一、需求

        Cesium支持加载gltf和3dtiles等三维数据模型，实现了很好的封装，往往只需要给一个uri就能加载模型文件，并实现贴图渲染等。但是好的封装带来的问题是如果开发者想要自定义贴图，那该怎么办？不得不从源码入手。

二、价值

        这篇文章的价值不仅仅是gltf增加纹理贴图，因为刚才说到的3dtiles其实也是基于gltf来实现的模型，那么如果想给3dtiles增加自定义贴图，是否也意味着可以走gltf这条路，并且从gltf这层实现之后，是否意味着对b3dm/i3dm/cmpt等的统一。

三、源码解读

1.框架：

       3dtiels中b3dm和i3dm是以gltf为基础进行加载和渲染的，因此Cesium在封装的B3dmLoader和I3dmLoader中都有调用_gltfLoader的地方，其中使用的是：gltfLoader.process

B3dmLoader.prototype.process = function(frameState) {
    ...
    const ready = this._gltfLoader.process(frameState);
    ...
  };
I3dmLoader.prototype.process = function(frameState) {
    ...
      ready = gltfLoader.process(frameState);
    ...
  };

在这个函数中抛开异常处理逻辑，关键函数在于：loadResources5(this, frameState):

async function loadResources5(loader, frameState) {
    //给出Json指引
    const gltf = loader.gltfJson;
    //具体的资源加载
    const promise = parse(loader, gltf, supportedImageFormats, frameState);
    ...
    //注意这里由于模型资源加载完成后是不需要中间数据的，为了减少内存的消耗，Cesium这里对Json信息进行了清理
    if (defined_default(loader._gltfJsonLoader) && loader._releaseGltfJson) {
      ResourceCache_default.unload(loader._gltfJsonLoader);
      loader._gltfJsonLoader = void 0;
    }
    return promise;
  }

再来看parse：

function parse(loader, gltf, supportedImageFormats, frameState) {
    //...拓展项相关的数据处理
    //注意一下结构，实际就是按照Json的指引，将具体的数据请取出，熟悉gltf的Json项自然就明白了各项的含义
    const nodes = loadNodes(loader, gltf, supportedImageFormats, frameState);
    const skins = loadSkins(loader, gltf, nodes);
    const animations = loadAnimations(loader, gltf, nodes);
    const articulations = loadArticulations(gltf);
    const scene = loadScene(gltf, nodes);
    const components = new Components2();
    const asset = new Asset2();
    const copyright = gltf.asset.copyright;
    ...
    //将取出的数据存储在components中
    components.asset = asset;
    components.scene = scene;
    components.nodes = nodes;
    ...
    loader._components = components;
    ...
  }

至此，数据的读取，处理就完成了，意味着渲染隐含与其中。下面将重点分析。

2.渲染逻辑

A.纹理资源的加载

如果了解gltf的管理方式：

不难看出node是总览全局的，那么进入 node处理部分：loadNodes::loadNode

function loadNode(loader, gltf, gltfNode, supportedImageFormats, frameState) {
   ...
    //一个node对应一个meshId，用于获取对应的mesh
    const meshId = gltfNode.mesh;
    if (defined_default(meshId)) {
      const mesh = gltf.meshes[meshId];
    //mesh中又包括多个图元
      const primitives = mesh.primitives;
      const primitivesLength = primitives.length;
      for (let i = 0; i < primitivesLength; ++i) {
        node.primitives.push(
        //图元是最小的渲染可调度单位
          loadPrimitive(
            loader,
            gltf,
            primitives[i],
            defined_default(node.instances),
            supportedImageFormats,
            frameState
          )
        );
      }
     ...
    }
    return node;
  }

在最小的渲染单位primitive中：

function loadPrimitive(loader, gltf, gltfPrimitive, hasInstances, supportedImageFormats, frameState) {
    ...
    //从图元取得MaterialId
    const materialId = gltfPrimitive.material;
    if (defined_default(materialId)) {
    //加载材质的入口，也意味着材质的管理（增删改）都可以从这里找到
      primitive.material = loadMaterial(
        loader,
        gltf,
        gltf.materials[materialId],
        supportedImageFormats,
        frameState
      );
    }
    ...
    return primitive;
  }

