Assigning Shader To Faces Via Api 2

[mikelem]

Hi,

I would like to assign different shaders to group of faces of a mesh I have created. I followed Maya API How-To #08 How do I assign polygon objects (and individual face components) to a Shading Group? but when I try to use the AssignToShadingGroup function Maya returns this message:

"Cannot add the following items to the set since they would break the exclusivity constraint: myMesh.f[0:299]"

As mentioned in (MEL How-To #104 How do I assign an object or face component to a material?) this kind of error can be overcome in MEL by the -forceElement tag. Is there a way to fix this also via API? Why am I given this message? What am I doing wrong?

I have also tried to use this MGlobal::exectuteCommand call:
MString comando = "sets -e -forceElement blinn3SG"; but I got this result:

// Error: line 1: Connection not made: 'myMeshShape.instObjGroups[0]' -> 'initialShadingGroup.dagSetMembers[1]'. Connections cannot be modified during evaluation. //
// Error: line 1: Cannot add the following items to the set since they would break the exclusivity constraint: //
// Error: file: C:/AW/Maya7.0/scripts/others/timeSlider.mel line 40: Cannot add the following items to the set since they would break the exclusivity constraint: myMesh.f[0:299]

CODE
MItMeshPolygon pIt(existingMeshNodeDagPath);
MSelectionList selList;
selList.clear();
int indexPolygon =0;pIt.reset();
for (indexPolygon; !pIt.isDone(); pIt.next(),indexPolygon++ )
{
//here I am selecting the first 300 polygons
if(indexPolygon < 300 )stat = selList.add ( existingMeshNodeDagPath,
pIt.polygon() , true);
if(stat != MS::kSuccess)cerr << "\nFails to add polygon to shaderGroup \n";
}
indexPolygon++;
MGlobal::setActiveSelectionList ( selList, MGlobal::kReplaceList);

//MString comando = "sets -e -forceElement blinn3SG";
//MGlobal::executeCommand(comando);

indexPolygon = 0;
MItSelectionList iter(selList);
if(stat != MS::kSuccess)cerr << "\nFails to initialize Selection List \n";
MDagPath dagPathEl;
MObject component;
int lungh = selList.length();
for(; !iter.isDone(); iter.next())
{
iter.getDagPath(dagPathEl, component);
dagPathEl.extendToShape();
AssignToShadingGroup(m_oaOutMeshShadingGroup[0], dagPathEl, component);
}

using this function taken from Maya API How-To #08:
CODE
MStatus MShadingGroup::AssignToShadingGroup(
  const MObject & shadingGroup,
  const MDagPath & dagPath,
  const MObject & component )
{
  MStatus                     status;
  MFnSet                      fnSG( shadingGroup, &status );

  if ( fnSG.restriction() != MFnSet::kRenderableOnly )
    return MS::kFailure;

  status = fnSG.addMember( dagPath, component );
  if ( status != MS::kSuccess )
  {
    cerr << " ! MShadingGroup::assignToShadingGroup could not add Dag/Component to SG ! " << endl;
  }

  return status;
}

If you can help me somehow I would be so happy cause at the moment I am really stuck on this thing.

Mikelem

[rob-bateman]

uhm, are you doing this within a compute function?

[mikelem]

Yes indeed, I am taking different meshes in input and copying into a new one as an output. The compute reads the input data and returns a new computed mesh as an output. Where am I suppose to do this?

mikelem

[thomaslandvik]

I would also like to know this. I can understand why you can't break connections in the compute function. Do you alwas make and break connections when manipulating shading groups? I'm making a node that alters the a mesh's topology, so I need to update shading gropus. Tried removing all components from the old mesh in the shading groups, then add the new components, still fails.

I noticed the polyModifier node, which is superclass for similar nodes native in Maya (polyReduce, triangulate etc.), have faceIdMap, which is controlling whether the node is checking for face ID modification. Is this because they are registrering a callback for the mesh shape node, and having the modifier node update data (i.e. shading groups) when faces are changing? This is the only alternative design I can think of, if not updating shading groups in the compute function. Then again, I'm fairly new to Maya...

Thomas

[joojaa]

Well this is very simple, your compute is part of the entire networks calculation. Because the network defines how your calculation takes place, if you now change the network you change your calculation, Wich means all you've done this far is in jeopardy, you can edn up updating entire maya several times. This is something the entire design is there to prevent.

