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Vertex groups for hair (length etc.): effect not always satisfactory
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System Information
iMac 27-in late 2015; 16GB; AMD Radeon R9 M395X 4GB

Blender Version
Broken: recent builds of 2.80 and 2.79 (feb 2018); 2.78c; 2.75a. Worked: none yet

When you open the attached .blend file,

you will see two objects with particle-based hair.

The lower one has a vertex group for length, that covers the entire object,
whose weights go linearly from 1 to almost 0, and its effect on hair length
is not quite what one would expect, but close:
weights of 0.9 and 0.8 appear to have the same effect as 1.0,
and the shortest hairs are about 1/4 the length of the longest, as opposed to almost 0,
but there is, clearly, a linear dropoff from vertex weight 0.8 on downwards.
This I observe with interpolated child particles, no kink,
or with kink = Curl, Radial, Wave, or Braid.
Note: Kink = Spiral does produce a length dropoff that looks impeccable!

The upper object is a simplified scalp from my WIP here:

It also has vertex groups for density, length, and kink, but here the discrepancy
between the expected hair lengths and what one sees is much greater.
In this case, even kink = Spiral fails to satisfy length expectations; moreover,
less than half the hairs receive a spiral, even when I’m not using any vertex group for kink.

Event Timeline

I'm not entirely sure what the cause is, but it seems to related to the number of hair segments interacting strangely with interpolation. There are plans to replace particles with a new system, so not sure if this will end up getting fixed in the current system or not. In the mean time increasing the 'steps' parameter should give you a better result.

Thank you, Mai.

The “Steps” parameter is specifically for kink=Spiral, and appears to affect only the presentation of the spiral,
but the “Segments” parameter that shows up near the top of the Particles Properties does influence hairs
in remarkable ways. My .blend file has it set at its default value, 5. If you select the scalp object
(named HeadLenTest) and bring Segments down to 4, you will see that, in response,
many hairs now extend their endpoints into the scalp, even though the object’s normals all point outward.
This effect is most obvious with kink=Spiral: you will then notice that all the hairs that extend inwards
have their spiral on the “in” side, which may provide a partial explanation to why, when Segments=5,
many hairs appear to have no spiral.

When Segments=2, no hairs project into the scalp, and the range of lengths appear to be ok.

Raising the Segments value from 5 to 10, or 20, appears to have no effect other than to shorten the hairs.

The “Steps” parameter is specifically for kink=Spiral, and appears to affect only the presentation of the spiral,

Mai is not talking about this setting present in Children panel.

In Display panel and Render Panel, there is also a "Steps" parameter.
This steps are set to a value of 2 or 3. If you increase them to 10, you obtain a better render result.

Steps are controlling visual segments of hair path.
Step to 0, you obtain a straight line. Steps to 1, you see a polyline of 2 segments. Steps to 2, a polyline of 4 segments, Steps to 3, a polyline of 8 segments. etc.... It is a power of 2.

You can define 20 physical segments that can be edited in Particle Edit mode and would be used by hair dynamics simulation.
If number of steps stays equal to 2, path stays visually a polyline of 4 segments.
And any particle emitted from face is not emitted from a vert. It has a weight higher than 0. So, first step is shown for most of particles.

Increasing number of steps on a path of same length means reducing length of this first step.

Increasing default value for "Steps" parameter, everywhere, to 4 would probably be more pleasant for users and lead to less questions.

Sergey Sharybin (sergey) triaged this task as Normal priority.

Caused by rB4f1c0a1. Will give a closer look on what's wrong in that commit later.