tri
    select triangular, linear slope (default)

qsin
    select quarter of sine wave

hsin
    select half of sine wave

esin
    select exponential sine wave

log
    select logarithmic

ipar
    select inverted parabola

qua
    select quadratic

cub
    select cubic

squ
    select square root

cbr
    select cubic root

par
    select parabola

exp
    select exponential

iqsin
    select inverted quarter of sine wave

ihsin
    select inverted half of sine wave

dese
    select double-exponential seat

desi
    select double-exponential sigmoid

switch (curve) {
case QSIN:
    gain = sin(gain * M_PI / 2.0);
    break;
case IQSIN:
    /* 0.6... = 2 / M_PI */
    gain = 0.6366197723675814 * asin(gain);
    break;
case ESIN:
    gain = 1.0 - cos(M_PI / 4.0 * (CUBE(2.0*gain - 1) + 1));
    break;
case HSIN:
    gain = (1.0 - cos(gain * M_PI)) / 2.0;
    break;
case IHSIN:
    /* 0.3... = 1 / M_PI */
    gain = 0.3183098861837907 * acos(1 - 2 * gain);
    break;
case EXP:
    /* -11.5... = 5*ln(0.1) */
    gain = exp(-11.512925464970227 * (1 - gain));
    break;
case LOG:
    gain = av_clipd(1 + 0.2 * log10(gain), 0, 1.0);
    break;
case PAR:
    gain = 1 - sqrt(1 - gain);
    break;
case IPAR:
    gain = (1 - (1 - gain) * (1 - gain));
    break;
case QUA:
    gain *= gain;
    break;
case CUB:
    gain = CUBE(gain);
    break;
case SQU:
    gain = sqrt(gain);
    break;
case CBR:
    gain = cbrt(gain);
    break;
case DESE:
    gain = gain <= 0.5 ? cbrt(2 * gain) / 2: 1 - cbrt(2 * (1 - gain)) / 2;
    break;
case DESI:
    gain = gain <= 0.5 ? CUBE(2 * gain) / 2: 1 - CUBE(2 * (1 - gain)) / 2;
    break;
}