Flexcomp: deformable body support (grid, mesh, direct)#18
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jonathanembleyriches wants to merge 3 commits intomainfrom
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Flexcomp: deformable body support (grid, mesh, direct)#18jonathanembleyriches wants to merge 3 commits intomainfrom
jonathanembleyriches wants to merge 3 commits intomainfrom
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UMjFlexcomp component parses MuJoCo <flexcomp> XML into UE properties (type, dim, count, spacing, mass, elasticity, contact, pin, etc). RegisterToSpec serializes the attributes back to a standalone MJCF fragment, parses it via mj_parseXMLString, and attaches the resulting expanded sub-spec (flex + generated bodies + slider joints) into the parent body via mjs_addFrame + mjs_attach. MuJoCo's own parser handles the macro expansion, so all geometry types (grid, box, mesh, direct) and DOF modes (full, radial, trilinear, quadratic) work correctly. Mesh-type flexcomps: welds per-face-split UE render verts, writes an OBJ to the wrapper's VFS, references it by filename in the generated XML. Python preprocessor (clean_meshes.py) extended to convert flexcomp-referenced meshes to GLB during import. XML parser creates a child UStaticMeshComponent for mesh-type flexcomps by resolving the file attribute relative to the XML directory, preferring GLB over raw OBJ. 7 automated tests covering grid (1D/2D/3D) compilation, flex vertex count, pinning, import attribute parsing, and elasticity propagation.
Each UMjFlexcomp creates a UDynamicMeshComponent on Bind that mirrors the source static mesh (positions, UVs, normals from the LOD render data) and hides the original static mesh. Every tick, the deformed vertex positions are pulled from mjData.flexvert_xpos, converted to UE local space, and pushed via FastNotifyPositionsUpdated. Raw UE vertex attributes (UVs, normals, material) are preserved via a RawToWelded remap table, so the source mesh's UV seams and smooth shading carry over to the deforming display. The source material is copied directly onto the dynamic mesh. Tangents are auto-calculated by GeometryFramework via MikkTSpace to avoid crashes from manually populated tangent overlays. Reads of flexvert_xpos are guarded by the physics engine's callback mutex to prevent torn reads while the async physics thread steps. Known limitation: TAA ghosts slightly on fast deformation because FLocalVertexFactory (used by UDynamicMeshComponent) has no previous- position vertex stream. For flex-heavy scenes, set FXAA via r.AntiAliasingMethod=1.
Add user-facing guide covering authoring, import, visualization, DOF modes, and the TAA ghosting limitation with FXAA workaround. Update MJCF schema coverage with all supported flexcomp attributes and sub-elements, noting pingrid/pinrange as the remaining gaps.
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Summary
Adds support for MuJoCo's
<flexcomp>macro — deformable ropes (1D), cloth (2D), and volumetric bodies (3D).Three commits:
UMjFlexcompcomponent wraps MuJoCo's flexcomp. Rather than reimplementing MuJoCo's macro expansion, the component serialises itself back to an MJCF fragment, parses it viamj_parseXMLString, and attaches the sub-spec withmjs_attach. This means every DOF mode (full/radial/trilinear/quadratic) and geometry type (grid/box/cylinder/ellipsoid/square/disc/circle/mesh/direct) works without plugin-side reimplementation. Mesh type sources geometry from a childUStaticMeshComponent— welded vertices are exported to an OBJ in the VFS on registration.Bindcreates aUDynamicMeshComponentmirroring the source static mesh (geometry, UVs, normals copied from LOD 0). Each tick, deformed vertex positions are read frommjData.flexvert_xposunderCallbackMutexand pushed viaFastNotifyPositionsUpdated. Source static mesh is hidden.Known limitation
TAA ghosting on fast deformation.
FLocalVertexFactoryhas no previous-position vertex stream and there is no public API to provide one, so CPU-deformed meshes produce zero motion vectors — TAA accumulates stale samples. Workaround: setr.AntiAliasingMethod=1(FXAA). Proper fix would require a custom vertex factory + forked shader (~1000+ LoC). Documented in docs/guides/flexcomp.md.