GLTF Blender exporting Quads

[jclarkk]

Seems like no matter what we do we're not able to export the mesh as Quads in GLTF.
Even though we have the mesh as Quads in Blender the export seems to convert them into Triangles.

When exporting the same mesh as OBJ it is still represented as Quads.

Is there a reason behind this?

Thanks!

[donmccurdy]

From the Blender addon docs –

glTF’s internal structure mimics the memory buffers commonly used by graphics chips when rendering in real-time, such that assets can be delivered to desktop, web, or mobile clients and be promptly displayed with minimal processing. As a result, quads and n-gons are automatically converted to triangles when exporting to glTF.

Support for quads has been proposed, but is currently only a proposal, and would be considerably slower for runtime use, which is the focus of glTF.

[donmccurdy]

There's a separate repository for the Blender exporter: https://github.com/KhronosGroup/glTF-Blender-IO. However, the glTF spec itself intentionally doesn't allow quads, so it's not just a matter of patching the exporter.

[lexaknyazev]

Support for quads has been proposed, but is currently only a proposal, and would be considerably slower for runtime use, which is the focus of glTF.

That proposal has no runtime cost if an implementation just renders the triangles.

[donmccurdy]

Good point! That particular proposal is clever about it; I should have read through.

[zellski]

Nonetheless, it is probably true that if @jclarkk assumes quad support, he probably also assumes de-facto support for it in the existing glTF ecosystem, and obviously there isn't any.

[ranbuch]

it is probably true that if @jclarkk assumes quad support, he probably also assumes de-facto support for it in the existing glTF ecosystem

Actually, a good reason to export in quads is the .bin (geometry) file size. Although WebGL doesn't support quads it can display it (as triangles, just is it's doing with other formats) and if there's no runtime cost it's a clear winner regarding loading time.

If an artists creates a model in quads, saving it in tris will resolve in twice the geometry size.

[lexaknyazev]

If an artists creates a model in quads, saving it in tris will resolve in twice the geometry size.

That's not necessarily the case. Vertex attributes (that use most of geometry space) will take the same size regardless of connectivity information. Index buffer that contains only quads could be smaller than the index buffer with triangles, but it would require runtime processing to be rendered with GPU APIs.

The extension proposed by @zellski encodes quads (or other n-gons) by using very specific order of triangles. This allows zero-processing rendering while preserving original connectivity information for tools that need it.

[donmccurdy]

Let's continue discussion in #1620.