A 3D detail placed at a 2D-looking offset ends up inside the mesh and vanishes silently

Fact

In 2D, draw order decides what is on top, so a face can be painted at any offset over a body and it will be visible. In 3D there is no draw order to lean on — position alone decides — and the offsets that look right in a flat sketch land inside the solid.

The gap is bigger than it feels. On an ellipsoid with semi-axes (a, b, c), the surface at a point that is 34% of the way across and 16% of the way up is still at z = c·√(1 − 0.34² − 0.17²) ≈ 0.87c. An eye placed at 0.62c — a perfectly reasonable-looking "on the front of the head" — is entirely swallowed, and so is anything smaller than the 0.25c it is short by.

Why it matters

Nothing reports it. The mesh exists, is added to the scene, is lit and is drawn; it is simply behind an opaque surface. So the symptom is a missing feature with a healthy scene graph, which reads as "the mesh never got added", "the material is broken" or "the parent transform is wrong" — three wrong places to look.

It bites hardest when porting layered 2D art, because every offset in the old code is a plausible-looking constant that now means something different, and the whole face disappears at once rather than one part at a time.

How to apply

• Project onto the surface instead of guessing: normalise a direction, scale it by the semi-axes, and orient the part outwards (lookAt the point doubled).
• Decide protrusion deliberately — sink a flat marking slightly in, push a rounded eye out by a fraction of its own radius.
• When a part is missing, rule out occlusion before existence: flip the parent's material to wireframe, or read the child's world position and compare it against the surface, rather than hunting the scene graph.
• The same applies to decals on any closed primitive, not just ellipsoids.