How to get pixel's coordinates from touch?

Hello!

I'm using my shader, that combines two textures, based on mask texture. I need to make some of mask's pixels "disappear" (Color.clear), but because my object's Mesh Collider is Convex, I can not use RaycastHit.textureCoord.

My question is - "Is there a way to use RaycastHit.textureCoord with convex mesh" or "what other methods I can use to get texture's coordinates from Touch?"

Thank You!