glTF defines an extensible, common publishing format for 3D content tools and services that streamlines authoring workflows and enables interoperable use of content across the industry.
可以将glTF理解为3D的图片。
glTF有两种（？）材料模型，一种是Metallic-Roughness Material mode，一种是Specular-Glossiness Material model。
Metallic（金属的）-Roughness Material model
- baseColor——base color
- metallic——metalness
- roughness——roughness
- simple to implement——mandated in core
Specular-Glossiness Material model
- diffuse——reflected diffuse color
- specular——specular color
- glossiness——glossiness
- silghtly more resources——optional extension