Stumbling Toward 'Awesomeness'

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Monday, January 7, 2013

Abusing ‘Blind Data’ in Maya

‘Blind data’ is custom data that you can store on any object or its components (vertex, edge, polygon, etc). The documentation says ‘Blind data is information stored with polygons which is not used by Maya in any way..’ I believe it is used when importing meshes from other apps that have properties that do not map to Maya, so that when you take them back to those apps, those properties remain.

Anyway, the important point here is that blind data is metadata (int, float, double, boolean, string, binary) that you can attach to any component. It matters not what happens to said component, you can extract a polygon from a mesh, it’s index will have changed, it’s object will have changed, but its blind data will remain with it. The only drawback can be that it can be painfully slow to write this data, but we will get to that later.

Simple Example

First let’s create a blind data template, this is required to store the data later. We use the command ‘blindDataType’ to create a template for a string type called ‘skinningInfo’ or ‘skin’ for short, giving it an ID 12344. Then we query the ID and it returns the blind data attribs we have created.

cmds.blindDataType(id=12344, dt='string', ldn='skinningInfo', sdn='skin')
print cmds.blindDataType(id=12344, tn=1, q=1)
>>['skinningInfo', 'skin', 'string']

So now we have our template, let’s try using it, this is more focused on getting the idea across than speed:

#query vertex # of mesh
v = cmds.polyEvaluate(node, v=1)
#loop through vertices
for vtx in range(0, v):
    #get influences
    infs = cmds.skinCluster(sc, inf=1, q=1)
    #get weights
    objVtx = node + ".vtx[" + str(vtx) + "]"
    vals = cmds.skinPercent(sc, objVtx, q=1, v=1)
    #build dict of influence:weight
    for i in range(0, len(infs)):
        weightDict[infs[i]] = vals[i]
    #write value to blind data
 cmds.polyBlindData(objVtx, id=12344, at='vertex', ldn='skinningInfo', sd=str(weightDict))

So here you have saved a dictionary per vertex that has key/value pairs of influence/weight. You can query like so:

#I have a vertex selected in component mode
print cmds.polyQueryBlindData(cmds.ls(sl=1), id=12344, showComp=1)
['polySurface2.vtx[64].skin', "{u'joint2': 0.49755714634259796, 'joint3': 0.49755714634259784, 'joint1': 0.0048857073148042395}"]

Now On To Something More Useful

So let’s create a function to store skinning data per-vertex, as you may have seen with the above, that was painfully slow. If you have written any skinning tools, you know that the solution to this (other than learning C++) is to apply your change to all vertices at once. Below we build two lists, one of vertices and one of weights, then we

def storeBlindSkinning(mesh, sc):
	'''
	mesh is a skinned mesh, and sc is the skincluster affecting the mesh
	'''
	vtxList = []
	v = cmds.polyEvaluate(mesh, v=1)
	infs = cmds.skinCluster(sc, inf=1, q=1)
	vtxWeights = []
	for vtx in range(0, v):
	    objVtx = mesh + ".vtx[" + str(vtx) + "]"
	    vtxList.append(objVtx)
	    vals = cmds.skinPercent(sc, objVtx, q=1, v=1)
	    for i in range(0, len(infs)):
        	weightDict[infs[i]] = vals[i]
            vtxWeights.append(str(weightDict))
        cmds.polyBlindData(vtxList, id=12344, at='vertex', ldn='skinningInfo', sd=vtxWeights)

Setting all the data at once is 1/3 faster, however, setting this data takes quite some time, and you may want to take a hit for a progress bar. (break it up into groups) On ~60,000 vertices this took 10min (15min doing it inside the loop). I don’t mind that hit if it means that I can now detach/alter/slice my mesh without losing skinning data. You can even extract faces and the new vertices created will get the same blind data as their original. (one becomes two)

As always, the C++ API is much faster, my colleague, Bogdan speed tested the above function and 50,000 vertices took only a few milliseconds, compared to 10 minutes in pythonland.

Remember, there are other ways to store skinning data, using UVs, position, vertex color channels, etc. I just wanted to introduce people to blind data in Maya and show a potential use.

posted by admin at 4:04 AM  

4 Comments »

  1. Hi! I just found this on LesterBanks.com.
    Thanks for sharing! Could this speed up laplacian and other fancy operations by storing values such as the cotan weights or eigenvalues as blind data?

    Comment by Michael Tuttle — 2013/01/08 @ 9:45 PM

  2. You could bake any information into this, a word of caution, Jacob Buck, an old colleague at ILM says that on mesh reductions he had blind data acting up. Our own tools guy says that blind data used to have issues, but these have been fixed (claimed by ADSK) in 2012.

    Comment by admin — 2013/01/09 @ 1:41 AM

  3. Hey Chris, do you actually know of any available C++ plugin for Maya capable of doing just that (loading/saving weights using blind data)?

    Comment by Seith — 2013/01/09 @ 2:20 PM

  4. No, unfortunately not. Almost makes me wanna learn C++ though!

    Comment by admin — 2013/01/14 @ 2:06 AM

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