFilter1 first;
    NfcFilter2 second;
    NfcFilter3 third;
    NfcFilter4 fourth;

public:
    /* NOTE:
     * w0 = speed_of_sound / (source_distance * sample_rate);
     * w1 = speed_of_sound / (control_distance * sample_rate);
     *
     * Generally speaking, the control distance should be approximately the
     * average speaker distance, or based on the reference delay if outputing
     * NFC-HOA. It must not be negative, 0, or infinite. The source distance
     * should not be too small relative to the control distance.
     */

    void init(const float w1) noexcept;
    void adjust(const float w0) noexcept;

    /* Near-field control filter for first-order ambisonic channels (1-3). */
    void process1(float *RESTRICT dst, const float *RESTRICT src, const int count);

    /* Near-field control filter for second-order ambisonic channels (4-8). */
    void process2(float *RESTRICT dst, const float *RESTRICT src, const int count);

    /* Near-field control filter for third-order ambisonic channels (9-15). */
    void process3(float *RESTRICT dst, const float *RESTRICT src, const int count);

    /* Near-field control filter for fourth-order ambisonic channels (16-24). */
    void process4(float *RESTRICT dst, const float *RESTRICT src, const int count);
};

#endif /* FILTER_NFC_H */