How can I correctly read a matrix attribute?

[Giulio Martella]

Hi all,

I'm trying without success to read an input matrix attribute of my custom node. Here an example of the method I'm using to read a matrix attribute (this also prints the matrix contents and outputs the sum of its elements).

import maya.api.OpenMaya as om

def maya_useNewAPI():
    pass

class Node(om.MPxNode):
    nodeName = 'test'
    nodeClass = 'general'
    nodeId = om.MTypeId(0xeff)

    aMatrix = om.MObject()
    aOut = om.MObject()

    def __init__(self):
        om.MPxNode.__init__(self)

    @staticmethod
    def create():
        return Node()

    @staticmethod
    def initialize():
        tAttr = om.MFnTypedAttribute()
        nAttr = om.MFnNumericAttribute()

        Node.aMatrix = tAttr.create(
            'matrix', 'mat', om.MFnMatrixData.kMatrix)
        tAttr.readable = False
        Node.addAttribute(Node.aMatrix)

        Node.aOut = nAttr.create(
            'out', 'o', om.MFnNumericData.kFloat, 0.0)
        nAttr.writable = False
        Node.addAttribute(Node.aOut)

        Node.attributeAffects(Node.aMatrix, Node.aOut)

    def compute(self, plug, data):

        if plug == Node.aOut:

            mat = data.inputValue(Node.aMatrix).asMatrix()
            print mat

            s = 0
            for i in range(15):
                print mat[i]
                s += mat[i]

            outHandle = data.outputValue(Node.aOut)
            outHandle.setFloat(s)

            data.setClean(plug)

# Plug-in initialization omitted

To me, this seems ok, but Maya does not appear to like it. In fact if I connect the matrix attribute to a transform at the origin with zero rotation, scale and shear, the matrix values are completely wrong:

(((6.92524e-310, 1.39063e-309, 2.3342e-312, 0), (3.95253e-322, 1.66006e-321, 3.32254e-316, 3.32813e-316), (2.52962e-321, 0, 1.39063e-309, 0), (6.92524e-310, 1.39065e-309, 0, 1)))

6.92524378327e-310
1.39062872165e-309
2.33419537006e-312
0.0
3.95252516673e-322
1.66006057003e-321
3.32253771394e-316
3.32813251331e-316
2.52961610671e-321
0.0
1.39062872165e-309
0.0
6.92523883349e-310
1.39064994161e-309
0.0

What am I missing?

Thanks in advance,

Giulio

[Angelo Sta. Catalina]

you're getting the matrix successfully when you call "print mat"

If you compare your input matrix and the values printed from "print mat", they should be identical.

As far as the other stuff goes, I'm not exactly sure what you're trying to accomplish.

if you're trying to return the input matrix as an array of elements from your input matrix, then you need to use MDataBuilder to build the array before your set it to the outHandle. The outHandle also has to be declared an array as well.

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[Giulio Martella]

Nope, the "print mat" instruction should have printed the identity matrix in that case; the thing printed is a matrix filled with values very near to zero, not the identity for sure.
What I'm trying to accomplish is far more complex than this barebone plugin I've written just to explain my problem easily. However, to be precise, I need to read a matrix and then multiply it by a point. So long, I've obtained only matrices filled with zeroes, resulting in zeroing also the point I was multiplying.

--
Giulio

[Angelo Sta. Catalina]

Oh. I see a problem. In your initialize method, you shouldn't use om.MFnTypedAttribute to create a matrix attribute. Use om.MFnMatrixAttribute.

mAttr = om.MFnMatrixAttribute()

Node.aMatrix = mAttr.create('matrix', 'mat')

Try doing that.

Giulio

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[Angelo Sta. Catalina]

Also, to multiply a point, you can simply just cast an MPoint and multply that directly into your MMatrix variable: mat. Maya supports MPoint to MMatrix multiplication.

om.MPoint() * om.MMatrix() will work.

On Thu, Nov 23, 2017 at 7:19 PM, Angelo Sta. Catalina <angelost...@gmail.com> wrote:

Oh. I see a problem. In your initialize method, you shouldn't use om.MFnTypedAttribute to create a matrix attribute. Use om.MFnMatrixAttribute.

mAttr = om.MFnMatrixAttribute()

Node.aMatrix = mAttr.create('matrix', 'mat')

Try doing that.

On Thu, Nov 23, 2017 at 7:01 PM, Giulio Martella <giuliomar...@gmail.com> wrote:

Nope, the "print mat" instruction should have printed the identity matrix in that case; the thing printed is a matrix filled with values very near to zero, not the identity for sure.
What I'm trying to accomplish is far more complex than this barebone plugin I've written just to explain my problem easily. However, to be precise, I need to read a matrix and then multiply it by a point. So long, I've obtained only matrices filled with zeroes, resulting in zeroing also the point I was multiplying.

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Giulio

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[Giulio Martella]

Thanks for your help.
Now everything works. Damn, I feel ashamed I've wasted so much time to figure out a mistake so trivial...

About the multiplication: yeah that's cool. Fortunately OpenMaya has a fairly good interface for its numerical types.

Bye,
Giulio

Il giorno venerdì 24 novembre 2017 02:22:58 UTC+1, Angelo ha scritto:

Also, to multiply a point, you can simply just cast an MPoint and multply that directly into your MMatrix variable: mat. Maya supports MPoint to MMatrix multiplication.

om.MPoint() * om.MMatrix() will work.

On Thu, Nov 23, 2017 at 7:19 PM, Angelo Sta. Catalina <angelost...@gmail.com> wrote:

Oh. I see a problem. In your initialize method, you shouldn't use om.MFnTypedAttribute to create a matrix attribute. Use om.MFnMatrixAttribute.

mAttr = om.MFnMatrixAttribute()

Node.aMatrix = mAttr.create('matrix', 'mat')

Try doing that.

On Thu, Nov 23, 2017 at 7:01 PM, Giulio Martella <giuliomar...@gmail.com> wrote:

Nope, the "print mat" instruction should have printed the identity matrix in that case; the thing printed is a matrix filled with values very near to zero, not the identity for sure.
What I'm trying to accomplish is far more complex than this barebone plugin I've written just to explain my problem easily. However, to be precise, I need to read a matrix and then multiply it by a point. So long, I've obtained only matrices filled with zeroes, resulting in zeroing also the point I was multiplying.

--

Giulio

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