 加载材质部分主要包括：初始化一个空材质+往材质模板中填充数据

  function loadMaterial(loader, gltf, gltfMaterial, supportedImageFormats, frameState) {
    //首先进来的第一件事先创建一个空材质用于填充数据
    const material = new Material3();
    ...
    //直接计算的填充
    material.unlit = defined_default(extensions.KHR_materials_unlit);
...
    //针对特定类型材质特定参数计算，最后再填充
      specularGlossiness.glossinessFactor = pbrSpecularGlossiness.glossinessFactor;
      material.pbrSpecularGlossiness = pbrSpecularGlossiness;
    ...
        //重头戏就是这里的加载纹理
        metallicRoughness.baseColorTexture = loadTexture(
          loader,
          gltf,
          pbrMetallicRoughness.baseColorTexture,
          supportedImageFormats,
          frameState
        );
      ...
    return material;
  }

 加载纹理的逻辑：

function loadTexture(loader, gltf, textureInfo, supportedImageFormats, frameState, samplerOverride) {
    //检查Image是否可用
    //纹理加载器
    const textureLoader = ResourceCache_default.getTextureLoader({
      gltf,
      textureInfo,
      gltfResource: loader._gltfResource,
      baseResource: loader._baseResource,
      supportedImageFormats,
      frameState,
      asynchronous: loader._asynchronous
    });
    //纹理解释器
    const textureReader = GltfLoaderUtil_default.createModelTextureReader({
      textureInfo
    });
    //将相关加载放入总加载器管理
    loader._textureLoaders.push(textureLoader);
    ...
    loader._textureState = GltfLoaderState.FAILED;
    loader._textureErrors.push(error);
    loader._texturesPromises.push(promise);
    loader._textureCallbacks[index]...
    return textureReader;
  }

