As the equation shows, an a value of 0 results in a completely transparent overlay, whereas a value of 1 results in a completely opaque overlay that disregards the background image entirely.

• 该方程表明，a值为0时，可以实现完全透明的叠图，a值为1时，则叠图完全是不透明的，完全不显示相应区域的背景图像。