seen = scrCamSees(objPlayer.x, objPlayer.y);
    if (seen) {
        with (objPlayer) {
            var wallpaper = instance_position(x, y, objWallpaper);
            if (instance_exists(wallpaper)
                    && wallpaper.image_blend == image_blend) {
                other.seen = false;
            } else {
                scrShapeshift(false);
            }
        }
    }

Go play!