The node is really very easy to write, and the longest part of it is actually setting up the attributes on the node and how they affect each other. Once you have a template for a MPxNode you can just copy and paste it as a starting point. However it is always nice to write this out and make sure you understand it rather than just copying and pasting it.
So this stretch node has 3 inputs. the startPoint and endPoint, as well as a length attribute which the user sets to the length at which stretching will start. There are then two outputs. Distance, which just returns the distance between the two in points at any given time (useful for determining the point at which stretching will occur), and the main output which outputs the final stretch value which will generally get piped into the scaleX of the necessary joints.
What awesome things did I learn when writing this plugin. The attribute that enables MPoint to return the distance between two points.
posA = OpenMaya.MVector()
posB = OpenMaya.MVector()
p0 = OpenMaya.MPoint(posA)
p1 = OpenMaya.MPoint(posB)
distance = p0.distanceTo(p1)#returns double
Next awesome thing.In the initialize() function we call this:
plugin = OpenMayaMPx.MFnPlugin(obj,'Tim Forbes','0.0','Any')
We then register the plugin type. Now this is different depending on what plugin you are creating, and will require different input data. Somewhere to get caught out, especially if you copy your template from a OpenMayaMPx.MPxCommand type plugin...like I did :p
The differences here are below:
OpenMayaMPx.MPxCommand will be plugin.registerCommand('command name', creator)
The command only requires the command name and the creator function, however the node requires a few extra things as seen below.
OpenMayaMPx.MPxCommand will be plugin.registerNode('stretchNode', stretchNode.kPluginNodeId,creator,initialize)
Because we are creating a node that will be connected to the dependency graph we need a unique node ID, we need the node name, the creator function and also the initialize function. The initialize function is where all the node attributes are added. The creator function is where our pointer is created to access the plugin within maya.
The unique Node Id is created using the inbuilt maya function: kPluginNodeId = OpenMaya.MTypeId(0x00000882)
For home use I have generated an arbitrary ID, however in production you would want to make sure you used an ID that would not clash with in house node IDs.Currently I built the node to take on the translates of the two input points. This was for ease of building and proof of concept. My next version will take wither the world matrix or transforms, thus making the node more adaptable to different rigging scenarios. Anyway, I built this node into a rig and you can view it working below.
Maya Python API Stretch Node
Next Up. Add in matrix connections to the StretchNode. Following that I'm going to make a MPxCommand plugin that traverses the dependency graph to find connected skin clusters. This will be useful in a convoluted node graph if you need to manupulate the skin cluster in a specific way, or really any specific node I guess:-)