B.纹理资源应用 

当纹理加载完成，就要考虑如何消费纹理，即编写shader和处理：

纹理的使用往往是在FragmentShader中，这块的编码在Cesium中为：

var MaterialStageFS_default = "// If the style color is white, it implies the feature has not been styled.\nbool isDefaultStyleColor(vec3 color)\n{\n    return all(greaterThan(color, vec3(1.0 - czm_epsilon3)));\n}\n\nvec3 blend(vec3 sourceColor, vec3 styleColor, float styleColorBlend)\n{\n    vec3 blendColor = mix(sourceColor, styleColor, styleColorBlend);\n    vec3 color = isDefaultStyleColor(styleColor.rgb) ? sourceColor : blendColor;\n    return color;\n}\n\nvec2 computeTextureTransform(vec2 texCoord, mat3 textureTransform)\n{\n    return vec2(textureTransform * vec3(texCoord, 1.0));\n}\n\n#ifdef HAS_NORMALS\nvec3 computeNormal(ProcessedAttributes attributes)\n{\n    // Geometry normal. This is already normalized \n    vec3 ng = attributes.normalEC;\n\n    vec3 normal = ng;\n    #if defined(HAS_NORMAL_TEXTURE) && !defined(HAS_WIREFRAME)\n    vec2 normalTexCoords = TEXCOORD_NORMAL;\n        #ifdef HAS_NORMAL_TEXTURE_TRANSFORM\n        normalTexCoords = computeTextureTransform(normalTexCoords, u_normalTextureTransform);\n        #endif\n\n        // If HAS_BITANGENTS is set, then HAS_TANGENTS is also set\n        #ifdef HAS_BITANGENTS\n        vec3 t = attributes.tangentEC;\n        vec3 b = attributes.bitangentEC;\n        mat3 tbn = mat3(t, b, ng);\n        vec3 n = texture(u_normalTexture, normalTexCoords).rgb;\n        normal = normalize(tbn * (2.0 * n - 1.0));\n        #elif (__VERSION__ == 300 || defined(GL_OES_standard_derivatives))\n        // If derivatives are available (not IE 10), compute tangents\n        vec3 positionEC = attributes.positionEC;\n        vec3 pos_dx = dFdx(positionEC);\n        vec3 pos_dy = dFdy(positionEC);\n        vec3 tex_dx = dFdx(vec3(normalTexCoords,0.0));\n        vec3 tex_dy = dFdy(vec3(normalTexCoords,0.0));\n        vec3 t = (tex_dy.t * pos_dx - tex_dx.t * pos_dy) / (tex_dx.s * tex_dy.t - tex_dy.s * tex_dx.t);\n        t = normalize(t - ng * dot(ng, t));\n        vec3 b = normalize(cross(ng, t));\n        mat3 tbn = mat3(t, b, ng);\n        vec3 n = texture(u_normalTexture, normalTexCoords).rgb;\n        normal = normalize(tbn * (2.0 * n - 1.0));\n        #endif\n    #endif\n\n    #ifdef HAS_DOUBLE_SIDED_MATERIAL\n    if (czm_backFacing()) {\n        normal = -normal;\n    }\n    #endif\n\n    return normal;\n}\n#endif\n\nvoid materialStage(inout czm_modelMaterial material, ProcessedAttributes attributes, SelectedFeature feature)\n{\n    #ifdef HAS_NORMALS\n    material.normalEC = computeNormal(attributes);\n    #endif\n\n    vec4 baseColorWithAlpha = vec4(1.0);\n    // Regardless of whether we use PBR, set a base color\n    #ifdef HAS_BASE_COLOR_TEXTURE\n    vec2 baseColorTexCoords = TEXCOORD_BASE_COLOR;\n\n        #ifdef HAS_BASE_COLOR_TEXTURE_TRANSFORM\n        baseColorTexCoords = computeTextureTransform(baseColorTexCoords, u_baseColorTextureTransform);\n        #endif\n\n    baseColorWithAlpha = czm_srgbToLinear(texture(u_baseColorTexture, baseColorTexCoords));\n\n        #ifdef HAS_BASE_COLOR_FACTOR\n        baseColorWithAlpha *= u_baseColorFactor;\n        #endif\n    #elif defined(HAS_BASE_COLOR_FACTOR)\n    baseColorWithAlpha = u_baseColorFactor;\n    #endif\n\n    #ifdef HAS_POINT_CLOUD_COLOR_STYLE\n    baseColorWithAlpha = v_pointCloudColor;\n    #elif defined(HAS_COLOR_0)\n    vec4 color = attributes.color_0;\n        // .pnts files store colors in the sRGB color space\n        #ifdef HAS_SRGB_COLOR\n        color = czm_srgbToLinear(color);\n        #endif\n    baseColorWithAlpha *= color;\n    #endif\n\n    material.diffuse = baseColorWithAlpha.rgb;\n    material.alpha = baseColorWithAlpha.a;\n\n    #ifdef USE_CPU_STYLING\n    material.diffuse = blend(material.diffuse, feature.color.rgb, model_colorBlend);\n    #endif\n\n    #ifdef HAS_OCCLUSION_TEXTURE\n    vec2 occlusionTexCoords = TEXCOORD_OCCLUSION;\n        #ifdef HAS_OCCLUSION_TEXTURE_TRANSFORM\n        occlusionTexCoords = computeTextureTransform(occlusionTexCoords, u_occlusionTextureTransform);\n        #endif\n    material.occlusion = texture(u_occlusionTexture, occlusionTexCoords).r;\n    #endif\n\n    #ifdef HAS_EMISSIVE_TEXTURE\n    vec2 emissiveTexCoords = TEXCOORD_EMISSIVE;\n        #ifdef