Autotile

//Check for surrounding objects
var xx, yy, obj, check, terrain;
terrain = argument0;
obj = argument1;
xx = argument2;
yy = argument3;
check = 0;

//This is the checking
if !instance_position(xx+20,yy,obj) {check += 2;}
if !instance_position(xx,yy-20,obj) {check += 4;}
if !instance_position(xx-20,yy,obj) {check += 8;}
if !instance_position(xx,yy+20,obj) {check += 16;}

//Now check diagonally
if check = 0 {
if !instance_position(xx+20,yy-20,obj) {check += 32;} else
if !instance_position(xx-20,yy-20,obj) {check += 64;} else
if !instance_position(xx-20,yy+20,obj) {check += 128;} else
if !instance_position(xx+20,yy+20,obj) {check += 256;}
}

//Generate tile accordingly
execute_string(global.t_add[check],terrain,xx,yy);