Frustrating bald patches on your animal model? Use Triangles!

[Jaco_Pretorius]

TL;DR: If you find your interpolated child hairs are sticking into your quad-model, use a Triangulate modifier before your particle system, and switch on the “Use Modifier Stack” option in the Particle settings.

So, I don’t see a lot of people posting finished animals and I have a suspicion why - Blender’s child interpolation is broken on narrow cylindrical geometry, and therefore the “grooming” tools are pretty much useless there… no matter what you do your animal will always look as if it is suffering from some kind of skin disease… a large portion of your hairs will be inside your animal (especially on the legs and tails) and there is very little you can do about it except “add more hairs”.

The workarounds are tedious and not fun. I hope they fix this.

Test geometry is just the tail of my cat model (face orientation view is on):

Top view with 4 examples with 2 pairs of identical hair particle systems (apart from 1 setting, more on that later):

Geometry A and B are identical, except that geometry A has a mirror modifier while geometry B had that modifier applied. The hair particle system is applied after the mirror modifier and the settings are the same:

Using same amount of particles as there are faces. In the above image, the side-geometries show the particle systems without interpolation; they demonstrate that each face does indeed have a hair particle. Geometry B has “use modifier stack” setting turned off (the particle system is the only thing in the modifier stack for geometry B).

Another view:

The expectation of any reasonable person is that the interpolation on geometry B should be identical to geometry A… yet this image clearly demonstrates that applying a mirror modifier will completely break the interpolation and cause about 30% of your hair particles to just go inside your model.

The Blender developer forums are inundated with reports of this and similar bugs, so I was dissuaded from re-reporting this bug myself.

Only started on the journey of figuring out workarounds and the only one that guaranteed results right now is to just not use the interpolation feature: i.e. get a super-computer and simulate millions of hairs… mortal PCs will just have to groom at one frame every 2 minutes.

Will be working on workarounds in the next couple of days… will report back if I find a method that’ll have a better chance of working on a potato.

[Jaco_Pretorius]

OK, found a solution quicker than expected

Seems to work in general…

Having a WeightedNormal modifier before the hair system sorts out the issue where interpolated child hair go into your model for some reason. If you keep the mirror modifier, then the interpolation seems to also work (Remember to switch on the Use Modifier Stack option). Also works with grooming.

Without the WeightedNormal modifier, notice the “banding” in the tail, and the skin patches showing through on the legs (if you look inside the model you’ll see a bunch of hairs sticking through):

Now with the WeightedNormal modifier:

Much better (and almost no hairs inside the model).

No idea why this works, but it does. I think if you kept your base-model mirrored, you probably would not have run into this issue.

Hope this helps someone.

[NP5]

Great problem, and detective work for a solution!
Bald patches, forcing more hairs frequently, and the ‘odd’ inside ones are an annoying part of doing hair.
Summerising as:-

Better particle hair placement and control.

Modifier stack.

1. Mirror (if used)
2. WeightedNormal
3. Particle hair.

Use Modifier Stack required.

[Jaco_Pretorius]

Update: Found that using a triangulate modifier before the particle settings works the best if the base mesh has quads. Forget what I said before, hair seems to want triangles… pretty sure Rick mentioned that but I must have been day-dreaming and forgot.

WIP cross-eyed cat - It is rigged using the default cat riggify rig (armature modifier needs to be at start of stack). Hair does not clump well at bits that deform quite a lot (like by the haunches). But that’s for another day to sort out.

Almost done with all the blender courses and will probably have to say goodbye soon.

[NP5]

Looks a bit scruffy cat! lol.
Needs a good brushing or stroking.
Hmm triangulate, is that actually a benefit of doubling the faces smaller?

Can’t say goodby there is always new stuff to do, ever changing Blender. Hard to find good little communities like this.

[Miss_B]

@Jaco_Pretorius

Almost done with all the blender courses and will probably have to say goodbye soon.

Nooooo! I’ve been here over 4 years, and (so far) have only taken this Blender course. I’m sure I’ll do others at some point. Besides @NP5 is right, they’re always updating something.

[Jaco_Pretorius]

@NP5

is that actually a benefit of doubling the faces smaller?

No, that doesn’t seem to be the case:

In the original test geometry, adding the triangulate modifier fixes the issue without increasing the particle-count… seems to be something to do with how Blender interprets normal data and how you need some type of modifier (like the weightedNormals modifier) to bubble that info through to the particle system (in the case of quad-heavy geometry, that is). The triangulate modifier just happens to give the most consistent results in my testing.

@Miss_B: Thanks for being part of what kept me sane in 2020, and don’t worry, pretty sure I’ll pop back every now and then, but I don’t feel like a beginner anymore and I think it is almost time to leave the nest…

[Jaco_Pretorius]

Progress:

Getting there.

[NP5]

Gorgeous fur!
Tail looks a bit too ‘fluffy’?

[Jaco_Pretorius]

Thanks… for a Birman, the tail is actually not fluffy enough… will need to fix that.

[FedPete]

Beautiful! Real-life-like cat.

[Alex_Rusu]

Thanks for sharing this. I’ll look into it. My computer is quite decent and had a lot of time wasted with either hairs not showing, patches of skin and hairs groomed that were looking funny. Not to mention the number of crashes and things I forgot to save after doing some minor changes, tedious ones. I was also wondering if weight paints are not supposed to show per payer. I had 3 colours for my render and the weight paint was the same for all layers, which didn’t seem the right thing. Had a lot of small adjustments to do by jumping from object mode into particle edit because I couldn’t see what I needed into the viewport. Also found a literal ball of hair inside my mesh’s head. It was poking out through it’s eyes. Might just be that the hair particle system is difficult to master, but still seems buggy. Anyway thanks for the workaround tip!

[godofwar]

your cat looks absolutely stunning I was hoping you could send the final settings like checkmarking the modifier stack or did you use the triangulate modifier or weighted normal modifier I need help as I’m really confused

[Jaco_Pretorius]

Note that this bug only seems to happen with interpolated “child” hair on some quad topology.
To fix:

• Add a Triangulate Modifier before the hair-particle system
• Check Use Modifier Stack in Particle Properties.
• Make sure you have enough particles (roughly the number of faces * 2)

A 180-faced geometry. Left does not have children, middle has interpolated children, right is triangulated.

So, for the rightmost one, the setup is:

… using about 50 child particles here.

Good luck.

[godofwar]

thank you

[FedPete]

Welcome back

[NP5]

Welcome Back Jaco, I had forgotten this ‘trick’.

[Jaco_Pretorius]

… I never left …

Spoiler

I pop in every couple of weeks and sometimes even vote in the challenges.