GMS Shadows

///draw shadows
//manage surface
if(surface_exists(surf_shadow)){
    surface_set_target(surf_shadow);
    draw_clear_alpha(0, 0);
}else{
    surf_shadow = surface_create(room_width,room_height);
    surface_set_target(surf_shadow);
}

//handle every shadow in the room
with(o_5_char_shadow){
    if(parent>-1){
        if(instance_exists(parent)){
            if(parent.visible){
                //transformation points
                var px = parent.x;
                var py = parent.y;
                var w = sprite_get_width(parent.sprite_index)*global.zoom;
                var h = parent.footground*global.zoom;
                var wd = 50; //horizontal iclination
                var hd = h/1.5; //vertical inclination
                var xoff = (sprite_get_xoffset(parent.sprite_index)+1)*global.zoom;
                var yoff = parent.sprite_height-(parent.footground+1)*global.zoom;
                //check if there's the necessity of duplicating a new sprite
                if(parent.sprite_index!=last_sprite){
                    //delete previous duplicated sprite
                    if(sp!=-1) sprite_delete(sp);
                    //create new duplicated sprite
                    sp = sprite_duplicate(parent.sprite_index);
                    sprite_collision_mask(sp,false, 1, 0, 0, 0, 0, 1, 0);
                    last_sprite = parent.sprite_index;
                }
                sprite_index = sp;
                //draw skewed sprite
                draw_sprite_pos(sprite_index,
                                parent.image_index,
                                px+wd-xoff,
                                py+h+hd,
                                px+w+wd-xoff,
                                py+h+hd,
                                px+w-xoff,
                                py+h-yoff,
                                px-xoff,
                                py+h-yoff,
                                1);
            }
        }else{
            instance_destroy();
        }
    }
}

//reset drawing target
surface_reset_target();
//draw the modified shadow sprites with black blending color and 20% alpha
draw_surface_ext(surf_shadow, 0, 0, 1, 1, 0, c_black, 0.1);