I am doing this tutorial where CG Geek shows how to build a realistic city. After about 45 minutes the city has basically been built. And now a few small buildings are created on a separate layer and a Hair particle system is created to grow the small buildings (as a group) out of the roofs of the high rise buildings

enter image description here

Not all of those small buildings are placed within the boundaries of the roofs. How can I control the positioning of these hairs so that all small buildings are neatly placed at the roofs ?

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    $\begingroup$ I think ideally, when having a problem with a complex tutorial you need to try and contact the tutorial maker. It will be very difficult for people who haven't already followed the tutorial to work out a solution which fits in to the techniques already used, particularly seeing as there is no indication what techniques were used to create this (without following the tutorial). At that the very least I think it requires you to upload your blend file. $\endgroup$ Oct 31 '16 at 17:14
  • $\begingroup$ @RayMairlot As you probably know I upload My blend file almost always when I ask à question. I throught iT wouldnot contribute much this time and I tried to formulate My question so that knowledge of the tutorial would be less important. But perhaps You are right $\endgroup$
    – Old Man
    Oct 31 '16 at 22:39
  • $\begingroup$ @RayMairlot About asking THE tutorial makers... I find they most Often don't answer your question, especially the popular tutorial makers $\endgroup$
    – Old Man
    Oct 31 '16 at 22:41
  • $\begingroup$ @RayMairlot I have added the file, thanks for your feedback $\endgroup$
    – Old Man
    Oct 31 '16 at 22:49

You can achieve this with several different ways:

A) Don't have the wrong particles (Chose one or more)

  • Make the houses smaller
  • Make less houses
  • On rotation, check "normal". If you made your normals right, the houses should align along the faces decreasing the probability of going of an edge
  • Adjust the seed until all houses fit in the areas visible in the final render

B) Cut them in particle edit

C) Vertex groups

  • Add a new vertex group
  • Assign all faces where a house is possibly allowed to stand (leaving a maring of the maximum size of a house in every direction) to that group
  • In the particle settings -> Vertex groups, select this group under "density"

D) Texture (Cycles only)

  • Create a new, empty image.
  • Go to orthographic top-down view of your city.
  • Select your mesh.
  • Go into texture paint mode.
  • Paint the areas where houses are allowed.
  • Go to particle settings -> Texture
  • Create a new texture and go to texture properties
  • Link that texture to the image you painted
  • Under influence uncheck all but "density"

E) Remove them manually

  • Apply your particle system.
  • Delete wrong houses
  • Select the remaining ones and join them (optional)

That's about all the possible ways I could think of. Chose the one you like the most! If I missed a good way, please add it. If something is unclear or you need screenshots, I can add them.

  • $\begingroup$ Thanks a lot for your very clear answer. I will try the various methods tomorrow $\endgroup$
    – Old Man
    Oct 31 '16 at 22:51

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