Add example for creating a triangle gltf file from scratch

examples/build-gltf/create_triangle_gltf.cpp

diff --git a/examples/build-gltf/create_triangle_gltf.cpp b/examples/build-gltf/create_triangle_gltf.cpp
new file mode 100644
index 0000000..be92981
--- /dev/null
+++ b/examples/build-gltf/create_triangle_gltf.cpp
@@ -0,0 +1,121 @@
+// An example of how to generate a gltf file from scratch. This example
+// was translated from the pygltlib documentation in the pypi project page,
+// which in turn is based on the Khronos Sample Models at:
+//
+// https://github.com/KhronosGroup/glTF-Sample-Models
+//
+// This example is released under the MIT license. 
+//
+// 2021-02-25 Thu
+// Dov Grobgeld <dov.grobgeld@gmail.com>
+
+
+// Define these only in *one* .cc file.
+#define TINYGLTF_IMPLEMENTATION
+#define STB_IMAGE_IMPLEMENTATION
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+
+// #define TINYGLTF_NOEXCEPTION // optional. disable exception handling.
+#include "tiny_gltf.h"
+
+int main(int argc, char **argv)
+{
+  // Create a model with a single mesh and save it as a gltf file
+  tinygltf::Model m;
+  tinygltf::Scene scene;
+  tinygltf::Mesh mesh;
+  tinygltf::Primitive primitive;
+  tinygltf::Node node;
+  tinygltf::Buffer buffer;
+  tinygltf::BufferView bufferView1;
+  tinygltf::BufferView bufferView2;
+  tinygltf::Accessor accessor1;
+  tinygltf::Accessor accessor2;
+  tinygltf::Asset asset;
+
+  // This is the raw data buffer. 
+  buffer.data = {
+    // 6 bytes of indices and two bytes of padding
+    0x00,0x00,0x01,0x00,0x02,0x00,0x00,0x00,
+    // 36 bytes of floating point numbers
+    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+    0x00,0x00,0x00,0x00,0x00,0x00,0x80,0x3f,
+    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+    0x00,0x00,0x00,0x00,0x00,0x00,0x80,0x3f,
+    0x00,0x00,0x00,0x00};
+
+  // "The indices of the vertices (ELEMENT_ARRAY_BUFFER) take up 6 bytes in the
+  // start of the buffer.
+  bufferView1.buffer = 0;
+  bufferView1.byteOffset=0;
+  bufferView1.byteLength=6;
+  bufferView1.target = TINYGLTF_TARGET_ELEMENT_ARRAY_BUFFER;
+
+  // The vertices take up 36 bytes (3 vertices * 3 floating points * 4 bytes)
+  // at position 8 in the buffer and are of type ARRAY_BUFFER
+  bufferView2.buffer = 0;
+  bufferView2.byteOffset=8;
+  bufferView2.byteLength=36;
+  bufferView2.target = TINYGLTF_TARGET_ARRAY_BUFFER;
+
+  // Describe the layout of bufferView1, the indices of the vertices
+  accessor1.bufferView = 0;
+  accessor1.byteOffset = 0;
+  accessor1.componentType = TINYGLTF_COMPONENT_TYPE_UNSIGNED_SHORT;
+  accessor1.count = 3;
+  accessor1.type = TINYGLTF_TYPE_SCALAR;
+  accessor1.maxValues.push_back(2);
+  accessor1.minValues.push_back(0);
+
+  // Describe the layout of bufferView2, the vertices themself
+  accessor2.bufferView = 1;
+  accessor2.byteOffset = 0;
+  accessor2.componentType = TINYGLTF_COMPONENT_TYPE_FLOAT;
+  accessor2.count = 3;
+  accessor2.type = TINYGLTF_TYPE_VEC3;
+  accessor2.maxValues = {1.0, 1.0, 0.0};
+  accessor2.minValues = {0.0, 0.0, 0.0};
+
+  // Build the mesh primitive and add it to the mesh
+  primitive.indices = 0;                 // The index of the accessor for the vertex indices
+  primitive.attributes["POSITION"] = 1;  // The index of the accessor for positions
+  primitive.material = 0;
+  primitive.mode = TINYGLTF_MODE_TRIANGLES;
+  mesh.primitives.push_back(primitive);
+
+  // Other tie ups
+  node.mesh = 0;
+  scene.nodes.push_back(0); // Default scene
+
+  // Define the asset. The version is required
+  asset.version = "2.0";
+  asset.generator = "tinygltf";
+
+  // Now all that remains is to tie back all the loose objects into the
+  // our single model.
+  m.scenes.push_back(scene);
+  m.meshes.push_back(mesh);
+  m.nodes.push_back(node);
+  m.buffers.push_back(buffer);
+  m.bufferViews.push_back(bufferView1);
+  m.bufferViews.push_back(bufferView2);
+  m.accessors.push_back(accessor1);
+  m.accessors.push_back(accessor2);
+  m.asset = asset;
+
+  // Create a simple material
+  tinygltf::Material mat;
+  mat.pbrMetallicRoughness.baseColorFactor = {1.0f, 0.9f, 0.9f, 1.0f};  
+  mat.doubleSided = true;
+  m.materials.push_back(mat);
+
+  // Save it to a file
+  tinygltf::TinyGLTF gltf;
+  gltf.WriteGltfSceneToFile(&m, "triangle.gltf",
+                           true, // embedImages
+                           true, // embedBuffers
+                           true, // pretty print
+                           false); // write binary
+  
+  exit(0);
+}