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So I'm making trees on a hill. But from what I know, trees grow straight up, and not at an angle. So how would you make the hair particles go straight up and not at an angle. Here's an example of the problem I'm having.

Tree Example Thing

I would like the trees instead of going outwards, to go upwards.

How could you do this?

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    $\begingroup$ Note that trees don't always go straight upwards, though it is of course by far the most usual configuration. $\endgroup$ – Lightness Races with Monica Sep 27 '15 at 21:00
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    $\begingroup$ @LightnessRacesinOrbit Thanks! I'll try to find some reference to see the angle for the trees $\endgroup$ – Chickenator Sep 27 '15 at 21:19
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Enable Advanced and Rotation, then set the Initial Orientation to Global Z:

enter image description here

Note that you'll have to rotate your objects so they are facing either the +X or +Y direction instead of the +Z direction. See Why are the objects in my hair particle system not sitting on top of the ground?.

By using this technique the orientation of the particles is kept independent of their size, and you can add some random rotations via the Particle settings > Rotation > Random settings:

enter image description here

Adding such randomizations will help make repeated use of the same models less noticeable.

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    $\begingroup$ This solution is more correct and clean. That's the way it was meant to achieve that goal. It deserves more upvotes and to be mark as accepted. $\endgroup$ – Carlo Sep 28 '15 at 9:00
  • $\begingroup$ I'm unsure whether to mark this one or the other one. Since the other has more upvotes I'll stick with it but if everyone prefers this question I'll mark it as correct $\endgroup$ – Chickenator Oct 4 '15 at 13:43
  • $\begingroup$ @Chickenator The answer you mark as accepted is entirely up to you. The question isn't which answer other people prefer, it's what answer you prefer. Votes are there to show what the majority prefer, but you don't have to follow them ;) $\endgroup$ – gandalf3 Oct 4 '15 at 18:55
  • $\begingroup$ Well since your answer also showed random rotation and better ways to do it I'll mark it. $\endgroup$ – Chickenator Oct 4 '15 at 19:02
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By enabling Advanced settings checkbox in the Hair partices system...

enter image description here

...you'll be able to see the Velocity panel, where the Hair emission vector's components can be defined.

By default hairs are emitted toward the face's normal.

enter image description here

Set the Normal value to 0 and increase the value of the wanted emitted vector axis, in your case Z.

enter image description here

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