说明:绘图区域x轴(0->1),y轴(0->1);
1.y = 0.5 + sqrt(x * (1 - x))
2.y = smoothstep(a , b , x)
y = smoothstep(0.9 , 0.3 , x):
y = smoothstep(0.3 , 0.9 , x)
