void gradient_horizontal(Surface *sf, int w, int h);
void gradient_diagonal(Surface *sf, int w, int h);
void gradient_crossdiagonal(Surface *sf, int w, int h);
void gradient_horizontal(Surface *sf, int w, int h);
void gradient_diagonal(Surface *sf, int w, int h);
void gradient_crossdiagonal(Surface *sf, int w, int h);