webgpu-app/gltf.js

function jsonChunk(buffer, offset, length)
{
  const decoder = new TextDecoder();
  const array = new Uint8Array(buffer);
  const object = JSON.parse(decoder.decode(array.subarray(offset, offset + length)));
  //console.log(object);
  return object;
}

function parseGltfChunks(buffer)
{
  const view = new DataView(buffer);

  const magic = view.getUint32(0, true);
  const version = view.getUint32(4, true);
  const length = view.getUint32(8, true);
  console.assert(magic === 0x46546C67);
  console.assert(version === 2);

  const gltf = {};

  var offset = 12;
  while ((length - offset) > 0) {
    const chunkLength = view.getUint32(offset + 0, true);
    const chunkType = view.getUint32(offset + 4, true);
    console.log("chunkLength", chunkLength);
    console.log("chunkType", chunkType);
    offset += 8;

    console.assert(chunkType === 0x4E4F534A || chunkType === 0x004E4942);
    if (chunkType === 0x4E4F534A) {
      const object = jsonChunk(buffer, offset, chunkLength);
      console.assert(!("json" in gltf));
      gltf.json = object;
    }
    if (chunkType === 0x004E4942) {
      console.assert(!("bin" in gltf));
      gltf.bin = {buffer: buffer, offset: offset, length: chunkLength};
    }

    offset += chunkLength;
  }

  console.assert(offset == length);
  return gltf;
}

function componentCount(componentType)
{
  switch (componentType) {
  case "SCALAR": return 1;
  case "VEC2": return 2;
  case "VEC3": return 3;
  case "VEC4": return 4;
  default:
    console.assert(false, componentType);
    break;
  }
}

function typeStride(type)
{
  switch (type) {
  case 5120: // signed byte
    return 1;
  case 5121: // unsigned byte
    return 1;
  case 5122: // signed short
    return 2;
  case 5123: // unsigned short
    return 2;
  case 5125: // unsigned int
    return 4;
  case 5126: // float
    return 4;
  default:
    console.assert(false, type);
    break;
  }
}

function attributeStride(componentType, type)
{
  const t = typeStride(componentType);
  const c = componentCount(type);
  return c * t;
}

function vertexFormat(componentType, type)
{
  switch (componentType) {
  case 5120: // signed byte
    if (type === "SCALAR")    return "sint8";
    else if (type === "VEC2") return "sint8x2";
    else if (type === "VEC4") return "sint8x4";
    else console.assert(false, [componentType, type]);
  case 5121: // unsigned byte
    if (type === "SCALAR")    return "uint8";
    else if (type === "VEC2") return "uint8x2";
    else if (type === "VEC4") return "uint8x4";
    else console.assert(false, [componentType, type]);
  case 5122: // signed short
    if (type === "SCALAR")    return "sint16";
    else if (type === "VEC2") return "sint16x2";
    else if (type === "VEC4") return "sint16x4";
    else console.assert(false, [componentType, type]);
  case 5123: // unsigned short
    if (type === "SCALAR")    return "uint16";
    else if (type === "VEC2") return "uint16x2";
    else if (type === "VEC4") return "uint16x4";
    else console.assert(false, [componentType, type]);
  case 5125: // unsigned int
    if (type === "SCALAR")    return "uint32";
    else if (type === "VEC2") return "uint32x2";
    else if (type === "VEC3") return "uint32x3";
    else if (type === "VEC4") return "uint32x4";
    else console.assert(false, [componentType, type]);
  case 5126: // float
    if (type === "SCALAR")    return "float32";
    else if (type === "VEC2") return "float32x2";
    else if (type === "VEC3") return "float32x3";
    else if (type === "VEC4") return "float32x4";
    else console.assert(false, [componentType, type]);
  default:
    console.assert(false, [componentType, type]);
    break;
  }
}

function primitiveVertexBufferLayouts(gltf, primitive)
{
  const accessors = [
    gltf.json.accessors[primitive.attributes["POSITION"]],
    gltf.json.accessors[primitive.attributes["NORMAL"]],
    gltf.json.accessors[primitive.attributes["TEXCOORD_0"]],
  ];

  const layouts = [];

  for (let i = 0; i < accessors.length; i++) {
    const componentType = accessors[i].componentType;
    const type = accessors[i].type;

    const bufferView = gltf.json.bufferViews[accessors[i].bufferView];

    const arrayStride = "byteStride" in bufferView ? bufferView.byteStride : attributeStride(componentType, type);
    const offset = "byteOffset" in accessors[i] ? accessors[i].byteOffset : 0;
    const layout = {
      arrayStride: arrayStride,
      attributes: [{
        format: vertexFormat(componentType, type),
        offset: offset,
        shaderLocation: i,
      }],
      stepMode: "vertex",
    };
    layouts.push(layout);
  }
  return layouts;
}

export { parseGltfChunks, primitiveVertexBufferLayouts };