HAS_EMISSIVE_TEXTURE_TRANSFORM\n        emissiveTexCoords = computeTextureTransform(emissiveTexCoords, u_emissiveTextureTransform);\n        #endif\n\n    vec3 emissive = czm_srgbToLinear(texture(u_emissiveTexture, emissiveTexCoords).rgb);\n        #ifdef HAS_EMISSIVE_FACTOR\n        emissive *= u_emissiveFactor;\n        #endif\n    material.emissive = emissive;\n    #elif defined(HAS_EMISSIVE_FACTOR)\n    material.emissive = u_emissiveFactor;\n    #endif\n\n    #if defined(LIGHTING_PBR) && defined(USE_SPECULAR_GLOSSINESS)\n        #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE\n        vec2 specularGlossinessTexCoords = TEXCOORD_SPECULAR_GLOSSINESS;\n          #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE_TRANSFORM\n          specularGlossinessTexCoords = computeTextureTransform(specularGlossinessTexCoords, u_specularGlossinessTextureTransform);\n          #endif\n\n        vec4 specularGlossiness = czm_srgbToLinear(texture(u_specularGlossinessTexture, specularGlossinessTexCoords));\n        vec3 specular = specularGlossiness.rgb;\n        float glossiness = specularGlossiness.a;\n            #ifdef HAS_SPECULAR_FACTOR\n            specular *= u_specularFactor;\n            #endif\n\n            #ifdef HAS_GLOSSINESS_FACTOR\n            glossiness *= u_glossinessFactor;\n            #endif\n        #else\n            #ifdef HAS_SPECULAR_FACTOR\n            vec3 specular = clamp(u_specularFactor, vec3(0.0), vec3(1.0));\n            #else\n            vec3 specular = vec3(1.0);\n            #endif\n\n            #ifdef HAS_GLOSSINESS_FACTOR\n            float glossiness = clamp(u_glossinessFactor, 0.0, 1.0);\n            #else\n            float glossiness = 1.0;\n            #endif\n        #endif\n\n        #ifdef HAS_DIFFUSE_TEXTURE\n        vec2 diffuseTexCoords = TEXCOORD_DIFFUSE;\n            #ifdef HAS_DIFFUSE_TEXTURE_TRANSFORM\n            diffuseTexCoords = computeTextureTransform(diffuseTexCoords, u_diffuseTextureTransform);\n            #endif\n\n        vec4 diffuse = czm_srgbToLinear(texture(u_diffuseTexture, diffuseTexCoords));\n            #ifdef HAS_DIFFUSE_FACTOR\n            diffuse *= u_diffuseFactor;\n            #endif\n        #elif defined(HAS_DIFFUSE_FACTOR)\n        vec4 diffuse = clamp(u_diffuseFactor, vec4(0.0), vec4(1.0));\n        #else\n        vec4 diffuse = vec4(1.0);\n        #endif\n    czm_pbrParameters parameters = czm_pbrSpecularGlossinessMaterial(\n      diffuse.rgb,\n      specular,\n      glossiness\n    );\n    material.diffuse = parameters.diffuseColor;\n    // the specular glossiness extension's alpha overrides anything set\n    // by the base material.\n    material.alpha = diffuse.a;\n    material.specular = parameters.f0;\n    material.roughness = parameters.roughness;\n    #elif defined(LIGHTING_PBR)\n        #ifdef HAS_METALLIC_ROUGHNESS_TEXTURE\n        vec2 metallicRoughnessTexCoords = TEXCOORD_METALLIC_ROUGHNESS;\n            #ifdef HAS_METALLIC_ROUGHNESS_TEXTURE_TRANSFORM\n            metallicRoughnessTexCoords = computeTextureTransform(metallicRoughnessTexCoords, u_metallicRoughnessTextureTransform);\n            #endif\n\n        vec3 metallicRoughness = texture(u_metallicRoughnessTexture, metallicRoughnessTexCoords).rgb;\n        float metalness = clamp(metallicRoughness.b, 0.0, 1.0);\n        float roughness = clamp(metallicRoughness.g, 0.04, 1.0);\n            #ifdef HAS_METALLIC_FACTOR\n            metalness *= u_metallicFactor;\n            #endif\n\n            #ifdef HAS_ROUGHNESS_FACTOR\n            roughness *= u_roughnessFactor;\n            #endif\n        #else\n            #ifdef HAS_METALLIC_FACTOR\n            float metalness = clamp(u_metallicFactor, 0.0, 1.0);\n            #else\n            float metalness = 1.0;\n            #endif\n\n            #ifdef HAS_ROUGHNESS_FACTOR\n            float roughness = clamp(u_roughnessFactor, 0.04, 1.0);\n            #else\n            float roughness = 1.0;\n            #endif\n        #endif\n    czm_pbrParameters parameters = czm_pbrMetallicRoughnessMaterial(\n      material.diffuse,\n      metalness,\n      roughness\n    );\n    material.diffuse = parameters.diffuseColor;\n    material.specular = parameters.f0;\n    material.roughness = parameters.roughness;\n    #endif\n}\n";

