Converting a model between formats loses whatever the destination cannot express. The losses are predictable.
Materials
The most common casualty. A node-based procedural material becomes, at best, a base colour texture. PBR values survive glTF-to-glTF and approximate everywhere else. Assume materials need rebuilding after any conversion involving FBX or OBJ.
Animation
Survives FBX-to-FBX and glTF-to-glTF reasonably. Constraint-driven and IK-driven animation does not survive at all unless baked first, because the destination has no way to evaluate the rig.
Scale and orientation
Every format and application has conventions. Y-up versus Z-up, metres versus centimetres, left versus right handed. Conversions apply corrections, and applying them twice is as common as not applying them at all.
Vertex order
Rarely thought about, and it matters: blend shapes are tied to vertex order. A conversion that reorders vertices invalidates every morph target silently.
Smoothing and normals
Some formats store explicit normals; some store smoothing groups; some store neither and rely on an angle threshold. Round-tripping through a format that drops the information means normals are recalculated, and a model that had crisp panel edges arrives smoothed.
Multiple UV sets
Supported by glTF and FBX, dropped by OBJ. A model with a lightmap layout loses it going through OBJ, and nothing warns you.
Practical rules
- Convert once, in one direction. Round-tripping compounds losses.
- Keep the authoring file as the source of truth. Exports are derivatives.
- Verify after conversion, against a checklist: scale, orientation, normals, UV sets, materials, animation.
- Bake anything procedural before export rather than hoping it survives.