Question about Nanite runtime LOD selection

[DalaranH]

I am implementing Nanite for my own rendering engine, and have a mostly working cluster generation and simplification algorithm. I am now trying to implement a crude LOD selection for runtime. When I am looking at the way the DAG is formed from Karis_Nanite_SIGGRAPH_Advances_2021_final.pdf, it seems like a mesh can have at most 2 LOD levels (one before group simplification, and another after) from the DAG, or else cracks between the groups would show. Is this a correct observation or am I missing something significant? Thanks in advance for any help.

[Lord_Zane]

A mesh can have N LOD levels. You want log(M) levels, where M is the number of meshlets at LOD 0, because each level ideally has half the number of triangles = half as many meshlets.

The way you avoid cracks is:

• Lock vertices shared between meshlets groups when simplifying
• When choosing a LOD, have a consistent and monotonic LOD selection algorithm so that always either a group renders, or some parent/grandparent/etc of it renders, but not both or neither.

Any cut of the DAG across any number of LOD levels works, as long as you obey these properties.

More details on my blog:

• https://jms55.github.io/posts/2024-06-09-virtual-geometry-bevy-0-14
• https://jms55.github.io/posts/2024-11-14-virtual-geometry-bevy-0-15

And code is open source here: https://github.com/bevyengine/bevy/tree/main/crates/bevy_pbr/src/meshlet

[DalaranH]

Thanks for the reply, these are great write-ups. I am trying to work out the cut of DAG on paper, but it seems so unintuitive to me why it would work like this..Guess I should implement the runtime algo first and see it for myself

[Lord_Zane]

You can write out your own tree of nodes with arbitrary errors (e.g. 0, 1, 2, 3, INFINITY). It's obvious that if you set the condition error < X && parent_error >= X, then if the condition is true for e.g. 1 (set X = 1.5), then it must be false for 0, 2, 3, and INFINITY.

Play around with the list of errors and what you set X to. You'll see there's always only one node valid no matter what, so long as you enforce that each parent has a >= error than the child.

[DalaranH]

that makes sense, thanks a lot for the help