Checking Dirty Plugs

[teddie]

I have an input attribute that is supposed to affect 4 outputs in a MPxSurfaceShape node I'm working on. The node is created and a connection is made from the MEL script. I would have thought that making the connection would make the input dirty so that all the outputs would be computed. But only 1 of the 4 output plugs are computed. In addition, compute is called only when the node I created finally comes into view, not when the first connection to the node is made. How can I check that the input is in fact being marked dirty in functions other than compute, so I can track what's going on? I thought a good place to check it would be in MPxNode::connectionMade.
I also tried using setDependentsDirty but it never get called. Maya is supposed to call that function right?

thanks

[daniel-faust]

When you connect to the input and change value or when it gets changed because it got connected, then the input attribute is marked dirty. Your 4 outputs get also marked dirty, because of the attributeAffects(input,output1...4) initialization.

So keep in mind, that all the five attributes are marked dirty at this moment.

Now Maya does some checking and asks itself "which nodes do I need to update?" Probably only one output is connected to something which actually controls something visible in the scene, something which affects the rendered image. It gets to one of your outputs and says "ok, I need this one, is it dirty? Yes, so we have to call the compute method because it knows how to get that attribute clean" The compute gets called (and keep in mind that this is because something on the screen needs to be redrawn, which does not neccesarily occur when your input attribute changes its value), the compute gets called and requests an evaluation of the input attribute, since this has been marked dirty as well.

Oh, no, the input attribute was not dirty, my mistake, the input attribute got set clean as soon as the connection was made, or as soon as the value got changed. It is clean since the value it contains is a valid one. So, after the connection only the outputs are dirty.

The compute method does compute your new output value, and returns to Maya. But you have to keep in mind, that the compute method got called because one of the outputs was requested to update. It got called only for one output, not for all four.

Imagine your compute method would do a computation which lasts 1 hour, each output. If maya only updates the value it needs for the rendering, you save 3 hours of work. That is the idea behind it. Therefore you should connect your other 3 outputs to something which affects the rendering or the viewport.

Or you can set up callbacks which do a getValue on the 3 plugs on each refresh, that could pull the data out of them and request a computation for those plugs.

=========== a couple of hours later =======

well, I'm stuck in a similar situation.

i've got code like

CODE
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
MStatus CCollisionLinkShape::compute(const MPlug &plug, MDataBlock &data)
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

{
 print::p("CCollision - Link - Shape -- compute(%s)", plug.name().asChar());
 MStatus status;
 if( plug == m_moPull ) {
   data.inputValue(m_moCurrentState).asBool();
   //...
   data.outputValue(m_moPull).setClean();
 }
 else if( plug == m_moCurrentState ) {
   data.inputValue(m_moInputMesh1);
   data.inputValue(m_moInputMesh2);
   data.inputValue(m_moInputMatrix1);
   data.inputValue(m_moInputMatrix2);
   //...

and when m_moPull gets called to update, m_moCurrentState does not update (I thought it should since m_moPull is requesting the value in the data.inputValue(m_moCurrentState).asBool(); call.

[soopagenius]

You should review the DG updating rules. In particular, you have to grasp the notion of "lazy evaluation." Your node's compute function will not be called until 1)one of it's outputs is requested; and 2) the output is dirty -- that is, it's inputs have changed, requiring the output be recomputed.

Both of those have to be true. If the output is requested but isn't dirty, a cached value is returned (though you can set the node behaviour to recompute every time), and compute isn't called; if the output is dirty, but isn't requested, compute isn't called.

"I would have thought that making the connection would make the input dirty so that all the outputs would be computed."

Well, in Maya parlance, it's the output that is considered "dirty". And yes, the output would indeed have to be recomputed, but only when the output was requested elsewhere in the DG. Nuther possible instance is if getAttr is called on the node in a MEL script, or through the api. In fact, that's a good way to test your compute function, is to hook up some inputs, or call setAttr on the inputs in MEL, and then call getAttr on an output. In this particular case, when the the node comes into view, Maya calls your compute function so it can draw your shape.

The reason only 1 of the 4 output plugs is computed is likely because it's the only one that has a downstream connection. Other possibility is that there is a problem with your compute function.

Hope that helps,

Kevin Atkinson

[teddie]

thanks so much for the suggestions and the explaining the lazy evaluation. It really helped me track down what was going on.