pgtp - a Blender plugin for procedurally generating trees

Overview:
    PGTP is a procedural tree-mesh generator plugin for Blender that
    generates trees using the space-colonization algorithm described by
    Runions, Lane, and Prusinkiewicz in their paper "Modeling Trees with a
    Space Colonization Algorithm." In brief, a set of points are first generated
    within a defined bound. A root is then grown upwards towards these
    points in steps, and the points are removed as the tree comes within a
    certain distance of them.
    
    The advantage of this algorithm over recursion/fractal based algorithms is 
    that branches are guaranteed not to intersect. Recursive models, in which 
    branches are usually generated independent of each other often have this
    problem, particularly in complicated outputs. The space colonization
    algorithm also has the advantage of introducing natural-looking irregularity
    and asymmetry into the generated tree. This too can be difficult to achieve
    with other standard algorithms.

Installing PGTP:
    PGTP is a Blender plugin, thus to use it, you must first install Blender
    on your computer. The latest version of Blender and instructions for 
    installing it for your operating system can be found at www.blender.org.
    
    Please note: PGTP uses API features introduced in Blender 2.63 and is
    incompatible with all earlier versions.

    To load PGTP into Blender once it has been installed, first change the
    screen lay-out from "Default" to "Scripting" on the top menu bar. Open the
    pgtp script with the text editor that will now be on the right hand side of
    the screen and click "Run script". You can now run PGTP from the top menu 
    bar with "Add > Mesh > Add_Tree".

Using PGTP:
    Once PGTP has been installed, you can access it through the "Add > Mesh"
    menu. If the left tool bar is not visible, you can toggle it on with "T".
    In this tool bar you will be presented with a list of parameters, changes
    to which will be visible in real-time in the 3D viewport! The following is
    an overview of what each parameter does. (You can also hover your mouse
    over the parameter in Blender for a short description.)
    
    - Canopy Bound Mesh Name:
        Name of the bound for the ellipsoid that will define the canopy.
        All calculations using this mesh are done relative to the origin, so
        in most situations it should be centered on the z-axis, hovering above
        the xy-plane.
    
    - Seed:
        Change to generate a new tree without changing the other parameters.
    
    - Attraction Points:
        The number of attraction points that will be generated in the bounds.
        Increasing will increase the density of the tree.
    
    - Iterations:
        The number of iterations the growth algorithm. Adds detail and density.
    
    - Initial Branch Segment Length:
        The length of each branch segment. Smaller gives a nicer tree, but make
        sure to increase Attraction Points and Max Iterations to spread through
        the full canopy.
    
    - Final Branch Segment Length:
        The length of the segments at the tips. Lengths of branches between the
        root and tip are calculated by linear interpolation.        
    
    - Influence Radius:
        The radius at which attraction points attract branches. Make sure this
        is high enough for the root to reach the lowest points in the canopy.
    
    - Kill Radius:
        The radius at which a branch is close enough to an attraction point for
        it to be removed. This number should usually be slightly lower than
        Branch Resolution.
    
    - Root Location:
        Translates the base of the trunk without moving the canopy.
    
    - Tip Radius:
        The radius of the outermost branches. This value will cascade all the
        way down the tree to determine the radius of the trunk.
    
    - Branch Faces:
        The number of faces in the n-gon that traces the branches. More will
        give "rounder" branches. (Also useful to decrease the number of polys
        in a dense tree.)
    
    - Align To View:
        Aligns the tree axis with your view.
    
    - Location:
        Translates the whole tree. Can also be done after generation.
    
    - Rotation:
        Rotates the whole tree. Can also be done after generation.
    
Using Custom Bounding Meshes:
    Unfortunately, there is currently a bug in Blender that causes ray_cast to
    freeze or crash Blender when certain updates are performed. Because 
    ray_cast is needed to test if a point is within an arbitrary mesh, this 
    means that when using canopies defined by such meshes, PGTP cannot show a 
    preview in real time. PGTP does, however, still have a way to use custom 
    bounds, although it requires directly editing the script's code.
    
    In the text editor on the left side of the window, staring on line 47, there
    is a commented out template to specify the data necessary to use a custom 
    mesh. Supply this data and uncomment the code. Then press "Run Script" to 
    load the new settings. In the "Add > Mesh" menu, there will be an additional
    "Add Tree" option (the lowest is always the newest) which will generate your
    custom tree!