Primitive Mode TRIANGLE_STRIP and TRIANGLE_FAN implementation, LINE_LOOP render fix

[Hexer611]

According to these two issues;

• Primitive Mode TRIANGLE_STRIP and TRIANGLE_FAN atteneder/glTFast#316
• Support for mesh primitive modes LINE_LOOP atteneder/glTFast#689

These primitive modes are not implemented:

• TRIANGLE_STRIP
• TRIANGLE_FAN

And this primitive mode is not rendered correctly:

• LINE_LOOP

TRIANGLE_STRIP and TRIANGLE_FAN implementation

First of all, we need example assets for the two primitive mode implementations (TRIANGLE_STRIP, TRIANGLE_FAN)
We can find those assets from these two links;

• https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/MeshPrimitiveModes
• https://github.com/KhronosGroup/glTF-Asset-Generator/tree/main/Output/Positive/Mesh_PrimitiveMode

First link includes a gltf file with all the primitive modes embedded in one file. Second one has seperate gltf files where each one is a different primitive mode.

In order to correctly implement these triangle primitive modes gltf-2.0 specification documentation of Khronos Group was used;

• https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html

From this documentation these two formulas was used;

Triangle Strips
One triangle primitive is defined by each vertex and the two vertices that follow it, according to the equation:

• pi = {vi, vi+(1+i%2), vi+(2-i%2)}

Triangle Fans
Triangle primitives are defined around a shared common vertex, according to the equation:

• pi = {vi+1, vi+2, v0}

Since it was necessary to create the indices from scratch this change was necessary;
before

var usage = (
    primitive.mode == DrawMode.Triangles
    || primitive.mode == DrawMode.TriangleStrip
    || primitive.mode == DrawMode.TriangleFan
    )
? AccessorUsage.IndexFlipped
: AccessorUsage.Index;

after

var usage = ( primitive.mode == DrawMode.Triangles )
? AccessorUsage.IndexFlipped
: AccessorUsage.Index;

In order to check if the results was correct gltf viewer of Khronos group was used;

• https://github.khronos.org/glTF-Sample-Viewer-Release/

Since I couldn't find any other example assets I used blender to generate them. These are the steps to do so;

• Import or draw any shape in Blender (I used Blender 4.2)
• Export using gltf seperate option
• Add primitive mode to the glft file (or change the existing from 4 TRIANGLE to 5 TRIANGLE_STRIP or 6 TRIANGLE_FAN)
• Import the asset to Unity and gltf viewer of Khronos group, and compare them
• The results were looking the same even though the topology was messed up

I am not sure if using blender was correct way to check that but the original assets were correctly imported.

LINE_LOOP render display fix

As stated in the issue it was needed to add one more index to the end of the original index list. In line 3661 Array.Copy was used (I don't know if Job system was supposed to be used here too)

Unity version 2022.3.38f1 was used in this PR

[unity-cla-assistant]

All committers have signed the CLA.

[atteneder]

Excellent work and pull request!

I've given it minor finishing touches and merged it internally. It'll land in an upcoming release.

Thanks a lot!

[Hexer611]

Thank you @atteneder !
I'll see if I can contribute more in the future :D
Is it possible for me to see those changes at the moment, or do I wait for the release?