Digital Signal Processing | Interpolation

void interpolate(float *x, int y, int s, SDL_Point p1, SDL_Point p2) {
    if ((p2.y - p1.y) == 0) return;
    *x = s + (float)((p2.x - p1.x) * (y - p1.y)) / (float)(p2.y - p1.y);
}