enter image description hereI want to use particle system to put particles on the inside of this hollow cylinder (to project from inner surface to the hollow cavity or lumen) but when i use particle system it automatically covers the outside of the cylinder with particles.

  • $\begingroup$ "..to put particles on the inside of this hollow cylinder"... which one? maybe will be better explain what kind of effect you are looking for, or attach some images to let people evaluate your problem. $\endgroup$ – mike Nov 20 '19 at 20:10
  • $\begingroup$ @mike I edited the question. $\endgroup$ – mohamed Nov 20 '19 at 20:28

Although you can give hair a negative length,

enter image description here

it is better to change normal directions

enter image description here

or solidify a tube and choose what faces to generate particles on.

enter image description here

  • $\begingroup$ very nice answer $\endgroup$ – mohamed Nov 21 '19 at 8:20

The particle system is following the direction of the normals on the cylinder faces. You can flip the cylinder face normals so they point in instead of out.

If you select the cylinder, switch to edit mode, select all with "A", and then do Mesh -> Normals -> Flip, that'll flip all the normals. Go back to object mode, and your hairs should be growing into the cylinder (though you may need to decrease their length to keep them fully within the cylinder.

If you need faces with outward pointing normals on your cylinder, too (e.g., if you want to properly shade the outside of the cylinder), then you should give your cylinder both inner and outer faces. The Solidify modifier can do this, and if you put it in the modifier stack after the particle system, the particle system will continue to grow only on the inside of the cylinder.

However, if you need to Apply your Solidify modifier at some point, you'll have particles growing on both the inside and outside, because they'll be coming out in the normal direction from all the faces. To fix this, you can use a vertex group to define which faces should emit particles. It's a little complicated, but basically:

  • Go into the Cylinder's Mesh Object Data pane.
  • Under Vertex Groups at the top, add a new group named "Inside".
  • In edit mode, select all the inner vertices (or inner faces), and click "Assign" under the Vertex Groups panel. Go back to object mode -- it'll still look the same.
  • Switch to the Particle Properties pane, and under the Vertex Groups panel, select "Inside" as the "Density Group"
  • Now, the hairs should only be growing from the inside.

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