cesium压平

Cesium的"flatten"操作通常用于将3D场景中的模型或几何体转换为单个平面网格。这个过程涉及到将所有三角形面片合并成一个网格，并重新计算每个顶点的坐标。

在Cesium中，可以使用以下代码来压平一个3D模型：

var entity = viewer.entities.getById(id); // 获取模型实体

var geometry = entity.geometry;
var attributes = geometry.attributes;

// 创建一个新的几何体
var newGeometry = new Cesium.Geometry({
    attributes: attributes,
    indices: Cesium.IndexDatatype.createTypedArray(attributes.position.values.length / 3, geometry.indices),
    primitiveType: Cesium.PrimitiveType.TRIANGLES,
});

// 将原始模型实体替换为新的压平几何体
viewer.entities.remove(entity);
viewer.entities.add({
    id: id,
    position: entity.position,
    orientation: entity.orientation,
    model: {
        uri: entity.model.uri,
        scale: entity.model.scale,
        minimumPixelSize: entity.model.minimumPixelSize,
        maximumScale: entity.model.maximumScale,
        silhouetteColor: entity.model.silhouetteColor,
        silhouetteSize: entity.model.silhouetteSize,
        distanceDisplayCondition: entity.model.distanceDisplayCondition,
        color: entity.model.color,
        colorBlendMode: entity.model.colorBlendMode,
        colorBlendAmount: entity.model.colorBlendAmount,
        allowPicking: entity.model.allowPicking,
        show: true,
        distanceDisplayCondition: entity.model.distanceDisplayCondition,
        shadows: entity.model.shadows,
        heightReference: entity.model.heightReference,
        silhouetteSupported: entity.model.silhouetteSupported,
        clampAnimations: entity.model.clampAnimations,
        nodeTransformations: entity.model.nodeTransformations,
        // 使用新的压平几何体
        geometryInstances: new Cesium.GeometryInstance({
            geometry: newGeometry,
            attributes: {
                color: Cesium.ColorGeometryInstanceAttribute.fromColor(entity.model.color),
            },
            id: entity.id,
        }),
    },
});

这段代码将获取指定ID的模型实体，然后使用Cesium的Geometry对象创建一个新的压平几何体。最后使用新几何体替换原始模型实体的geometryInstances属性，从而实现将模型压平的效果。