There are no commands to do it. The best solution is to use mel. The API is not what you think it is. read this: http://www.highend3d.com/boards/index.php?showtopic=242667

You can manipulate the vertex colours using the MFnMesh function set. But that doesn't work in most cases because an input node will be overriding the mesh data 99% of the time (i.e. polyCube, polySmooth, skinCluster etc). The result will be that as soon as you change it, the node will be flagged dirty and the computation will be re-evaluated deleting your change.

What you should be doing is creating a node that takes a mesh input, modifies the incomming data, and then outputs that data to an output mesh attribute.

this one sweedish guy has a node thats awfully close to this, or actually he calculates the stretchiness of points but should be trivial to change, now if i only could remember the address. I must consult my bookmarks, only it has to wait till i get home. It sounds like a demo group name (i think ive actually met this guy in person once at assembly).

So you can skip almost all the coding if you can accept his licensing agreement. I actually used this node in renders because in mr its easy to reference vertex colors at render time.

ANYWAY: since your baking this into maps maya knows how to calculate this at shading time by comparing reference geometry world position with current world position. No need to code anything, just make shading network. And it woudl work even on sbdivson surfaces at phenomenal precision. So may acan bake this to points aswell as maps at not much sloer speed than firing of the calculation form a deformer.

PS the recipie how to make this in shader connection is nontrivial, Make object then make reference geometry of it (in the render menuset under texturing). Make any 3d teture and its 3d placement node. Then heres the tick connect the ref point obj t a plus minus average node. Then connect a sampler info's point object to the plus minus average and put it to subtract. Now this is the raw values, to make them magnitde based connect the sum out to a multiply divide node and multiply output with self then sum the components and power up to 0.5 thats 7 nodes ancd it computes just about as fast as reading just the vertex values render time

QUOTE(cyberman2 @ 01/09/09, 06:21 PM) [snapback]298562[/snapback]
If I could only understand how Maya's API stores the Color information of a mesh (as shown in my image above) I could manage to do it ...

Its just an array. Anyway you just make a MPx node, make 2 mesh datas in, one mesh data out. grab that nodes input push it into mfnMesh and copy this to the output with changes. (the other input is for the original if you make it caching you dont need to store anything in a file)

Yes their site is:

http://www.madcrew.se/blog/

But appears they have taken the node down (as their blog gets a 1 year cutback). now searching for its source on my disk, not found. I have to ask for permission to ship it to you, or actually im asking them to ship it. You can ask the guys at madcrew blog directly to.

Anyway i can find the page on web history repository but not the source:

http://web.archive.org/web/20061215011642/...adcrew.se/blog/

QUOTE
Well, if it does that it's not working for me because I want it to calculate the tension between each vertex and it's neighbors vertex to calculate the real distortion that is happening.

No but see the thing is thats what you asked earlier:
QUOTE
Scan each vertex at a given pose and calculate how much the vertex's distance changed -> give a value from black to white for each vertex. (Grey = neutral, White = stretched, Black = compress)

Had you asked for tension i would have made that if possible, And indeed you could do that with a expression on top of thsi shading calc but id rather not go there. (its not taht its slow its just that its something i would like to avoid teaching peopel as its actually hacking maya not USING it)

QUOTE
I know it's an array but I don't understand how maya is sorting it.

You don't need to.

read the docs on MFnMesh

MFnMesh:: setVertexColors ( MColorArray &colors, MIntArray &vertexList, MDGModifier *modifier)

what you do is you pass a array of colors and a array of vertices that correspond to each other. The color in the array will be passed to the corresponding vertex in the int array, wich is just the number of the index color is to be assigned to. You Dont need to set them all although you certainly can! Maya figures out the rest.

PS: You shouldnt actually be concerned how the internal store is made because it has tendency to mutate as maya goes along. BUt yes its a index array that points to a array.

Did you ever figure this out ? I'm stuck on a similar problem and would love to see the solution.

Thanks

import maya.api.OpenMaya as api
import maya.OpenMaya as OpenMaya
import pymel.core as pm
 
 
 
sel = pm.selected()[0]
 
shape = sel.getShape()
 
mobj = shape.__apimobject__()
 
meshFn = OpenMaya.MFnMesh(mobj)
 
vertexColorList = OpenMaya.MColorArray()
normalsList = OpenMaya.MFloatVectorArray()
 
meshFn.getVertexColors(vertexColorList)
meshFn.getNormals(normalsList)
 
 
lenVertexList = vertexColorList.length()
 
 
fnComponent = OpenMaya.MFnSingleIndexedComponent()
fullComponent = fnComponent.create( OpenMaya.MFn.kMeshVertComponent )
 
fnComponent.setCompleteData( lenVertexList );
 
vertexIndexList = OpenMaya.MIntArray()
fnComponent.getElements(vertexIndexList)
 
 
for k in range(lenVertexList):
    vertexColorList[k].r = normalsList[k].x
    vertexColorList[k].g = normalsList[k].y
    vertexColorList[k].b = normalsList[k].z
 
meshFn.setVertexColors(vertexColorList,vertexIndexList,None )