Light2D has a wrong shape

Archive
1
:information_source: Attention Topic was automatically imported from the old Question2Answer platform.
:bust_in_silhouette: Asked By D.Q.Li

I am a Godot begginner and is trying to achieve an effect that allows the player to see only a small region around the character. To achieve this I used a light 2D and a canvaslayer.

enter image description here

This is the texture I provided for light2D
However, the result turnsout to be this:

enter image description here
enter image description here1024×328 33 KB

I don’t know whats going on but this is really annoying. Please help me out.