Thing is maya assumes nodes don't make any new connections, it is the core of the entire software if you circumvent this you might as well not use maya at all. (connections are rules for what to do next)

However the sahepe node contains things called groups inside it, wich are passed to the shading nodes as inputs, now you can affect those lists without changing any connections and achive same results.

Dont confuse creation with computation.

[thomaslandvik]

QUOTE(Joojaa @ 12/02/06, 12:58 PM) [snapback]253771[/snapback]
Well this is very simple, your compute is part of the entire networks calculation. Because the network defines how your calculation takes place, if you now change the network you change your calculation, Wich means all you've done this far is in jeopardy, you can edn up updating entire maya several times. This is something the entire design is there to prevent.

Exactly, this I understood, why not to make and break connections in compute.

QUOTE(Joojaa @ 12/02/06, 12:58 PM) [snapback]253771[/snapback]
However the sahepe node contains things called groups inside it, wich are passed to the shading nodes as inputs, now you can affect those lists without changing any connections and achive same results.

What I did was:
loop through all the shading groups (retrieved with getConnectedShaders prior to new mesh creation)
create a MFnSet for the shading group
remove all components in that set associated with the current shape node
loop through all shading groups
add the new components (from the reduction) to the shading groups
based on their old faceID and the shading group associated with that face

When doing this I get a exclusivity constraint error message, just like mikelem.

So I should manipulate the instObjGroups[*].objectGroups of the mesh shape node instead?
By updating the objectGrpCompList attribute of each objectGroup? Guess that would make sense...

Thanks again Joojaa,
Thomas

[rob-bateman]

QUOTE(thomaslandvik @ 12/04/06, 02:02 AM) [snapback]253920[/snapback]
Exactly, this I understood, why not to make and break connections in compute.

Because you are in the middle of computing the result of the DG, and that DG is required (by design) to be thread safe (ie, network rendering). If inside your compute you reconnect the DG, which inturn changed the requirements for what should be calculated, all other pc's on your render farm would be wasting their time.

QUOTE
What I did was:
loop through all the shading groups (retrieved with getConnectedShaders prior to new mesh creation)
create a MFnSet for the shading group
remove all components in that set associated with the current shape node
loop through all shading groups
add the new components (from the reduction) to the shading groups
based on their old faceID and the shading group associated with that face

The first rule of compute is: Never access any data outside of the node's own attributes. ie, no calls to mel scripts, no access of other nodes, no creating new nodes, no deleting nodes etc.

Second rule of the compute is: Never access any data ......

QUOTE
So I should manipulate the instObjGroups[*].objectGroups of the mesh shape node instead?
By updating the objectGrpCompList attribute of each objectGroup? Guess that would make sense...
Thanks again Joojaa,

yes

[joojaa]

QUOTE(Rob Bateman @ 12/04/06, 04:28 PM) [snapback]253947[/snapback]
Because you are in the middle of computing the result of the DG, and that DG is required (by design) to be thread safe (ie, network rendering). If inside your compute you reconnect the DG, which inturn changed the requirements for what should be calculated, all other pc's on your render farm would be wasting their time.

Not just the other pc's but allso your pc, in a simlliar way. because the moment yopu change the dg you change what the compute should be doing, midway of doing it. So what you suddenly have is a total new computation scenario going on. So all you did thus far is wasted.

So if the connections looked like this

a->b->c.

and b changed the connectiopns mid compute to

q->a->b->c

a wich already computed would need to recompute which would lead b to recompute, and that could cause something else to be connected somewhere else, this would cause maya to start over (or actualy not necceserily but yud need to force this reupdate so what you essentialy would do is increase the workload by 2/3 or 1/2 depending on i

[thomaslandvik]

Ok, I'm convinced, threw away all code accessing external data in compute(). Let the bastards wait for their damn plug-in (no offense Terje).

And actually solved the problem in the process. The groupParts are auxillary nodes that pass along object group information between shape intermediate nodes (in construction history). So what I did was to use MFnMeshData to copy object groups from the input mesh data attribute to the output mesh data attribute, then update group components using the old face indices stored during the reduction process. I'm guessing this is what Alias did in their own reduction node as well. Quite simple really, almost embarrassing...

Thanks again Joojaa and Rob
Thomas