The groom is the source. Cards are a bake.

A groom is resolution-independent and restylable. Cards are a disposable engine-facing derivative,
re-baked when the budget changes, when an LOD tier is added, or when the tool changes. Everything on
this page follows from that one sentence, including which folder each file goes in.

Hair uses two of the three stages of the Raw / Source / Exported mirror. Raw is for
unprocessed captures and a groom is not one.

→ Project Folder Structure > The one idea

Sisir makes the groom, the cards and the atlas. Blender bakes the channel data, transfers the
normals and runs the checks. Unity gets an .fbx and a texture set, and nothing else.

Grooming is guide-based: you style a small number of guide curves, and the strands interpolate
between them. That is what makes a groom restylable. A parting is moved by moving four guides, not
by touching ten thousand hairs.

Cards come out of the same groom. Sisir clusters the strands, fits a strip to each cluster and maps
it into an atlas strip. The groom stays; the cards are its output.

It writes OBJ for the cards, and the atlas maps as ordinary image files.

We need exactly three things out of Sisir, and OBJ carries all three:

Alembic's advantage over OBJ is arbitrary geometry parameters and animation, and we want neither.

We do not want the parameters because we bake the channels ourselves. A per-strand random or a
width that Sisir wrote is a number whose definition lives in Sisir's release notes. Our contract is
arc length along this card, a hash of the quantised root centroid, and a stiffness somebody
decided, and Sisir computes none of those three. Carrying its params across would give us a second
set of numbers that look like ours, mean something else, and disagree silently.
→ Hair Cards

We do not want the animation because there is none to carry. The cards ship static and the movement
happens in the shader, from the channels. → Hair Motion

What is left is a format every tool writes, that opens in a text editor when an import goes wrong.

OBJ is unit-less and declares no up-axis. A file that is metres and Z-up is byte-identical to one
that is centimetres and Y-up. That is a real cost and it is answered three times, because one answer
would not hold.

The third is the one worth knowing. The first two stop a bad import; the third stops the bad import
being papered over.

Cards lit by their own flat normals read as a pile of planes. Each card catches light as the
quad it is, the ones facing the light go bright together, and the silhouette the alpha works so hard
for is contradicted by the shading behind it.

Blend the normals toward a scalp proxy (a smooth cap over the skull) and the same cards read
as one volume. The geometry does not change. Only what it tells the light does.

This is the biggest look win on the page for the least work, and it is a Blender step, so the
import does it on the way in when you name the Scalp Proxy in its sidebar. It runs on the joined mesh,
or on every object in turn when Join is off or the join fails. There is nothing in the cards that
says which way the skull under them faces, so with no proxy named the import says it skipped the
transfer rather than guessing one.

Transfer Hair Normals does the same job later, from the Hair Cards panel. It opens a dialog: pick
the proxy from a searchable list of the scene's objects, set Blend, and tick Apply Now only
when the look is settled. A live modifier can still follow a re-shaped scalp, and applying can't be
undone. The field opens on the proxy the cards already borrow from, and empty otherwise. It
never opens on the last groom's scalp or on a guess at the Body, because a wrong guess looks right until
the head is lit from the side, and an empty field just asks.

Pressing it again updates the transfer rather than stacking a second, because two transfers from two
proxies blend into a look nobody chose. It finds the existing transfer (a Data Transfer modifier carrying custom normals) even if you renamed
it, switches it back on if it was off, and keeps anything else it was set up to transfer.

The proxy is never counted as cards, whatever it is called: naming it here is saying it is the
scalp. It is only measured from (by the flip and placement checks) if its name says Body.
→ Hair > Which mesh is the cards, and which is the Body

requireNormalTransfer is on in the rules for the same reason: it is cheap, it is invisible when it
is missing until someone lights the head, and by then the cards are already in a prefab. MSH022
checks it on the mesh that ships, so a live transfer counts on a machine that exports with
modifiers and not on one that exports without them. When the cards ship flat, the finding names the
one repair that applies, because each sends you somewhere different:

None of them has a Fix button: which proxy the normals come from is a look decision, and the whole
body is the wrong answer for hair that hangs past it.

Always export through our add-on, never Blender's own FBX exporter. This is the existing house
rule and hair does not get an exception. → Bubbly Games/Workflows/3D Models/3D Models > Import and export

It runs the checks first, and it knows where the file belongs. MSH022 is among them, and like
every new rule it lands as a warning. It reports a flipped card, a card over two atlas strips
and a budget overrun, and it does not block. → Check Your Work > When a rule is wrong

Hair has a second reason. Only the add-on's export ships the attribute flagged for render. Unity
reads only the first colour set, and Blender's own exporter does not control which one that is. The
add-on puts the render-flagged attribute first for the export and leaves your mesh unchanged.
→ Hair Cards > The render flag decides which attribute ships

Overdraw is not measured on export. Press Measure Hair Overdraw before exporting.
→ Hair > Overdraw is an average, and it is measured on request