 相当的长，但是这中间有上述分析过程中对材质单个参数（粗糙度，金属度）和纹理的处理，不妨一读。那么这段Shader如何使用的呢？

MaterialPipelineStage.process = function(renderResources, primitive, frameState) {
    ...
    processMaterialUniforms(
      material,
      uniformMap2,
      shaderBuilder,
      defaultTexture,
      defaultNormalTexture,
      defaultEmissiveTexture,
      disableTextures
    );
    if (defined_default(material.specularGlossiness)) {
      processSpecularGlossinessUniforms(
        material,
        uniformMap2,
        shaderBuilder,
        defaultTexture,
        disableTextures
      );
    } 
    else {
      processMetallicRoughnessUniforms(
        material,
        uniformMap2,
        shaderBuilder,
        defaultTexture,
        disableTextures
      );    
    }
    ...
    shaderBuilder.addFragmentLines(MaterialStageFS_default);
    ...
  };

 是的，直接在最下方添加到ShaderBuilder。

看上去我们这个过程从数据获取到消费似乎是完成了，但是细心的人应该发现了MaterialStageFS_default 还有一些宏或者不同的纹理它的采样器uv这样的数据其实也是要告知shader的，那么这种处理实际是在processGldLightMapUniforms::processTexture2这个函数中：

function processTexture2(shaderBuilder, uniformMap2, textureReader, uniformName, defineName, defaultTexture) {
//添加Uniform变量
    shaderBuilder.addUniform(
      "sampler2D",//类型
      uniformName,//变量名
      ShaderDestination_default.FRAGMENT//添加到Fragment
    );
    uniformMap2[uniformName] = function() {
      return defaultValue_default(textureReader.texture, defaultTexture);
    };
    //shaderBuilder.addDefine用于在Shader中定义变量并给初值
    //(名称，默认值，添加位置)
    const textureDefine = `HAS_${defineName}_TEXTURE`;//宏
    shaderBuilder.addDefine(textureDefine, void 0, ShaderDestination_default.FRAGMENT);
    
    const texCoordIndex = textureReader.texCoord;
    const texCoordVarying = `v_texCoord_${texCoordIndex}`;
    const texCoordDefine = `TEXCOORD_${defineName}`;
    //uv
    shaderBuilder.addDefine(
      texCoordDefine,
      texCoordVarying,
      ShaderDestination_default.FRAGMENT
    );
   ...
  }

至此，整个流程才算完成，理解了以上流程之后，要想加一张纹理那就比较容易了。

3.实操添加一张贴图

这里给出步骤思路，具体实现自己写一遍应该会好很多：

a.新增一张纹理贴图，意味着Material要加新成员，对应的是loadMaterial中的空材质构造函数：

const material = new Material3();

b. 对空material填充需要loadTexture，这里要注意纹理解释器的丰富，封装在了getAllTextureReaders中；

c.加载好纹理之后就是纹理处理，也就是shader部分，这里一共又可以分为两步：

      processTexture2添加uniform数据资源，往shader压入变量及其值；

      编写shader代码：MaterialStageFS_default。直接在这里改就可以利用上ShaderBuilder的添加一步到位。