ModelMesh support custom vertex element and vertex buffer binding

[GuoLei1990]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Featutres:

• ModelMesh can get advanced data even if the data is set via Buffer and vice versa, It was a hack to implement advanced data reading before

engine.resourceManager.load<GLTFResource>({
      url: "https://gw.alipayobjects.com/os/OasisHub/267000040/9994/%25E5%25BD%2592%25E6%25A1%25A3.gltf",
      params: {
        keepMeshData: true,
      },
    })
    .then((glTF) => {
      // Get advanced data
      const mesh = glTF.meshes![0][0];
      const positions = mesh.getPositions();
      const normals = mesh.getNormals();
      const uvs = mesh.getUVs();

      ......
    });

• Buffer support store cpu data when it is readable

Additional supplement：
This is a function implementation for reading Mesh low-level data, considering the most complex vertex structure (interleaved and normalized), maybe the physical MeshCollider needs it！

/**
 * Get typed array of position data, consider buffer interleaved and normalized position data.
 * @param mesh - Mesh
 * @returns Typed array of position data
 */
function getPositionTypedArray(mesh: ModelMesh): TypedArray | null {
  const vertexElements = mesh.vertexElements;
  const bufferBindings = mesh.vertexBufferBindings;
  for (let i = 0, n = vertexElements.length; i < n; i++) {
    const vertexElement = vertexElements[i];
    if (vertexElement.semantic === "POSITION") {
      const formatMetaInfo = vertexElement._formatMetaInfo;
      const bufferBinding = bufferBindings[vertexElement.bindingIndex];

      if (!bufferBinding) {
        return null;
      }

      const stride = bufferBinding.stride;
      const formatType = formatMetaInfo.type;
      const verticesData = mesh._getVertexTypedArray(
        bufferBinding.buffer.data.buffer,
        formatType
      );
      const BYTES_PER_ELEMENT = verticesData.BYTES_PER_ELEMENT;

      if (stride === BYTES_PER_ELEMENT * 3) {
        if (formatMetaInfo.normalized) {
          const normalizedVerticesData = verticesData.slice();
          const scaleFactor = formatMetaInfo.normalizedScaleFactor;

          for (let i = 0, n = verticesData.length; i < n; i++) {
            normalizedVerticesData[i] *= scaleFactor;
          }
          return normalizedVerticesData;
        } else {
          // return origin data directly
          return verticesData;
        }
      } else {
        const vertexCount = mesh.vertexCount;
        const positionBuffer = new ArrayBuffer(BYTES_PER_ELEMENT * 3);
        const positionData = mesh._getVertexTypedArray(
          positionBuffer,
          formatType
        );
        const elementOffset = vertexElement.offset;
        const scaleFactor = formatMetaInfo.normalizedScaleFactor;

        for (let i = 0; i < vertexCount; i++) {
          const pOffset = i * 3;
          const vOffset = (i * stride + elementOffset) / BYTES_PER_ELEMENT;
          if (formatMetaInfo.normalized) {
            positionData[pOffset] = verticesData[vOffset];
            positionData[pOffset + 1] = verticesData[vOffset + 1];
            positionData[pOffset + 2] = verticesData[vOffset + 2];
          } else {
            positionData[pOffset] = verticesData[vOffset] * scaleFactor;
            positionData[pOffset + 1] = verticesData[vOffset + 1] * scaleFactor;
            positionData[pOffset + 2] = verticesData[vOffset + 2] * scaleFactor;
          }
        }
        return positionData;
      }
    }
  }
  return null;
}

[yangfengzzz]

Add a enum refer to "POSITION", make it easy to use

[GuoLei1990]

done!

[yangfengzzz]

It will better to be writable, If user modify data in GPU and read back

[GuoLei1990]

User use setData() to write data, data will automatically sync

[cptbtptpbcptdtptp]

Unused buffers are not destroyed.

    // Repro code
    modelMesh.setVertexBufferBinding(posBuffer, 12, 0);
    modelMesh.setVertexBufferBinding(norBuffer, 12, 2);
    modelMesh.setVertexBufferBinding(uvBuffer, 8, 3);
    modelMesh.setNormals(null);
    modelMesh.uploadData(false);
    console.log(“buffers”, modelMesh._vertexBufferBindings);

[GuoLei1990]

VertexBufferBindings shouldn't clear( user-custom-set and even interleaved buffer), but vertexElements not remove normal vertexElement, this is a bug and has been fixed!

[cptbtptpbcptdtptp]

+1