three或cesium只让在相机视野内的模型显示，看不见的隐藏，并分别给出详细代码

对于Three.js，可以使用Frustum（锥形体）来实现只让在相机视野内的模型显示，看不见的隐藏。具体代码如下：

// 创建相机
var camera = new THREE.PerspectiveCamera(45, window.innerWidth / window.innerHeight, 1, 1000);
camera.position.z = 5;

// 创建Scene
var scene = new THREE.Scene();

// 创建Cube
var geometry = new THREE.BoxGeometry();
var material = new THREE.MeshBasicMaterial({ color: 0x00ff00 });
var cube = new THREE.Mesh(geometry, material);
scene.add(cube);

// 创建渲染器
var renderer = new THREE.WebGLRenderer();
renderer.setSize(window.innerWidth, window.innerHeight);
document.body.appendChild(renderer.domElement);

// 动画循环
function animate() {
    requestAnimationFrame(animate);
    renderer.render(scene, camera);
}
animate();

// 判断Cube是否在相机视野内
function isObjectVisible(object, camera) {
    var frustum = new THREE.Frustum();
    var cameraViewProjectionMatrix = camera.projectionMatrix.multiply(camera.matrixWorldInverse);
    frustum.setFromMatrix(cameraViewProjectionMatrix);
    return frustum.intersectsObject(object);
}

// 每一帧判断Cube是否在相机视野内
function checkObjectVisibility() {
    if (isObjectVisible(cube, camera)) {
        cube.visible = true;
    } else {
        cube.visible = false;
    }
    requestAnimationFrame(checkObjectVisibility);
}
checkObjectVisibility();

对于Cesium，可以使用CullingVolume（裁剪体）来实现只让在相机视野内的模型显示，看不见的隐藏。具体代码如下：

// 创建Viewer
var viewer = new Cesium.Viewer('cesiumContainer');

// 创建Box
var box = viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883),
    box: {
        dimensions: new Cesium.Cartesian3(400000.0, 300000.0, 500000.0),
        material: Cesium.Color.RED
    }
});

// 判断Box是否在相机视野内
function isObjectVisible(object) {
    var cullingVolume = viewer.scene.camera.frustum.computeCullingVolume(viewer.scene.camera.positionWC, viewer.scene.camera.directionWC, viewer.scene.camera.upWC);
    return cullingVolume.computeVisibility(new Cesium.BoundingSphere(object.position.getValue(), Cesium.Cartesian3.magnitude(object.box.dimensions.getValue()))) !== 0;
}

// 每一帧判断Box是否在相机视野内
function checkObjectVisibility() {
    if (isObjectVisible(box)) {
        box.show = true;
    } else {
        box.show = false;
    }
    requestAnimationFrame(checkObjectVisibility);
}
checkObjectVisibility();

需要注意的是，这两段代码只是简单的示例，实际应用中可能需要更复杂的逻辑来判断模型是否在相机视野内。