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22 | Wayfair's Iridescent Dish with Olives (CC BY) converted to USD+MaterialX with guc and rendered in hdStorm (left). 23 | The same model in Khronos's glTF Sample Viewer (right). 24 |
25 | 26 | ### Build 27 | 28 | You need USD v24.03+ (e.g. v25.02) with MaterialX support enabled. 29 | 30 | Do a recursive clone of the repository and set up a build folder: 31 | ``` 32 | git clone https://github.com/pablode/guc --recursive 33 | mkdir guc/build && cd guc/build 34 | ``` 35 | 36 | Pass following parameters in the CMake generation phase: 37 | ``` 38 | cmake .. -Wno-dev -DCMAKE_BUILD_TYPE=Release 39 | ``` 40 | 41 | Build the executable: 42 | ``` 43 | cmake --build . -j8 --config Release 44 | ``` 45 | 46 | > Note: set `BUILD_SHARED_LIBS` for shared builds, and `CMAKE_MSVC_RUNTIME_LIBRARY` to USD's MSVC ABI. 47 | 48 | ### Usage 49 | 50 | ``` 51 | guc 0.5 - glTF to USD converter 52 | 53 | Usage: guc [options] [--]
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22 | 23 | 24 | ```json 25 | { 26 | "asset" : { "version" : "2.0" }, 27 | "scene": 0, 28 | "scenes" : [ { "nodes" : [ 0 ] } ], 29 | "nodes" : [ { "mesh" : 0 } ], 30 | "meshes" : [ 31 | { 32 | "primitives" : [ { 33 | "attributes" : { 34 | "POSITION" : 1 35 | }, 36 | "indices" : 0, 37 | "material" : 0 38 | } ] 39 | } 40 | ], 41 | "materials" : [ 42 | { 43 | "pbrMetallicRoughness": { 44 | "baseColorFactor": [ 1.000, 0.766, 0.336, 1.0 ], 45 | "metallicFactor": 0.5, 46 | "roughnessFactor": 0.1 47 | } 48 | } 49 | ], 50 | "..." 51 | } 52 | ``` 53 | 54 | | 55 |
59 | A render of the golden glTF triangle (left), and its corresponding JSON (right). 60 |
61 | 62 | ### Asset and Scenes 63 | 64 | Upon conversion, guc creates a root prim, `/Asset`, under which scenes and materials reside. 65 | Scenes contain Xform, mesh, camera and light prims. 66 | 67 | For the "golden triangle" example above, following high-level scene structure is generated: 68 | ``` 69 | #usda 1.0 70 | ( 71 | defaultPrim = "Asset" 72 | doc = "Converted from glTF with guc 0.2" 73 | metersPerUnit = 1 74 | upAxis = "Y" 75 | ) 76 | 77 | def Xform "Asset" ( 78 | customData = { 79 | string version = "2.0" 80 | } 81 | kind = "component" 82 | ) 83 | { 84 | def Xform "Scenes" 85 | { 86 | def Xform "scene" 87 | { 88 | # ... 89 | ``` 90 | 91 | Only the default scene is authored as visible, and if no scenes exists, the glTF file is a _library_. 92 | guc then generates an asset-level `/Nodes` prim. 93 | 94 | ### Nodes 95 | 96 | Nodes are glTF graph elements with an optional transformation, which can either contain a number of nodes, or a reference to a mesh, camera, or light. Nodes are translated to Xforms under the scene prim. Meshes, cameras and lights are "instanced" across scenes through references, but instancing of node trees does not exist. 97 | 98 | ### Meshes 99 | 100 | The glTF buffer, bufferView and accessor concepts are reduced to a single prim, 101 | in the case of the example above, following mesh prim: 102 | 103 | ``` 104 | def Mesh "submesh" ( 105 | prepend apiSchemas = ["MaterialBindingAPI"] 106 | ) 107 | { 108 | float3[] extent = [(0, 0, 0), (1, 1, 0)] 109 | int[] faceVertexCounts = [3] 110 | int[] faceVertexIndices = [0, 1, 2] 111 | rel material:binding = 112 | rel material:binding:preview = 113 | normal3f[] normals = [(0, 0, 1), (0, 0, 1), (0, 0, 1)] ( 114 | customData = { 115 | dictionary guc = { 116 | bool generated = 1 117 | } 118 | } 119 | interpolation = "vertex" 120 | ) 121 | point3f[] points = [(0, 0, 0), (1, 0, 0), (0, 1, 0)] 122 | color3f[] primvars:displayColor = [(1, 0.766, 0.336)] ( 123 | customData = { 124 | dictionary guc = { 125 | bool generated = 1 126 | } 127 | } 128 | interpolation = "constant" 129 | ) 130 | uniform token subdivisionScheme = "none" 131 | } 132 | ``` 133 | 134 | Depending on material parameters and available accessors, guc creates following primvars: 135 | 136 | Name | Description 137 | --- | --- 138 | color[N] | Vertex color set N 139 | displayColor | Display color (constant or per-vertex) 140 | displayOpacity | Display opacity (constant or per-vertex) 141 | opacity[N] | Vertex opacity set N 142 | st[N] | Texture coordinate set N 143 | tangents | Three-component tangent vectors 144 | bitangentSigns | Bitangent handedness 145 | 146 | Additionally, a material binding relationship is always authored on the prim and its 147 | overrides, potentially binding a default material. 148 | 149 | ### Materials 150 | 151 | guc authors UsdPreviewSurface and MaterialX material collections on an asset-level `/Materials` prim. 152 | The exact path is motivated by the behaviour of UsdMtlx, which implicitly creates prim scopes. 153 | 154 | The generated UsdPreviewSurface material for the example above looks like this: 155 | ``` 156 | def Material "mat" 157 | { 158 | token outputs:surface.connect = 159 | 160 | def Shader "node" 161 | { 162 | uniform token info:id = "UsdPreviewSurface" 163 | float3 inputs:diffuseColor = (1, 0.766, 0.336) 164 | float inputs:metallic = 0.5 165 | float inputs:roughness = 0.1 166 | token outputs:surface 167 | } 168 | } 169 | ``` 170 | 171 | While the generated MaterialX document is approximately the following: 172 | ```xml 173 | 174 |