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Accessing MASH Point Data with the Maya API

by | Aug 15, 2017 | MASH | 0 comments

Good news, MASH only uses Maya’s default data types, so all MASH data can be accessed via normal Maya APIs.

Ordinarily this isn’t needed as MASH provides two built in ways to extract data:

  1. For the non scripter there’s the Breakout node, which can feed MASH data into float/vector attributes in Maya via connections.
  2. The Python node gives users access to the raw data which they can extract and manipulate in any way they like.

However, for the performance conscious, or for those wanting to write their own MASH nodes or export MASH data in some custom/ game engine format, it might help to know a few things about Maya’s data types and APIs, so here are some simple recipes to get you going. In the following example we’ll get a MASH node (a Waiter), get it’s output data, and print a list of all the channels it contains along with the MASH point positions.

Before we get started, please remember, data getting/setting in Maya should only be done with connections otherwise Parallel Evaluation performance will suffer, so what I’m about to show you should never be run in an expression node.

MASH Data can only be extracted with either the CAPI or PyAPI 1.0. The PyAPI 2.0 isn’t finished yet and thus can’t be used with MASH.

Step 1: Get the MObject for the MASH node.

Get the MASH Node Object
Python
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import maya.OpenMaya as om
 
# Create an empty selection list
sel = om.MSelectionList()
# Add the waiter (named "MASH") to the selection list
sel.add("MASH")
# Create an empty MObject
mashNode = om.MObject()
# Get the Waiter's node object, and save it to the empty MObject
sel.getDependNode( 0, mashNode )

Step 2: Get a list of channel data and print it.

Get the channel data
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# Attach a function set to the MASH node
mashNodeFn = om.MFnDependencyNode(mashNode)
# Find the attribte that we want
pointsAttribute = mashNodeFn.attribute("inputPoints")
# Get the attribute plug
pointsPlug = om.MPlug(mashNode, pointsAttribute)
# Get the plug data
handleData = pointsPlug.asMDataHandle().data();
# Attach an array attribute function set to the handle data
inputPointsData = om.MFnArrayAttrsData(handleData)
# Get the names of every channel in the data
channels = inputPointsData.list()
# Print them
print channels

Step 3: Get a specific channel. In C++ getting the channel data type is trivial, however I’ve never managed to do this in Python (it seems the data type enum get’s mangled when the Python API is auto-generated), so a bit of educated guessing on your part may be needed.

A. Double channel eg. Visibility.

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# Get the visibility double data and return a copy of it
# WARNING NO ERROR CHECHING
doubleList = inputPointsData.getDoubleData("visibility")[:]
# Print it
print doubleList

B. Vector array, e.g. Position.
First we need a function to copy the MVectorArray into a Python list:

Python
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def channelToList(channel):
    if channel == None or channel.length() == 0:
        return
 
    if channel.__class__.__name__ == "MVectorArray":
        listOfVectors = []
        for i in range (0, channel.length(), 1):
            vec = (channel[i].x, channel[i].y, channel[i].z)
            listOfVectors.append(vec)
        return listOfVectors

Now we can use that to get the data:

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# Get the dynamic array data
dynamicArray = inputPointsData.getVectorData("position")
# Copy it to a list of tuples
listOfVectors = channelToList(dynamicArray)
# Print it
print listOfVectors

And you’re done. In C++, everything’s easier, you would check the channel data type first like so and then just copy the array:

C++
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MVectorArray position_pp;
if (inputPointsData.checkArrayExist("position", dataType) && (dataType == MFnArrayAttrsData::kVectorArray))
{
    position_pp = inputPointsData.getVectorData("position");
}

Any questions let me know.