Pixel-Composer/scripts/line_intersect/line_intersect.gml

function line_intersect(x1, y1, x2, y2, x3, y3, x4, y4) {
	var d = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4);
	if(d == 0) return false;
	
	var px = (x1 * y2 - y1 * x2) * (x3 - x4) - (x1 - x2) * (x3 * y4 - y3 * x4);
	var py = (x1 * y2 - y1 * x2) * (y3 - y4) - (y1 - y2) * (x3 * y4 - y3 * x4);
	
	return [ px / d, py / d, d ];
}

function line_is_intersect_ccw(x0, y0, x1, y1, x2, y2) { return (y2 - y0) * (x1 - x0) > (y1 - y0) * (x2 - x0) }

function line_is_intersect(ax0, ay0, ax1, ay1, bx0, by0, bx1, by1) {
	return line_is_intersect_ccw(ax0, ay0, bx0, by0, bx1, by1) != line_is_intersect_ccw(ax1, ay1, bx0, by0, bx1, by1) && 
	       line_is_intersect_ccw(ax0, ay0, ax1, ay1, bx0, by0) != line_is_intersect_ccw(ax0, ay0, ax1, ay1, bx1, by1)
}