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I am seeing hair emitted from faces outside the vertex group. I created a simple UV sphere, a vertex group (colored), a hair particle system using the vertex group as the density, no children. You can see below hair growing out of faces adjacent to the vertex group and I don't know why. Anyone?

enter image description here

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  • $\begingroup$ Are there any modifiers on the object except for the Particle System modiifer? $\endgroup$
    – Mr Zak
    Nov 17, 2017 at 19:21
  • $\begingroup$ Nope. No other modifiers. $\endgroup$
    – stav_nan
    Nov 17, 2017 at 19:38
  • $\begingroup$ Then it most likely happens because of weight paint interpolation. If you preview that vertex group in Weight Paint mode you'll see something like this where vertex group is only where pure red light is. The outer face loop has fading out red color which is interpolating weight paint between 0 and 1 but it's enough for particles to be emitted. I don't really think one can avoid this other than making selection smaller or detaching part of mesh $\endgroup$
    – Mr Zak
    Nov 17, 2017 at 19:56
  • $\begingroup$ Ah, I think you're right. The weight paint mode shows exactly what you predict; the outer face loop is fading and thus allowing some hairs. $\endgroup$
    – stav_nan
    Nov 17, 2017 at 20:16

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Mr Zak has the right answer. The outer edge loop of my vertex group is weighted and allows some hairs to be emitted.

One workaround is to duplicate the faces in the mesh, assign them to a new vertex group and use that for the particle system:

enter image description here

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  • $\begingroup$ You can assign a texture for the particles density (and have texture resolution to be new limitation as well as uv map). Or you can paint some weights out (Subtract mode of the weight brush). Or make mesh geometry more dense (subdivide it in other words). Note that if duplicating geometry avoid removing it as double faces (if it is part of the same object) $\endgroup$
    – Mr Zak
    Nov 17, 2017 at 20:28
  • $\begingroup$ I prefer subdividing affected faces and adjusting the vertex group is a better solution for me. I'd rather avoid duplicating geometry for the reasons you mention. Thanks! $\endgroup$
    – stav_nan
    Nov 17, 2017 at 20:50

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