Edge flow is the direction the loops in a mesh travel across a surface. Good flow follows the underlying structure; bad flow cuts across it.
On a face
Loops circle the eyes and the mouth, radiate outward, and follow the cheekbone and jaw. That is not aesthetic preference — it is because those are the lines the face moves along. A smile pulls the mouth loops; a blink closes the eye loops. Geometry aligned to the motion deforms cleanly; geometry running across it shears.
On mechanical objects
Flow follows the panel lines, the edges, the direction of manufacture. A bevelled edge needs supporting loops parallel to it, close enough to hold the highlight under subdivision.
On limbs
Loops circle the limb, perpendicular to its length, with concentrations at each joint. Three loops at an elbow behave far better than one, because bending distributes across them rather than pinching at a single line.
Reading a wireframe
You can judge a model's fitness for animation from its wireframe alone. Look for:
- Loops circling the joints
- Even quad distribution across deforming areas
- Poles placed away from creases
- Density concentrated where curvature and motion are
If the wireframe looks like an even net draped over the shape with no relationship to its structure, it is a rendering mesh rather than an animation mesh.
Why generated meshes have none
Reconstruction fits a surface to observed shape. Nothing in that process knows where a joint is, which way a face moves, or where a panel line runs — so the resulting edges follow the algorithm's convenience. This is exactly the gap retopology closes, and why the level of detail chosen for a character matters more than for a prop.