I am a beginner and I want to add nails to a cactus I've modeled. How would I go about doing this? I'm after actual geometry not just a flat plane with a texture.
One way to go about this would be to use DupliFaces.
Duplication Faces or DupliFaces is the capability to replicate an object on each face of a parent object.
You would model your nail then parent it to the cactus object with Ctrl + P. You then select the cactus object and in the Object panel under Duplication and choose Faces.
This is not perfect and the outcome of this depends a lot on the topology of your parent object (the cactus) and how your child object (the nail) is scaled and or rotated etc. It can work with some tweaking however…
Another way would be to use a particle system, set the type to Hair and under Render, select the nail object. This is a bit harder to tweak if you don't have much experience with it etc. but can give you alot more control and variation as to how things can be placed.
Expanding on iKlsR's answer… if using the dupli-faces method, both objects have to be at
0, 0, 0 (xyz).
What might also be causing problems, is that orientation and the origin of the duplicated object are very important as well… for your needles the origin should be at the base of the needles.
For rotating the object, just experimenting is the easiest way just remember to either rotate in edit-mode, or if you rotate in object mode, Ctrl + A to apply the rotation, so that in the end both (duplicated and duplicating object) both have location
0, 0, 0 and rotation
0°, 0°, 0°
With the particle method you do indeed have a lot more control, but it takes more time to set it up properly.
Also, if you work with particles, you also have control over which faces emit the thorns.
(you just have to add the vertices of those faces to a vertex group and select this group for the emitting density)
for this method origin and orientation of the object that is to be duplicated are important as well
While I heartily agree that Duplifaces are a good way to go, I tend to combine them with a Shrinkwrap Modifier. You create a second mesh, consisting of tris and glue it to your target mesh with a Shrinkwrap. Then you parent the nail to the shrunk mesh and enable Duplifaces in the shrunk mesh's properties. Make sure your nail has the object origin at it's base.
Now you can shove the Tris around and you still can be sure that the nails are reasonably perpendicular. You'll have absolute control over the placement and density of your nails/thorns.
You can either give the shrunk mesh a transparent material or you can exclude it from rendering in the Outliner
You do not have to create the shrinkmesh by hand. You can separate all faces by applying an Edge Split Modifier set to 0° Split Angle. And you can decrease the size of the faces in Face Select mode by scaling them all with Individual Origins as Pivot Point.