wycinek z helpa - wsyzstko pisze, jak rozumiesz słowa precision, i not me to już wszystko jasne, nie trzeba nawet angielskiego znać, wystarczy strona http://slowniki.onet.pl/ do przetłumaczenia 3 wyrazów :/
collision_line(x1,y1,x2,y2,obj,prec,notme)
This function tests whether there is a collision between the line segment from (x1,y1) to (x2,y2) and entities of object obj. This is a powerful function. You can e.g. use it to test whether an instance can see another instance by checking whether the line segment between them intersects a wall.
(...)
The argument obj can be an object, the keyword all, or the id of an instance. The argument prec indicates whether the check should be precise or only based on the bounding box of the instance. Precise checking is only done when the sprite for the instance has the precise collision checking set. The argument notme can be set to true to indicate that the calling instance should not be checked. All these functions return either the id of one of the instances that collide, or they return a negative value when there is no collision.