接着这个吐槽继续： 

有空去翻了翻vgmstream的C代码，发现如下段落： 
void mixing_macro_downmix(VGMSTREAM* vgmstream, int max /*, mapping_t output_mapping*/) {
    mixer_t* mixer = vgmstream->mixer;
    int output_channels, mp_in, mp_out, ch_in, ch_out;
    channel_mapping_t input_mapping, output_mapping;
    const double vol_max = 1.0;
    const double vol_sqrt = 1 / sqrt(2);
    const double vol_half = 1 / 2;
    double mat...