Cesium进阶教程（1）customShader的使用

在 Cesium 中，CustomShader 是一个非常强大的接口，用来自定义图形渲染过程中的 GLSL 着色逻辑，让开发者可以对 3D Tiles、模型（Model）或 Primitive 等对象实现更灵活、复杂的视觉效果。

CustomShader 让你可以在 Cesium 的标准渲染管线中插入自己的 GLSL 代码，从而控制模型的 顶点变换 和 片元着色 过程。
也就是说，你可以让模型发光、流动、闪烁、根据高度变色、按时间变化、根据法线方向渐变等。

基本原理

在 Cesium 中，每个 3D 模型或 Tileset 都会使用一个默认的渲染着色器（Shader Program）。
而 CustomShader 的作用就是允许你在保留 Cesium 默认功能的前提下，往其中注入自定义的 GLSL 代码。

它主要能影响两个阶段：

• Vertex Shader 阶段（顶点处理）
• Fragment Shader 阶段（像素着色）

在这些阶段里面可以获取到一些重要的数据

| Corresponding Attribute in Model | variable in shader | Type    | Available in Vertex Shader? | Available in Fragment Shader? | Description                                                                                                                                                              |
| -------------------------------- | ------------------ | ------- | --------------------------- | ----------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `POSITION`                       | `positionMC`       | `vec3`  | Yes                         | Yes                           | Position in model coordinates                                                                                                                                            |
| `POSITION`                       | `positionWC`       | `vec3`  | No                          | Yes                           | Position in world coordinates (WGS84 ECEF `(x, y, z)`). Low precision.                                                                                                   |
| `POSITION`                       | `positionEC`       | `vec3`  | No                          | Yes                           | Position in eye coordinates.                                                                                                                                             |
| `NORMAL`                         | `normalMC`         | `vec3`  | Yes                         | No                            | Unit-length normal vector in model coordinates. Only available in the vertex shader                                                                                      |
| `NORMAL`                         | `normalEC`         | `vec3`  | No                          | Yes                           | Unit-length normal vector in eye coordinates. Only available in the fragment shader                                                                                      |
| `TANGENT`                        | `tangentMC`        | `vec3`  | Yes                         | No                            | Unit-length tangent vector in model coordinates. This is always a `vec3`. For models that provide a `w` component, that is removed after computing the bitangent vector. |
| `TANGENT`                        | `tangentEC`        | `vec3`  | No                          | Yes                           | Unit-length tangent vector in eye coordinates. This is always a `vec3`. For models that provide a `w` component, that is removed after computing the bitangent vector.   |
| `NORMAL` & `TANGENT`             | `bitangentMC`      | `vec3`  | Yes                         | No                            | Unit-length bitangent vector in model coordinates. Only available when both normal and tangent vectors are available.                                                    |
| `NORMAL` & `TANGENT`             | `bitangentEC`      | `vec3`  | No                          | Yes                           | Unit-length bitangent vector in eye coordinates. Only available when both normal and tangent vectors are available.                                                      |
| `TEXCOORD_N`                     | `texCoord_N`       | `vec2`  | Yes                         | Yes                           | `N`-th set of texture coordinates.                                                                                                                                       |
| `COLOR_N`                        | `color_N`          | `vec4`  | Yes                         | Yes                           | `N`-th set of vertex colors. This is always a `vec4`; if the model does not specify an alpha value, it is assumed to be 1.                                               |
| `JOINTS_N`                       | `joints_N`         | `ivec4` | Yes                         | Yes                           | `N`-th set of joint indices                                                                                                                                              |
| `WEIGHTS_N`                      | `weights_N`        | `vec4`  |

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