OS: Windows 7 x64
Blender version: 2.74 (although it shows up in .73 and possibly earlier)
The clumping of child hairs does not take into account the length of the parent hair as defined by the Length vertex group.
Steps to reproduce:
- Add a new particle system to an object, and set its type to 'Hair.'
- Enable either simple or interpolated children (the bug shows up with both).
- Set 'Clump' to 1.
- Switch to Weight Paint mode on the emitter object, and paint an area with a low weight.
- Under the Vertex Groups panel of the particle settings, select the newly created group for Length.
- Note that in the areas where the hair is shorter (i.e. the low-weight areas), the tips of the children do not converge at the ends.