getsetchannel

struct chantype{ enum { normal, mono, stereo }; };

// Returns the leftmost channel number of the take, 0-based, enum of type is normal, mono, stereo
int GetTakeChannel(MediaItem_Take* take, int* chanmode, int* type) {
    int chm, t;

    chm = GetMediaItemTakeInfo_Value(take, "I_CHANMODE");
    int ch = chm;

    // Get channel type
    if (chm == 0) t = chantype::normal;
    else if (in_range(chm, 2, 66)) t = chantype::mono;
    else t = chantype::stereo;

    // Get leftmost channel
    if (chm >= 67) ch = chm - 67;
    else if (chm >= 3) ch = chm - 3;
    else if (chm == 2) ch = chm = 0;

    if (chanmode != nullptr) *chanmode = chm;
    if (type != nullptr) *type = t;
    return ch;
}

void SetTakeChannel(MediaItem_Take* take, int type, int channel) {
    switch (type) {
    case chantype::normal:
        SetMediaItemTakeInfo_Value(take, "I_CHANMODE", 0);
        break;
    case chantype::mono:
        if (channel < 0) channel = GetTakeChannel(take);
        SetMediaItemTakeInfo_Value(take, "I_CHANMODE", clamp(channel+3, 3, 66));
        break;
    case chantype::stereo:
        if (channel < 0) channel = GetTakeChannel(take);
        SetMediaItemTakeInfo_Value(take, "I_CHANMODE", clamp(channel+67, 67, 129));
        break;
    }
}

vector<vector<double>> MultichannelToChannelMode(vector<vector<double>> audio, MediaItem_Take* take) {
    int nch = audio.size();
    vector< vector<double> > v;
    int chanmode;

    size_t frames = 0;
    for (int i = 0; i < audio.size(); ++i) frames = max(frames, audio[i].size());

    size_t ch = GetTakeChannel(take, &chanmode);

    // Stereo
    if (chanmode >= 67) {
        if (nch > ch) v.push_back(audio[ch]); else v.push_back(vector<double>(frames, 0));
        if (nch > ch+1) v.push_back(audio[ch+1]); else v.push_back(vector<double>(frames, 0));
    }

    // Mono
    else if (chanmode >= 3) {
        if (nch > ch) v.push_back(audio[ch]); else v.push_back(vector<double>(frames, 0));
    }

    // Mono Mix (L + R)
    else if (chanmode == 2) {
        vector<double> temp(frames, 0);
        if (nch > 1) for (int c = 0; c < nch; ++c) {
            for (int s = 0; s < audio[c].size(); ++s) temp[s] = audio[0][s] + audio[1][s];
            v.push_back(temp);
        }
        else v.push_back(audio[0]);
    }

    //Reverse Stereo
    else if (chanmode == 1) {
        if (nch > 1) v.push_back(audio[1]);
        v.push_back(audio[0]);
        for (int c = 2; c < nch; ++c) v.push_back(audio[c]);
    }

    //Normal
    else if (chanmode == 0) v = audio;

    return v;
}