cesium gltf 1.0 2.0的一些扩展

cesium中对于 gltf1.0、2.0中某些扩展需要进行解析，解析的过程是将gltf1.0的格式转换成gltf2.0的格式。
对于解析过程中使用的某些扩展的解释：


{"asset": {"version": "2.0"},"scene": 0,"scenes": [{"nodes": [0]}],"nodes": [{"children": [2,1],"matrix": [0.009999999776482582,0.0,0.0,0.0,0.0,0.009999999776482582,0.0,0.0,0.0,0.0,0.009999999776482582,0.0,0.0,0.0,0.0,1.0]},{"matrix": [-0.7289686799049377,0.0,-0.6845470666885376,0.0,-0.4252049028873444,0.7836934328079224,0.4527972936630249,0.0,0.5364750623703003,0.6211478114128113,-0.571287989616394,0.0,400.1130065917969,463.2640075683594,-431.0780334472656,1.0],"camera": 0},{"mesh": 0}],"cameras": [{"perspective": {"aspectRatio": 1.5,"yfov": 0.6605925559997559,"zfar": 10000.0,"znear": 1.0},"type": "perspective"}],"meshes": [{"primitives": [{"attributes": {"NORMAL": 1,"POSITION": 2,"TEXCOORD_0": 3,"_CUSTOM_ATTRIBUTE": 4},"indices": 0,"mode": 4,"material": 0}],"name": "LOD3spShape"}],"accessors": [{"bufferView": 0,"byteOffset": 0,"componentType": 5123,"count": 12636,"max": [2398],"min": [0],"type": "SCALAR"},{"bufferView": 1,"byteOffset": 0,"componentType": 5126,"count": 2399,"max": [0.9995989799499512,0.999580979347229,0.9984359741210938],"min": [-0.9990839958190918,-1.0,-0.9998319745063782],"type": "VEC3"},{"bufferView": 1,"byteOffset": 28788,"componentType": 5126,"count": 2399,"max": [96.17990112304688,163.97000122070313,53.92519760131836],"min": [-69.29850006103516,9.929369926452637,-61.32819747924805],"type": "VEC3"},{"bufferView": 2,"byteOffset": 0,"componentType": 5126,"count": 2399,"max": [0.9833459854125976,0.9800369739532472],"min": [0.026409000158309938,0.01996302604675293],"type": "VEC2"},{"bufferView": 3,"byteOffset": 0,"componentType": 5125,"count": 2399,"type": "SCALAR"}],"materials": [{"extensions": {"KHR_techniques_webgl": {"technique": 0,"values" : {"u_shininess": 10.0,"u_light0Color": [0.8,0.8,1.0],"u_ambient": [0.1,0.1,0.1,1.0]}}}}],"textures": [{"sampler": 0,"source": 0}],"images": [{"uri": "DuckCM.png"}],"extensions": {"KHR_techniques_webgl": {"programs": [{"fragmentShader": 0,"vertexShader": 1}],"shaders": [{"type": 35632,"uri": "duck0FS.glsl"},{"type": 35633,"uri": "duck0VS.glsl"}],"techniques": [{"program": 0,"attributes": {"a_normal":  {"semantic": "NORMAL"},"a_position": {"semantic": "POSITION"},"a_texcoord0": {"semantic": "TEXCOORD_0"},"a_custom": {"semantic": "_CUSTOM_ATTRIBUTE"}},"uniforms": {"u_ambient": {"type": 35666},"u_diffuse": {"type": 35678,"value": {"index": 0}},"u_shininess": {"type": 5126},"u_light0Color": {"type": 35665,"value": [1,1,1]},"u_light0Transform": {"semantic": "MODEL","node": 1,"type": 35676},"u_modelViewMatrix": {"semantic": "MODELVIEW","type": 35676},"u_normalMatrix": {"semantic": "MODELVIEWINVERSETRANSPOSE","type": 35675},"u_projectionMatrix": {"semantic": "PROJECTION","type": 35676}}}]}},"extensionsRequired": ["KHR_techniques_webgl"],"extensionsUsed": ["KHR_techniques_webgl"]
}

使用的扩展需要在 extensionsRequired、extensionsUsed中进行声明。

 "materials": [{"extensions": {"KHR_techniques_webgl": {"technique": 0,"values" : {"u_shininess": 10.0,"u_light0Color": [0.8,0.8,1.0],"u_ambient": [0.1,0.1,0.1,1.0]}}}}],

上面的格式中某个mesh使用了材质扩展一些效果，使用了KHR_techniques_webgl扩展，这个扩展定义了technique是一个引用，真正的数据在"extensions":"KHR_techniques_webgl":"techniques":中。

而如果materials中的technique不是一个索引，是一个光照模型，就比较清楚materials中的扩展的values是什么了，technique可以是“lambert”、“phong”、“blinn”等光照模型，如果是“lambert”需要“u_ambint”,"u_light0Color"等信息。这些信息代表着模型本身的属性，如：环境光反射率、灯光的反射率、模型对灯光的反射强度等。

{"extensions": {"KHR_materials_common" : {"lights" : {"ambient_light_id": {"ambient": {"color": [1.0,1.0,1.0],"extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"name": "user-defined name of ambient light","type": "ambient","extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"directional_light_id": {"directional": {"color": [1.0,1.0,1.0],"extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"name": "user-defined name of directional light","type": "directional","extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"point_light_id": {"name": "user-defined name of point light","point": {"color": [1.0,1.0,1.0],"constantAttenuation": 1.0,"linearAttenuation": 0.5,"quadraticAttenuation": 0.25,"extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"type": "point","extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"spot_light_id": {"name": "user-defined name of spot light","spot": {"color": [1.0,1.0,1.0],"constantAttenuation": 1.0,"fallOffAngle": 40,"fallOffExponent": 0.0,"linearAttenuation": 0.5,"quadraticAttenuation": 0.25,"extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     },"type": "spot","extensions" : {"extension_name" : {"extension specific" : "value"}},"extras" : {"Application specific" : "The extra object can contain any properties."}     }}}},"nodes" : {"light_node_id": {"children": [],"matrix": [ 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0 ],"name": "user-defined name of light node","extensions": {"KHR_materials_common" : {"light": "point_light_id"}}            }},"techniques" : {"technique_id" : {"name": "user-defined technique name","parameters": {"lightTransform": {"node": "directional_light_node_id","type": 35676},"modelViewMatrix": {"semantic": "MODELVIEW","type": 35676},"position": {"semantic": "POSITION","type": 35665},},"attributes": {"a_position": "position"},"program": "program_id","uniforms": {"u_lightTransformMatrix": "lightTransform"},}}
}

对于“KHR_materials_common”这个扩展，设置了灯光的颜色属性。

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