Hair info: tangent (direction)
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Authored by Mikhail Matrosov (ktdfly) on Mar 26 2021, 2:15 PM.

Details

Reviewers

None

Group Reviewers

Cycles
EEVEE & Viewport

Summary

Adds hair tangent (direction) output to hair info node.

Outputs a unit vector tangent to hair, pointing in the direction of hair growth. Works for Cycles and Eevee.

Why it is needed?

Enables numerous shading effects.
Hair tangent is impossible to derive mathematically from normal or view direction.
Tangent is evaluated in D10706: Eevee hair shader, but instead can be connected to this node.
Rightclickselect requiest and devtalk.

Naming

It is tangent to hair.
Name "direction" would be confusing for curly hair.
It is not "root normal" since it is not locked to a hair root, and is not constant along hair. Though, it is still works for a RCS request.

Diff Detail

Repository: rB Blender

Event Timeline

Mikhail Matrosov (ktdfly) requested review of this revision.Mar 26 2021, 2:15 PM

Mikhail Matrosov (ktdfly) created this revision.

Mikhail Matrosov (ktdfly) created this object with edit policy "Administrators".

Duarte Farrajota Ramos (duarteframos) awarded a token.Mar 26 2021, 2:31 PM

The tangent is currently exposed as part of the Geometry node, I think this is the same thing?

Maybe there is an argument that it's more discoverable as part of the Hair Info node, though so far we have avoided duplicating such things.

In D10829#275166, @Brecht Van Lommel (brecht) wrote:

The tangent is currently exposed as part of the Geometry node, I think this is the same thing?

No, it is not the same thing. In the Geometry node the exposed tangent is exactly Z-tangent and doesn't know anything about underlying hair structure. Here I propose basically a hair direction output socket.

In Cycles, the Geometry Tangent also returns normalize(sd->dPdu);.

In Eevee it seems to be doing something else indeed.

Stig Oskar (Stig) awarded a token.Mar 26 2021, 3:39 PM

jack conradsen (STANN.co) awarded a token.Mar 26 2021, 4:52 PM

In discussion with @Brecht Van Lommel (brecht) he suggested to fix Geometry info node for Eevee to output hair tangent (and to be consistent with Cycles) to avoid duplicating functionality. See D10841: Geometry info -> tangent: Make consistent with Cycles for hair

In D10829#275177, @Mikhail Matrosov (ktdfly) wrote:

In D10829#275166, @Brecht Van Lommel (brecht) wrote:

The tangent is currently exposed as part of the Geometry node, I think this is the same thing?

No, it is not the same thing. In the Geometry node the exposed tangent is exactly Z-tangent and doesn't know anything about underlying hair structure. Here I propose basically a hair direction output socket.

When you say the tangent is exactly Z-tangent.
Do you mean it get's the direction of the surface the hair is sitting on?

Cause that would still be very useful for toony shading effects. I've talked about it prior, but Here I've narrowed down and described it a bit better

In D10829#302193, @jack conradsen (STANN.co) wrote:

When you say the tangent is exactly Z-tangent.
Do you mean it get's the direction of the surface the hair is sitting on?

No, it's the tangent along the hair curve, not the surface it originated from.