cesium 局部区域等高线实现

要实现局部区域等高线，需要先获取该区域的高程数据。然后，可以使用Cesium的TerrainProvider来加载高程数据，并利用Cesium的Terrain-quantized-mesh来渲染地形。接着，将等高线数据转换成多边形，并使用Cesium的PolygonGeometry来渲染等高线。最后，通过调整等高线的颜色和透明度，使其更加清晰可见。

以下是一个实现局部区域等高线的示例代码：

// 加载地形
var terrainProvider = new Cesium.CesiumTerrainProvider({
    url : 'https://assets.agi.com/stk-terrain/world',
    requestVertexNormals : true
});
viewer.terrainProvider = terrainProvider;

// 获取局部区域高程数据
var rectangle = Cesium.Rectangle.fromDegrees(110, 20, 120, 30);
var terrainData = terrainProvider.requestTileGeometry(0, 0, 0);

// 转换等高线数据
var contourData = generateContourData(terrainData, rectangle, 100);

// 渲染等高线
var contourGeometry = Cesium.PolygonGeometry.fromContour({
    polygonHierarchy : new Cesium.PolygonHierarchy(contourData),
    height : 0,
    extrudedHeight : 0
});
var contourInstance = new Cesium.GeometryInstance({
    geometry : contourGeometry,
    attributes : {
        color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.RED.withAlpha(0.5))
    }
});
var contourPrimitive = new Cesium.Primitive({
    geometryInstances : contourInstance,
    appearance : new Cesium.PerInstanceColorAppearance()
});
viewer.scene.primitives.add(contourPrimitive);

// 生成等高线数据
function generateContourData(terrainData, rectangle, interval) {
    var longitudeExtent = Cesium.Math.toDegrees(terrainData.longitudeExtent);
    var latitudeExtent = Cesium.Math.toDegrees(terrainData.latitudeExtent);
    var terrainWidth = terrainData._uVertexCount - 1;
    var terrainHeight = terrainData._vVertexCount - 1;
    var gridWidth = longitudeExtent / terrainWidth;
    var gridHeight = latitudeExtent / terrainHeight;
    var minHeight = Number.MAX_VALUE;
    var maxHeight = Number.MIN_VALUE;

    var contourData = [];
    for (var i = 0; i < terrainWidth; i++) {
        for (var j = 0; j < terrainHeight; j++) {
            var x = rectangle.west + i * gridWidth;
            var y = rectangle.south + j * gridHeight;
            var height = terrainData.getHeight(i, j);
            minHeight = Math.min(minHeight, height);
            maxHeight = Math.max(maxHeight, height);
        }
    }

    for (var k = minHeight + interval; k <= maxHeight; k += interval) {
        var contourLine = [];
        for (var i = 0; i < terrainWidth; i++) {
            for (var j = 0; j < terrainHeight; j++) {
                var x = rectangle.west + i * gridWidth;
                var y = rectangle.south + j * gridHeight;
                var height = terrainData.getHeight(i, j);
                if (height >= k && height < k + interval) {
                    var p1 = Cesium.Cartesian3.fromDegrees(x, y, height);
                    var p2 = Cesium.Cartesian3.fromDegrees(x + gridWidth, y, terrainData.getHeight(i + 1, j));
                    var p3 = Cesium.Cartesian3.fromDegrees(x + gridWidth, y + gridHeight, terrainData.getHeight(i + 1, j + 1));
                    var p4 = Cesium.Cartesian3.fromDegrees(x, y + gridHeight, terrainData.getHeight(i, j + 1));
                    contourLine.push(p1, p2, p3, p4, p1);
                }
            }
        }
        if (contourLine.length > 0) {
            contourData.push(contourLine);
        }
    }

    return contourData;
}

需要注意的是，由于等高线的渲染是通过多边形来实现的，因此等高线的精度和效率都与多边形数量有关。因此，在实际应用中需要根据需求和性能做出权衡。