2D Painting and calculating the percentage

Hello guys, I’m working on 2D painting using UI Line Renderer. And this feature is, there will be an image like cake, phone, bottle, ect.

Like this, player has to fill this image with color.

How to check if the image is filled with color over certain percentage.
I’ve been searching for it for a while, but I don’t know what key word should I use to help me google it and get the answer.
Please do me a little favor.
Any ideas or even just a key word would also be appreciated.
Thanks !

key word “c# calculate photo transparency percentage”