Hi Rick,
In the parameter view for nodes you can find a button in the top left with a question mark. Click that and you will be taken to the online documentation for that node. There are still gaps in the documentation, but the population and object nodes are there. Having said that the population page needs updating and I'll get on to that today.
The values you mention don't really "come from" anywhere. You define them, although some objects are placed in a certain location depending on the camera view when you create them. Others are placed at the origin. They have default settings and you change them to suit your purposes.
With populations you don't need to worry about getting things at the correct elevation. In the Terrain tab of the populator there's a "Sit on terrain" checkbox. If this is checked then population instances (the objects in the population) are positioned on the terrain. The "terrain" is defined by the shader link fields in the Terrain tab. By default these are connected to the Compute Terrain node and the default planet. Unless you have special needs then this default setup is sufficient to have the population instances sit on the terrain. I should say that exactly how an object sets on the terrain depends on where the object origin is. For example the Xfrog tree models typically have their origin set just above the roots, where the ground would normally be on a tree. That means the roots are effectively "underground" when the model sits on the terrain.
To control where population instances appear you can use a density shader. This can be a distribution shader, an image map shader with a mask image, a painted shader or many of other sorts of shaders. The Painted Shader Guide has an example project using a density shader (Painted shader in that case) with a population:
http://www.planetside.co.uk/wiki/index.php?title=Painted_Shader_GuideGo down to the Examples section, it's the first one. Also try searching the forum for population and density, or similar.
Population instances are placed randomly. You can use the "Spacing variation in a,b" parameter to control how random the placement is. The default setting of 1 is "completely random". If used 0 it wouldn't be random at all, the instances would be placed on a grid the spacing of which is determined by the "Object spacing in a,b" parameter. Changing the seed value in the Seed tab lets you change the general pattern of the random placement.
You don't have to define any equations or anything to control the placement of population instances, with the exception of the density shader perhaps. It will try to place things randomly according to the parameters you set. Just play with it and see what happens.
If you wanted trees in specific places then it's best to place them yourself. The idea behind the populator is to make it easy to place lots of copies of the object across an area. The populator has the advantage that it uses a lot less memory than if you used invididual objects. If you're not too worried about absolutely exact placement you can use a density shader with the populator to control specific places where instances should appear. For example you might use the painted shader to paint some dots on the landscape where you want trees. With a bit of messing about you could get the populator to show trees in those places. It's up to you whether it's easier to place individual objects and their higher memory use or spend time getting the populator to do what you want and save some memory.
Regards,
Jo
