L-Systems and the stack in Maya

Question

So I understand the rules of an L-System and I've managed to create a Sierpinski Triangle. I'm now moving onto making different styles of trees.

The basic rules of this would be:

F: Draw forward
+: Rotate right by angle
-: Rotate left by angle
[: Push stack 
]: Pop stack

I'm using Maya to do this and I was unsure how to go about pushing and popping the stack. I know how to create a basic stack using a list as Maya has no default stack, but what exactly would I push / pop?

I can't seem to push the world matrix of an object. And as you cannot do something without creating an object first, the L-System process goes wrong. As a basic example:

F[+F]

...would not work due to not creating the object to push/rotate.

Any tips would be really useful, as this has been stumping me for a while.

If you were implementing this in pure Python, you would store the current position and heading. Having never used Maya before, I have no idea what you'd do there. — Kevin, Feb 15 '17 at 19:03

joojaa · Accepted Answer · 2017-11-25T12:32:58.633

First, let me point out to you that Maya does indeed have a graphics stack like this. Its just that since Maya is not a immediate mode drawing tool this stack is in fact a tree. This tree is called the DAG or in other words the object parent hierarchy. This tree serves the same purpose as a stack described by the L-system. However this may or may not help you since its not, directly, of much use for your evaluation.

So let us take a look what a simple L system rendering would look like in Maya. Lets borrow the base code from How to Think Like a Computer Scientist and let us use the slightly more exciting rule:

F -> F[-<<<<F][+>>>>F]

where > and < are rotations about a axis. Using base axiom:

with 4 iterations and a angle of 30 degrees results in:

Note the production ruleset is a bit redundant and tends to draw many stems at each iteration. But I am aiming at simplicity of understanding not elegance. Code used:

import maya.cmds as cmds

def applyRules(lhch):
    rhstr = ""
    if lhch == 'F':
        rhstr = 'F[+F][<<<<+F][>>>>+F]'
    else:
        rhstr = lhch  # no rules apply so keep the character
    return rhstr


def processString(oldStr):
    newstr = ""
    for ch in oldStr:
        newstr = newstr + applyRules(ch)
    return newstr


def createLSystem(numIters, axiom):
    startString = axiom
    endString = ""
    for i in range(numIters):
        endString = processString(startString)
        startString = endString
    return endString

def drawLsystem(instructions, angle, distance):
    parent = cmds.createNode("transform", n="L_Root_#")
    saved=[]
    for act in instructions:
        if act == 'F':
           cyl = cmds.cylinder(r=0.1, ax=[0,1,0], hr=1/0.1*distance)
           cyl = cmds.parent( cyl[0], parent, r=1)
           cmds.move(0, (distance/2.0), 0, cyl[0], os=1) 
           parent = cmds.createNode("transform", p=parent)
           cmds.move(0, (distance), 0, parent, os=1) 
        if act == '-':
           parent = cmds.createNode("transform", p=parent)
           cmds.rotate(angle, 0, 0, parent, os=1) 
        if act == '+':
           parent = cmds.createNode("transform", p=parent)
           cmds.rotate(-angle, 0, 0, parent, os=1) 
        if act == '<':
           parent = cmds.createNode("transform", p=parent)
           cmds.rotate(0, angle, 0, parent, os=1) 
        if act == '>':
           parent = cmds.createNode("transform", p=parent)
           cmds.rotate(0, -angle, 0, parent, os=1) 
        if act == '[':
           saved.append(parent)
        if act == ']':
           parent = saved.pop()  


drawLsystem(createLSystem(4, "F"),30,1)

PS: if you change the axiom to FX and rule X to [+FX][<<<<+FX][>>>>+FX] it generates less redundancy. Running the code without undo makes it light years faster due to Maya undo being pretty slow to update. And as a result you can run the rule set more times for a more complicated result. Added some scale operations to the generation rules and got this:

Thanks so much for this! I'm quite new to Maya so I was still learning about the DAG hierarchy. I've done some research on it so I'm getting better at understanding it. I've now adapted my code using the DAG hierarchy as you suggested. That's so much for the help! — TheCoder95, Feb 20 '17 at 13:37

L-Systems and the stack in Maya

1 Answers1