maya_api_cpp_node.md

Maya API CPP node

Inherit MPxNode

In head file - Declarations

#ifndef __CUSTOM_NODE_H
#define __CUSTOM_NODE_H

#include <maya/MPxNode.h>
#include <maya/MPlug.h>
#include <maya/MDataBlock.h>
#include <maya/MTypeId.h>

class CustomNode : public MPxNode
{
public:
    MPxNode();
    virtual ~MPxNode();
    static void * creator();
    static const char * kNodeName;
    static MStatus initialize();
    static MTypeId id;
    MStatus compute(const MPlug &, MDataBlock &) override;
    // Attributes
    static MObject moInput;
    static MObject moOutput;
    // Attribute name
    static const char * kInputSN;
    static const char * kInputLN;
    static const char * kOutputSN;
    static const char * kOutputLN;
};

#endif /* __CUSTOM_NODE_H */

In cpp file - Definitions

#include "customNode.h"
#include <maya/MPxNode.h>
#include <maya/MPlug.h>
#include <maya/MDataBlock.h>
#include <maya/MDataHandle.h>
#include <maya/MTypeId.h>
#include <maya/MFnNumericAttribute.h>

• Node name definition.
• Attribute name definitions.

const char * CustomNode::kNodeName= "customNode";
const char * CustomNode::kInputSN = "in";
const char * CustomNode::kInputLN = "input";
const char * CustomNode::kOutputSN= "out";
const char * CustomNode::kOutputLN= "output";

• Node Type Id definition.
• Id range is between 0x00000000 - 0x0007ffff.

MTypeId CustomNode::id(0x80012);

• Initialize attributes to NULL values.

MObject CustomNode::moInput;
MObject CustomNode::moOutput;

• Write a simple function to set input and output attributes.

MStatus make_input(MFnAttribute & attr)
{
    MStatus stat = MS::kSuccess;
    stat = attr.setKeyable(true);
    stat = attr.setStorable(true);
    stat = attr.setReadable(true);
    stat = attr.setWritable(true);
    return stat;
}

MStatus make_output(MFnAttribute & attr)
{
    MStatus stat = MS::kSuccess;
    stat = attr.setKeyable(false);
    stat = attr.setStorable(false);
    stat = attr.setReadable(true);
    stat = attr.setWritable(false);
    return stat;
}

• Constructor and Destructor.

CustomNode::CustomNode()
{
}

CustomNode::~CustomNode()
{
}

• Creator - return this new instance.

void * CustomNode::creator()
{
    return new CustomNode();
}

• Initialize your attributes - create, add and affects.
• The MFnNumericAttribute is derived from MFnAttribute.
  The MFnAttribute have many child class can be create attributes.
  In general, we need to provide full and brief name.
  
• MPxNode::addAttribute can be add attribute to this node
  
• MPxNode::attributeAffects() can be specifies that a particular input
  attribute affects a specific output attribute.
  The output attribute cannot be keyable, if they are keyable, this method will fail.

MStatus Custom::initialize()
{
    MStatus stat = MS::kSuccess;
    MFnNumericAttribute fnAttr;
    
    // Create attributes by MFnNumericAttribute::create().
    moInput = fnAttr.create(kInputLN, kInputSN, 0.0, &stat);
    if (stat != MS::kSuccess)
    {
        MGlobal::displayError("Failed to create attribute : " + kInputLN);
        return stat;
    }
    CHECK_MSTATUS_AND_RETURN_IT(make_input(fnAttr));
    moOutput = fnAttr.create(kOutputLN, kOutputSN, 0.0, &stat);
    if (stat != MS::kSuccess)
    {
        MGlobal::displayError("Failed to create attribute : " + kOutputLN);
        return stat;
    }
    CHECK_MSTATUS_AND_RETURN_IT(make_output(fnAttr));

    // Use MPxNode::addAttribute to add attributes.
    CHECK_MSTATUS_AND_RETURN_IT(addAttribute(omInput));
    CHECK_MSTATUS_AND_RETURN_IT(addAttribute(omOutput));

    // Use MPxNode::attributeAffects to make affects.
    CHECK_MSTATUS_AND_RETURN_IT(attributeAffects(omInput, omOutput));
    
    return stat;
}

• MPxNode::compute is core of MPxNode, we have to implement it.
• We must use MDataHandle to get or set values.

MStatus CustomNode::compute(const MPlug & plug, MDataBlock &)
{
    if (plug != omOutput)
    {
        return MS::kUnknownParameter;
    }
    MStatus stat;
    // Create a data handle from input data block.
    MDataHandle input_handle = data.inputValue(omInput, &stat);
    CHECK_MSTATUS_AND_RETURN_IT(stat);
    // Create a data handle from output data block.
    MDataHandle output_handle = data.outputValue(omOutput, &stat);
    CHECK_MSTATUS_AND_RETURN_IT(stat);
    // Calculate your data.
    float input_value = input_handle.asFloat();
    // Make input value double.
    float double_value = input_value * 2.0;
    // Set output data value, and set clean.
    output_handle.set(double_value);
    output_handle.setClean();
    //
    return stat;
}

About Maya Attributes

The Maya attribute is MObject.
By default, attributes are :

• Readable.
• Writable.
• Connectable.
• Storable.
• Cached.
• Not keyable.
• Not hidden.
• Not used as colors.
• Not indeterminant.
• Not arrays.
• Have indices that matter.
• Do not use an array builder.
• Set to disconnect behavior kNothing.

Attribute Notes

If attribute is keyable and writable, it will be displayed in the channel box.

MFnAttribute's children

Class	Description
MFnCompoundAttribute	set compound dependency node attributes.
MFnEnumAttribute	enumerated attributes.
MFnGenericAttribute	accept several types of data.
MFnLightDataAttribute	set light data attribute.
MFnMatrixAttribute	set matrix attributes.
MFnMessageAttribute	set message attributes.
MFnNumericAttribute	set numeric attributes.
MFnTypedAttribute	set typed attributes.
MFnUnitAttribute	fundamental types of Maya data.

How to check out attribute type use mel

attributeQuery -node "pCubeShape1" -at "uvSet";
// Result: compound //
attributeQuery -node "file1" -at "filterType";
// Result : enum //
attributeQuery -node "file1" -at "coverage";
// Result: float2 //

Set Disconnect Behavior

Use MFnAttribute::setDisconnectBehavior to set disconnection behavior

enum	Behavior
kDelete	Delete array element(array attribute only)
kReset	Reset the attribute to its default.
kNothing	Do nothing to the attribute's value.