finger
sc->finger = 0;
dx = sc->pos_x[sc->finger] - sc->pre_pos_x;
dy = sc->pos_y[sc->finger] - sc->pre_pos_y;
sc->finger = 0;
sc->finger == 0) {
sc->finger = 1;
sc->finger == 1) {
sc->finger = 0;
dx, dy, sc->finger);
sc->pre_pos_x = sc->pos_x[sc->finger];
sc->pre_pos_y = sc->pos_y[sc->finger];
uint8_t finger; /* 0 or 1 *, check which finger moving */