function getCurrentExtent() {
        // 范圍對象
        var extent = {};

        // 得到當前三維場景
        var scene = viewer.scene;

        // 得到當前三維場景的橢球體
        var ellipsoid = scene.globe.ellipsoid;
        var canvas = scene.canvas;

        // canvas左上角
        var car3_lt = viewer.camera.pickEllipsoid(new Cesium.Cartesian2(0, 0), ellipsoid);

        // canvas右下角
        var car3_rb = viewer.camera.pickEllipsoid(new Cesium.Cartesian2(canvas.width, canvas.height), ellipsoid);

        // 當canvas左上角和右下角全部在橢球體上
        if (car3_lt && car3_rb) {
            var carto_lt = ellipsoid.cartesianToCartographic(car3_lt);
            var carto_rb = ellipsoid.cartesianToCartographic(car3_rb);
            extent.xmin = Cesium.Math.toDegrees(carto_lt.longitude);
            extent.ymax = Cesium.Math.toDegrees(carto_lt.latitude);
            extent.xmax = Cesium.Math.toDegrees(carto_rb.longitude);
            extent.ymin = Cesium.Math.toDegrees(carto_rb.latitude);
        }

        // 當canvas左上角不在但右下角在橢球體上
        else if (!car3_lt && car3_rb) {
            var car3_lt2 = null;
            var yIndex = 0;
            do {
                // 這里每次10像素遞加，一是10像素相差不大，二是為了提高程序運行效率
                yIndex <= canvas.height ? yIndex += 10 : canvas.height;
                car3_lt2 = viewer.camera.pickEllipsoid(new Cesium.Cartesian2(0, yIndex), ellipsoid);
            } while (!car3_lt2);
            var carto_lt2 = ellipsoid.cartesianToCartographic(car3_lt2);
            var carto_rb2 = ellipsoid.cartesianToCartographic(car3_rb);
            extent.xmin = Cesium.Math.toDegrees(carto_lt2.longitude);
            extent.ymax = Cesium.Math.toDegrees(carto_lt2.latitude);
            extent.xmax = Cesium.Math.toDegrees(carto_rb2.longitude);
            extent.ymin = Cesium.Math.toDegrees(carto_rb2.latitude);
        }

        // 獲取高度
        extent.height = Math.ceil(viewer.camera.positionCartographic.height);
        return extent;
    }

viewer.camera.computeViewRectangle()