Find Ngons - How to optimize with OpenMaya?

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Chad_Fox

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Sep 6, 2016, 7:08:29 PM9/6/16
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Hi All

I recently started diving into OpenMaya to write a faster NGon check, but my results are still too slow @ 5.5 seconds in a 500k Polygon scene. 

I'd like to ask for any recommendations you can offer on how to improve this check script, if possible.  

Side note: the script cannot select anything. I previously used a command to run the PolyCleanup tool. Although it finds NGons in a microsecond, it doesnt give me a list, it selects them. 

Thoughts?

import maya.cmds as cmds
import maya.OpenMaya as om


violationPolygons = []

mIt_kMesh = om.MItDependencyNodes( om.MFn.kMesh )

while not mIt_kMesh.isDone():
    mObject = mIt_kMesh.thisNode()
   
    dagPath = om.MDagPath.getAPathTo( mObject )
   
    mFnMesh = om.MFnMesh( dagPath )
    mObject_numFaces = mFnMesh.numPolygons()
   
    for faceID in range(0, mObject_numFaces ):
        mIntArray = om.MIntArray()
        mFnMesh.getPolygonVertices( faceID, mIntArray )
       
        if mIntArray.__len__() > 4:
            if not dagPath.fullPathName() in violationMesh:
                violationMesh.append( dagPath.fullPathName() )
            violationPolygons.append( '%s.f[%d]' %( dagPath.fullPathName(), faceID ) )
                           
    mIt_kMesh.next()
   
cmds.select(violationPolygons)

Justin Israel

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Sep 6, 2016, 7:28:35 PM9/6/16
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Have you done any profiling to determine the slowest parts of this code? Beyond that, I could just throw out some suggestions from a pure python code review standapoint as opposed to the best way to approach this problem..

Where is violationMesh defined? Is it a typo and actually meant to be violationPolygons? If so, you are doing a membership test in a list that is growing over time. Your lookups will get longer and longer because it has to do a linear search to match. You may consider switching to using a set(), as membership tests will be much faster.

You could switch to use xrange() instead of range(), because in Python 2.x the range() command will expand to a complete list, whereas xrange() will be a generator. If you have tons of faces, this could make a difference.

This may get you nothing in return, but you could also create your mFnMesh just once before the loop, and change the object each time with mfnMesh.setObject(dagPath). It would save you whatever the cost is of calling the constructor and then throwing it away on each loop.

Justin

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Ian Jones

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Sep 6, 2016, 8:00:01 PM9/6/16
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Two recommendations: One I would use selection lists as they'll be faster to interact with during your loops and you can convert them to strings if needed. Second I would recommend looking at the mesh iterators as there are convenient access to things like vertex count per face found there that will save you querying an array just to then check the length. We recently wrote something similar at work like this:

bad_stuff = OpenMaya.MSelectionList()
meshIt = OpenMaya.MItMeshPolygon(dagPath)
while !meshIt.isDone:
     if meshIt.polygonVertexCount > 4:
          bad_stuff.add(dagPath,meshIt.currentItem())
     meshIt.next()

OpenMaya.MGlobal.setActiveSelectionList(bad_stuff)

Ian


Justin

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Chad_Fox

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Sep 6, 2016, 9:14:57 PM9/6/16
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Thanks Justin and Ian,

violationMesh = [ ] was indeed missing. My mistake not copying it over.

Regarding profiling, I'm now ashamed to say that I did not do a line by line profile because doing so just now I found that looping over and re-creating "mIntArray = om.MIntArray()" was  accounting for 80+% of the runtime. I switched to creating it first and clearing it in every loop and got... 80+% time reduction.  

With Ian's suggestion I assume runtime will only get better.

Thanks for all the other optimization tips too.


import maya.cmds as cmds
import maya.OpenMaya as om


violationMesh       = [ ]

violationPolygons = [ ]

mIt_kMesh = om.MItDependencyNodes( om.MFn.kMesh )

while not mIt_kMesh.isDone():
    mObject = mIt_kMesh.thisNode()
   
    dagPath = om.MDagPath.getAPathTo( mObject )
   
    mFnMesh = om.MFnMesh( dagPath )
    mObject_numFaces = mFnMesh.numPolygons()

    mIntArray = om.MIntArray()

    for faceID in range(0, mObject_numFaces ):
        mIntArray.clear()

Marcus Ottosson

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Sep 7, 2016, 3:02:59 AM9/7/16
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For completeness and to not miss anything obvious,

Side note: the script cannot select anything. I previously used a command to run the PolyCleanup tool. Although it finds NGons in a microsecond, it doesnt give me a list, it selects them.

What about selecting, and running cmds.ls(selection=True) afterwards? If you restore your selection afterwards, this would give you a list in that microsecond.

Chad Fox

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Sep 9, 2016, 5:06:01 PM9/9/16
to Python Programming for Autodesk Maya
Unfortunately I cannot cheat the no selection rule with the constructor system my code is working within. Kind of glad that it forced me to play with API though :)

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