I needed just a simple way to know all the places that a spot could “see” that were able to be got to by drawing an unbroken line to them.

	
    var degrees = 2 // number of degrees for each beam of light. Smaller numbers make more beams. 
    var maxlength = 30 // how many checks to do in a line
    var currentlength = 0
    var checklength = 10; // How long should each check be 

    var xcheck = x;
    var ycheck = y;
    var gottox = x;
    var gottoy = y;

    var i;

    for (i = 0;i < 360; i += degrees)  {

	currentlength = 0
	xcheck = x;
	ycheck = y;

	while (
	    ! collision_line(xcheck,ycheck,gottox,gottoy,obj_wall,true,true) and 
	    currentlength < maxlength) {

	    gottox = xcheck;
	    gottoy = ycheck;

	    xcheck = xcheck+lengthdir_x(checklength,i); 
	    ycheck = ycheck+lengthdir_y(checklength,i); 

	    currentlength ++

	} 

	draw_set_colour(c_yellow)
	draw_line(gottox,gottoy,x,y);

    }

	

Things I would improve.

• This version doesn’t cache the results like the final game did so if you go back to a spot it recalculates what it can see.
• Rather than drawing the line outwards from the centre I would have done a binary chop on the line as I think this is the most efficient way to find the place on the line there is a collisions.