Hi yuma,
what you are trying to do is not that easy, if I understood your questions correctly.
Quotei want to change the length/radius of segments based on the height of that segment from the ground plane, rather than using the parent primitive instance filter.
Do you mean that you want to make the leaves smaller or bigger (in radius or length) the further they are away from the ground plane? If so, this is not possible. Here's why: The ground plane isn't part of your plant. It is just a visual reference in the OpenGL view for an easier orientation. Thus, you cannot access its position, length or any of its other parameters and make other things depend on it.
QuoteI want to add some leaves to the branches but i want the size of the leaf to depend on the vertical height that the leaf is at rather than its position on the parent (in this case the position of the leaf along the parent segment (branch)
almost like a grandparent primitive instance filter?
For this, you have to use the "Parent parameters" control node. Add one to your graph (Control => Parent Parameters), select your "stalk" (or whatever node comes before your branch node in the graph hierarchy) and then "Position on primitive instance". With these settings, this node will yield a value of 0 for the bottom of your stalk and 1 for the top of your stalk. Now, go to the "Global scale" parameter on the "Transform" tab of your leaf node, click on the word "Global" and select "Connect parameter". This will extract the scale value of your leaf as an extra node. Now all that is left to do is to multiply the value from the "Parent parameter" control node with the value of your Global scale (the random range node that was just created when you selected "connect parameter"). Add a multiply node between your leaf node (Combiner => Multiply inputs) and your "random range" node and connect your "parent parameter" node to the multiply node.
You can take a look at the node setup and also how the leaves get bigger the higher they are in the tree in the screenshot that I attached. If you want the leaves to get smaller the higher they are in the plant, then just connect an Opposite Filter node (Filter => Opposite) between your "Parent Parameter" and your "Multiply inputs" node. This will invert the values from the PP node (0 top 1 bottom) and thus will make the leaves bigger at the bottom.
Whenever you need a grandparent dependency, or even a grand-grand-grand-parent dependency, the PP node is the way to go.
As for the TPF community, I'd suggest you have a look at this Facebook group which is all about 3D Plant Design in general, be it with TPF, XFrog, ngPlant, Speed Tree or whatever:
https://www.facebook.com/groups/699076176775692/There are users of every major plant software in this group, so you usually get an answer for most problems
.
Daniel
