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KhronosGroup / COLLADA2GLTF

COLLADA to glTF converter
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Use earcut to triangulate polylist/polygons #201

Open apptaro opened 6 years ago

apptaro commented 6 years ago

I have a box and a half transparent glass board in my scene in Blender:

test

Now, I export the scene to .dae without triangulation, then I convert it to .gltf with COLLADA2GLTF-v2.1.2-windows-Release-x64. Alpha transparency conversion is not supported yet (see #168) so I manually edit .gltf file to make my glass board transparent. The result is weird like this:

test-polygons

Now, I export the scene to .dae with triangulation, then I convert it to .gltf, edit .gltf for alpha transparency, then the result is correct like this:

test-triangles

Note that changes in .gltf file is marked by a red line in the editor. I suspect something is wrong with triangulation of polygons by COLLADA2GLTF. It looks as if triangle are doubled for a part of polygon. The same issue happens for polylist, as I confirmed.

apptaro commented 6 years ago

Associated data is here: data-transp.zip

lasalvavida commented 6 years ago

See the comment at: https://github.com/KhronosGroup/COLLADA2GLTF/blob/master/src/COLLADA2GLTFWriter.cpp#L633

This approach is very efficient in terms of runtime, but there are more correct solutions that may be worth considering.
Using a 3D variant of Fortune's Algorithm or something similar to compute a mesh with no overlapping triangles would be ideal.

This is the expected behavior currently; the triangulation performed is very simple. The overlapping nature of the triangle data that is generated is only visible with transparency.

lasalvavida commented 6 years ago

Looking at it again, I think I would use earcut to do this. Compute the plane that the polygon lies on -> project to 2D -> earcut -> map back to 3D