How to convert GLTF graphics elements in an iModel Connector

[PeterJBassett]

Hello @pmconne
I am hoping you can help again with a new question. I am developing the FutureOn - iTwin connector. It currently has code to convert GLTF graphics objects into iModel graphics elements, but I now need to update this to handle recent changes. I am wondering if it is possible to use the iTwin readGltf function (https://www.itwinjs.org/reference/core-frontend/tiles/readgltf/)

I can see that this is a frontend facility, is it possible to use this in a backend 'connector-framework' application to read a GLTF graphic object and convert it into an iModel graphic element?

[pmconne]

Hi Peter,

readGltf is only appropriate for creating graphics, not extracting geometry. We never really wanted to write our own glTF parser in iTwin.js, and the one we have is not fully spec-compliant. It pre-dates the standardizations of glTF 1.0 and 2.0. We plan to remove it eventually in favor of using a library specifically designed for reading glTF. I suggest you do the same. Here are a couple of suggestions:

• loaders.gl has modular support for a variety of formats, including glTF. We use it in iTwin.js for certain use cases, and it works great.
• glTF-Transform looks quite promising. I have not tried it yet, but he maintainer, Don McCurdy, is an expert in this area.

[PeterJBassett]

The latest asset library in FieldTwin uses vertex colors and uv textures; and then uses texture blending to achieve the surface color.

[pmconne]

The latest asset library in FieldTwin uses vertex colors

Specifically, does it contain vertices that have different colors from other vertices?

You could do per-vertex color by creating a texture containing the unique colors and assigning UVs to each vertex. But it sounds like you've also already got a texture, which rules out that option.

A second option would be to extract all triangles of each unique color into a set of separate meshes, but that's hardly ideal. The ideal would be for us to support per-vertex colors directly.

How specifically is the vertex and texture color blended? Is it just a straight ratio for the entire mesh?

[PeterJBassett]

Hello @pmconne thanks again for your reply.
The texture blending in FieldTwin is done using a multiply function.
Is it possible that iTwin can be upgraded to support vertex colors?

[pmconne]

Yes, but I don't have a timeline. @danieliborra @markschlosseratbentley

[danieliborra]

That is not in our roadmap. I'd recommend discussing this with Joe Granville, as our list of critical issues is long.