1. 由世界坐标转模型坐标

顶点着色器：

attribute vec3 position3DHigh;

attribute vec3 position3DLow;

attribute vec3 normal;

attribute vec2 st;

attribute float batchId;

varying vec3 v_positionEC;

varying vec3 v_normalEC;

varying vec2 v_st;

void main()

{

vec3 positionWC = position3DHigh + position3DLow; // 得到世界坐标

// 官方得到世界坐标(齐次)是这么做的，在三维模式下等价

// vec4 positionWC = czm_computePosition();

vec4 positionMC = czm_inverseModel * vec4(positionWC, 1); // 得到模型坐标

// 以下为官方代码，未改动，仅修改注释

vec4 p = czm_computePosition(); // 得到世界坐标

v_positionEC = (czm_modelViewRelativeToEye * p).xyz; // 得到相机坐标

v_normalEC = czm_normal * normal; // 得到相机坐标系下的法线向量

v_st = st; // 传递uv

gl_Position = czm_modelViewProjectionRelativeToEye * p; // 世界坐标到裁剪空间坐标

}

2. 由相机坐标转模型坐标

顶点着色器

attribute vec3 position3DHigh;

attribute vec3 position3DLow;

attribute vec3 normal;

attribute vec2 st;

attribute float batchId;

varying vec3 v_positionEC;

varying vec3 v_normalEC;

varying vec2 v_st;

void main()

{

vec4 p = czm_computePosition(); // 得到齐次世界坐标

v_positionEC = (czm_modelViewRelativeToEye * p).xyz; // 得到相机坐标

v_normalEC = czm_normal * normal;

v_st = st;

/** 此处开始添加计算模型坐标的代码 */

vec4 positionMC = czm_inverseModelView * vec4(v_positionEC, 1.0); // 得到模型坐标

/** 添加的代码结束 */

gl_Position = czm_modelViewProjectionRelativeToEye * p; // 世界坐标到裁剪空间坐标

}

3. 坐标陷阱：模型坐标系≠东北上坐标系

参考如下代码：

var viewer = new Cesium.Viewer("cesiumContainer");

viewer.scene.globe.depthTestAgainstTerrain = true;

viewer.camera.setView({

destination : new Cesium.Cartesian3(-2644963.9889313546, 5763731.142118295, 2199400.7089496767), //世界坐标系下的一个坐标点

orientation : {//旋转角度

heading :6.075,

pitch :-0.727,

roll : 6.283

}

});

const extrudedPolygon = new Cesium.PolygonGeometry({

polygonHierarchy : new Cesium.PolygonHierarchy(

Cesium.Cartesian3.fromDegreesArray([

112.41726298378288, 23.290411251106182,

113.67072522399741, 23.560312361463682,

114.09370956893551, 22.590768298743153,

112.83803246418894, 22.285610818885644

])

),

extrudedHeight: 30000

});

const instance = new Cesium.GeometryInstance({

geometry: extrudedPolygon,

id: 'box with height'

});

const m = new Cesium.Material({

fabric: {

type: 'Color',

uniforms: {

color: new Cesium.Color(216 / 255.0, 170 / 255.0, 208 / 255.0).withAlpha(0.618),

},

}

});

const aper = new Cesium.MaterialAppearance({

fragmentShaderSource:

` varying vec3 v_positionEC;

varying vec3 v_normalEC;

varying vec2 v_st;

void main()

{

vec3 positionToEyeEC = -v_positionEC;

vec3 normalEC = normalize(v_normalEC);

#ifdef FACE_FORWARD

normalEC = faceforward(normalEC, vec3(0.0, 0.0, 1.0), -normalEC);

#endif

czm_materialInput materialInput;

materialInput.normalEC = normalEC;

materialInput.positionToEyeEC = positionToEyeEC;

materialInput.st = v_st;

czm_material material = czm_getMaterial(materialInput);

material.diffuse = vec3(0.24313725490196078, 0.7372549019607844, 0.9333333333333333);

material.emission = vec3(0.0, 0.66666666, 0.0);

material.specular = 0.5;

material.shininess = 0.8;

#ifdef FLAT

gl_FragColor = vec4(material.diffuse + material.emission, material.alpha);

#else

gl_FragColor = czm_phong(normalize(positionToEyeEC), material, czm_lightDirectionEC);

#endif

}

`,

vertexShaderSource:

`

attribute vec3 position3DHigh;

attribute vec3 position3DLow;

attribute vec3 normal;

attribute vec2 st;

attribute float batchId;

varying vec3 v_positionEC;

varying vec3 v_normalEC;

varying vec2 v_st;

void main()

{

vec4 p = czm_computePosition();

v_positionEC = (czm_modelViewRelativeToEye * p).xyz; // position in eye coordinates

v_normalEC = czm_normal * normal; // normal in eye coordinates

v_st = st;

vec4 positionMC = czm_inverseModelView * vec4(v_positionEC, 1.0);

vec4 positionMC_new = vec4(positionMC.xy, positionMC.z + czm_frameNumber * 100.0, 1.0); // z轴向上平移动画

vec4 resultPosition = czm_modelViewInfiniteProjection * positionMC_new; // 一步直接算到 gl_Position 所需的坐标

gl_Position = resultPosition;

}

`,

});

var p = viewer.scene.primitives.add(new Cesium.Primitive({

geometryInstances: instance,

appearance: aper,

releaseGeometryInstances: false,

compressVertices: false,

}));

在顶点着色器处，我对模型坐标的z值进行了修改，达到z轴平移动画的效果

可是动画的效果并不是沿着地表的垂直向上的方向平移，换做是 x、y 平移也不是对应的正东方、正北方(如果平移量大，还要考虑曲率的问题)

所以，可以下结论：

顶点着色器中的模型坐标所用的局部坐标系，仅仅是原点在模型中心，但是三轴并不是沿着正东x、正北y、垂直朝上z这三轴的。

要格外注意这一点，这是我在使用 Primitive API 时发现的问题。