Double side shading by real-view-direction

main.js

const viewer = new Cesium.Viewer("cesiumContainer");
const primitives = viewer.scene.primitives;

const fs = `czm_material czm_getMaterial(czm_materialInput materialInput) {
  czm_material m = czm_getDefaultMaterial(materialInput);

  float side = dot(materialInput.positionToEyeEC, materialInput.normalEC);

  m.alpha = side > 0.0 ? 0.9 : 0.4;
  m.diffuse = side > 0.0 ? vec3(0.0, 1.0, 0.0) : vec3(1.0, 0.0, 0.0);

  return m;
}`;

const g = new Cesium.PolygonGeometry({
  closeBottom: false,
  closeTop: false,
  height: 400000,
  extrudedHeight: 800000,
  polygonHierarchy: new Cesium.PolygonHierarchy(
    Cesium.Cartesian3.fromDegreesArray([
      110.0, 20.0,
      115.0, 20.0,
      115.0, 25.0,
      112.5, 22.5,
      110.0, 25.0
    ])
  ),
});

const material = new Cesium.Material({
  fabric: {
    type : 'DoubleSideColor',
    source: fs,
  },
});
const a = new Cesium.MaterialAppearance({
  faceForward: false,
  material,
});
console.log(a.fragmentShaderSource);
const primitive = new Cesium.Primitive({
  geometryInstances: [new Cesium.GeometryInstance({
    geometry: g,
  })],
  appearance: a,
});
primitives.add(primitive);

着色器不能用 -Z 轴当视线的原因很简单.
-Z 轴与三角面平行的时候，透视视角下“里面”是可以看得到的，但是因为平行的关系会在边界处轻易判断成“外面”，参考点积的意义。
所以视线应为视空间下到片元的向量，也即 materialInput.positionToEyeEC