Render order of gaussian splats and meshes

[naichamilktea]

The 3DGS has an incorrect occlusion relationship with the mesh model.

I loaded the example scene inside sandbox and added a cube, and found that the occlusion relationship between the cube and the 3DGS building in the example was incorrect.

Reproduction steps

1. clone the Cesium 1.125 splat shader branch
2. build the source code
3. open the local sandbox, then open the 3d gaussian example, then add a primitve box near the building
4. observe the occlusion relationship between the example 3DGS building and the box

Sandcastle example

http://localhost:8080/Apps/Sandcastle/index.html#c=lVbfT9tWFP5XrOwBR42cBGhL+KWlIYApJJC4UFAkdLFv4gvXvp59A4QJaeum9aGjMKndplaTuj20k8Zapv1oC+36z5Ckfeq/sGM7ThzTPoyH4Otzvu9859xzz3UOO6RuSDIzJQ1XUZ3yrKpix1HYFjaFiYopCJUYbszpGzMqKZI5+caenC4Q2ZHN0mU1J1+Rt6yby7m5jAROn2kzW+AkDy6s5BuFFXm4UE4Nrq0sG/PKUmNNyfOiUtpcI6nBwibdLK4U9OLKMinM5Ml8bs5aA7IFZWmnoNTS7nr15hIpbuaHFvZW9wqbtWHAZiRbzuZzQyNX5+avrRYX+dBOukrJdWVlrabM5JZW5+jMCLs8O+coO6uV2FjFVJnpcGGb4B1sCxOCiXeEnJ/xsvdOrMRUb51jJkfExHYlFgecj5AcFZsYCrNRr5V1tjNtIwM7i9guYyDWgJDbdQzuQaBi6ZqsrJeykNP6dDanFEvgMzTWb16Uldzs+pS8LE/lC1PgMOIxUMwFFVOqMIisEIodzMHQkev/G5rqGKSqzQzYtKwDC1kTBzPpq0ODlxPC5+6OGWiXGHWjrNoYm2ULqThv28weFdKJirkfl7iOTVHkPlVcmJj0YdHobn5dHYLQVxPLJgbhZBs7EtI0MQqNhxF7jBkK68YbczW4NUa2oCNTo9GtCevexiaf9Z3EPgEqAgIn7pUumRTaD79rHh2/e/Wo9eh28/njd/eetm+9bN4+Oz+9c372T8XsBJLccplWHbqcE2aK1brpP2A3UNyvgyAA4du7z5uH33dIXh+0Xz9tP/i6ffywefBz8+Re86cjCOQ7u3moyOYgHsGRidSJqFuLzCG9IJLVWfo1EgRSFcRO5nAEoQehnAFdvCupI6t58k1ExNvXx62DX85fnLbPDtunx83Txz1AII3VbGTpRAV1QT+F3nrNlAtChoKP9TNRZtYIr2u4R7OAuC5xNoVrsGWOGI4ldd3jY65ykHdBG0X8/xB2vDt8H8hVx6Smu33bh/PfeqB3v/3ogXowBF3BxQFf3ftXDwaES6FELwkD719964cKjIFm3+Yzdmx+pKBu+9DoiY81tdKwsDSfn1bWc/Ny7rrfyP4UMBDHNkG0ryreG2+nXKQ4kGOU2QMuLHCX6iapMttwJNW1hfbaXfqb7DjeosxtYtZg+H1SrabgD4aetEO4nqWWjsSUlMm4xP5EMjm2+/vGa44hjzDYpxFp8EomIQxfkVLpdEJIpwK8RgxsOtDtTv8Z7/GI6ZQLCIMMpmEKOdtktxdZsZHp+Plh5PACs7l+w1LYNNnFmjeZRV9swKJbdolx5B61vkoCqy9+FiMNyrBIuKqXGKWiv20hlRc8+ru0BFYQJWZS8YSQklLeTxza7PzF7803X8HwaJ381Xz5Z+uHO3BKMymv9ULyorKGPVmBaC9h6j12lIUySlzQGqpoICWk5wBktJ+c+eEjQQ249YlFG2Ko7gk3WCK8ExeuxcgVcEHQYmAXOyOswmuYGZjbDRluQ2TCzTsaBsxErGJo9PWwo4H7NbYbIPxe7LZaGNgdD3DM8e409A/iXQq4yoNg3rnIWhZGtruUlvMlJX9zfbpYWsgqiYt8vcYeDT1HHPfjkReheo6GF4lksnV4dP7vQ/+qOX9xt/nkTeuLL5uHt1r3T/o5iDYK32Sh7CuxnkNfRNRNp6/OeUqJ5TCilet2FeZRL+to4dAG28YzNqubELOKqIOj+XRmz2j36SNSNGxxXcEO1D4SA5tog2Lgd7+lvCvu6Fn73q+t53/AcWn9fefts/shTp+ywvfjbivHErFxhzcongxcPiWGBZNBqNtUlKQkx4YFRwg7yY26ugXfTarjBON5PBmGjmtkG0o78YEPQkGlyHHAUq1TWiZ7uBKbHE+C/wUoZd68KEKzUdRw3fT05Lz/UpKk8SQsP4zkjNENZEeY/wM

Environment

Browser: Chrome 133.0.6943.59 64bit
CesiumJS Version: 1.125 (on the splat-shader branch )
Operating System: Windows 11

[naichamilktea]

When the opacity of this cube is 0.99, the cube always blocks the building, no matter from which direction it is viewed. When the opacity of the cube is 1, the building always blocks the cube , no matter which direction it is viewed from.

[ggetz]

@keyboardspecialist Could you please check out this bug report and determine if there is any action needed, either in your branch or in main?

[keyboardspecialist]

@ggetz No action needed on main. This is a known limitation right now just due to how splats are painted and draw order.

Development on this branch is frozen for now. It's definitely something we want to address in the future, but that will come with other updates/branches.

Splatting does not use the translucency pass, so it's possibly related to that.

[ggetz]

Thanks @keyboardspecialist! If it's useful to discuss this known limitation, I'm going to leave this issue open and tag this as "gaussian splats" for consideration in the future.

But let me know if you have other thoughts.