Mask with partial transparent image won't take an effect on child image

Unity Engine
1

Mask with partial transparent image won’t take an effect on child image

  1. What happened
    I generated an image to use as a mask but the mask is set by the bounderies of the image component instead of the image content

  2. How we can reproduce it using the example you attached
    i. Right click → UI → Image (= Image1)
    ii. Right click on Image1 → UI → Image (=Image2)
    iii. Add a sprite to Image1 & Image2
    iv. Add a mask component to Image1
    result : Image2 is masking only by the bounderies of image 1 instead of the image shape

** That’s was working fine on version 5.2.3f1 but never on later versions
** Current version 5.3.4f1

2

I met same problem, but found a similar question here:

still got no answers，all I can do is to avoid using partial transparent Image as mask, damn