gltfpack: Accelerate deduplication pass for scenes with a lot of primitives #790
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While on most scenes our strategy performs reasonably well, and we already have some potential N^2 processing elsewhere, there are cases where the existing N^2 algorithms in the pipeline are actually semi-linear, because mesh merging lazily and effectively aggregates most meshes; since deduplication is the first pass, it may actually be quadratic in the face of a scene with a lot of meshes. A simple fix is to track the number of occurences of each hash and only search for duplicates for meshes where this value is >1. We could of course implement more involved tracking but for now this works just fine.
Neither std::map nor std::unordered_map are particularly good containers, but since we're now using unordered_map for deduplication filtering we might as well use it for the primitive cache. This requires a custom hash function for pair<uint64, uint64>, but our hash halves are sufficiently decorellated that a xor suffices.
This almost never comes up in practice, but occasionally scenes might have the same geometry attached to the same nodes. This results in counterintuitive results because we create node lists with duplicate nodes for this, which never happened before, and end up producing more quantization sub-nodes than necessary. For now we can just filter these attachments out to avoid redundancy.
clang-tidy complains that we're dividing an integer by 3 before a float cast; this is correct as the division is exact, and a cast silences it.
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While on most scenes our strategy performs reasonably well, and we
already have some potential N^2 processing elsewhere, there are cases
where the existing N^2 algorithms in the pipeline are actually
semi-linear, because mesh merging lazily and effectively aggregates most
meshes; since deduplication is the first pass, it may actually be
quadratic in the face of a scene with a lot of meshes.
A simple fix is to track the number of occurences of each hash and only
search for duplicates for meshes where this value is >1. We could of
course implement more involved tracking but for now this works just
fine.
We also now prune the node list to avoid accidental duplicate attachments,
as it confuses the logic downstream in the pipeline.