Houdini Groom Fur/Hair

reference:

https://youtu.be/p4yUNgy5HSA?si=m0sZ1LR9qjeizaTD

https://youtu.be/csPolONCnxE?si=YFZs2IBbB7V2SP7S

Preparing 3 object for demonstration

original mesh – the static mesh that will put the guides on
collision object – animated tube for testing hair sim
deform mesh – the mesh which should have the same vertex ID as the original mesh but either being animated or deformed. In this case is being scaled

get a guide groom node
drop the static mesh out null to the SOP path

turn off the display as subdiv curve to speed up the viewport

need to adjust the VDB size. to do so, first needs to visualize it
make the guide groom node editable
go in side and look for the VDB from Poly node. visualize it

then go back out and adjust the size so the VDB to have more details
lock it back up after it is done

to apply a custom density mask or any type of mask. It can be done manually by adding the attribute create node and attribute paint node inside the SOP geo
or click on skin attribute from the density drop down of the guide groom node
click on the painting icon on the left
it’ll add it automatically.
Suggested to add it manually. to understand the process better. multiple masks with multiple different mask name can be added through the same way.

make sure that the name of the attribute create are the same as the mask name that is being recalled

to paint the mask, select the attribute paint mask and press enter at the viewport
the value can be beyond 1
to visualize it go to the visualizer icon and chooes the mask
under min and max adjust to what min/max is being painted on

inside the guide groom node add another guide groom node
hover to the centre and it’ll connect it by itself

the guide groom node is a paint node
select delete and over to the viewport and hit enter
delete all the guides

switch over to plant node and plant them manually
switch to sculpt mode to further shaping the guides

Guide partition: Currently still can’t find a way to make part lines to work in a guide deform/simulate stage.

to add parting line, add a guide partition node
hit draw parting line and it’ll add a draw curve node
draw the line on at the viewport

note that if the guides are not goign to deform to an animated mesh a rest node isn’t needed

GuideDeform: to reference the static mesh groom to the animated mesh

add a guide deform node
specify the deformed mesh
drag and drop the guide groom node to the groom object section

Guide Simulation: to put collision or simulation to the guides

simply drag and drop the guide deform node tot he groom object section. If the object doesn’t deform then put the guidegroom node in there instead.

by default the simulation is more for long human hair
for fur and short hair, either to test and increase the stiffness and damping ratio from the vellum constrain section or increase the vellum solver substeps to 2, or both

go to vellum collision to activate external collision.
put the collision object to the collide SOP section
better to turn on self collision as well

Drawing the hair: this part is the most difficult and needs a lot of custom adjustments to define the hair clumps

add a hair generation node
put the guide sim node or deform node to groom object
notice that if a parting line is added earlier there will be an error showing
adding the rest node from the guide groom node will solve this issue

increasing the influence radius will draw the hair around each guide wider

inside the hair generation node, add hair clump node to get clumps
more other nodes like “guide process” can enhance the look of it

Skin attribute: This is a type of mask that is like the density mask being added at the geometry SOP stage or it can be added under hair gen. In projection it is more comment to add attribute mask in SOP level and recall them to guide process node or clump node.

to add the attribute mask within hair generate node
for any drop down on right, choose skin attribute
it’ll predefine the name of the input. type in the attribute name it is pre-made in the SOP level

hitting the painting icon will add the 2 node inside hair generation node.

Guide Attribute: this one is similar to skin attribute but be a more procedural. A guide mask must be prepared in advance tobe recall

add a guide mask before the clump node
give the out attribute a name
in the additional mask tab, commonly will use noise mask or random mask

back to the hair clump or guide process node
pick guide attribute and type in the name of the guide mask
both guide and skin attribute can be use on those overides. So it’ll be driven by the mask instead of a flat float number.

additional fractal clumping can be added to the clump node without adding extra guide attribute

Limit to bounding box: generate hair in a selected area to speed up the process for lookdev

under hair gen node
optimization > limit to bounding box

Houdini Groom to Unreal:

source: https://www.youtube.com/watch?v=YL8oLH-EzGU

inside hair gen node
get a resample
change the segment or the length, make sure the vertex aren’t too much but keep the shape as close as possible as the original
set the treat poly as subdiv

add a view attribute
get attribute promote
set the original name
change class to vertex
new name to groom_width (that is needed for unreal and it is written in their documents)

ps: if the colour is needed for the groom. needs to promote the cd attribute as well. Refer to the following tutorial for it.
source: https://youtu.be/AbQImIzNtV8?si=X4HAr9dQcm-mO95Q