Vertex world/local space transforms not working as imagined

[themadmonster]

So a quick explination of what I was making.  A tool that will align verts along a slop, this went pretty quick and it worked great on my test poly plane.  I had to make sure that the pointPositoin used the local flag and all was fine.  So I moved to test on a lofted surface ( end result is a poly ).  This is where the chaos begins, and what I thought I new about Maya went out the window, which probable wasnt much.

Looking at the vert positions on the lofted ( poly ) surface, all the vert positions were 0,0,0.  Why is this?  Is there any way that I can get these points to be in world space?  Can I set the positions in world space? That being said, we also have a tool that will convert objects to world space.  When used on the object the points get values other then 0, but they are all the same value ( all verts are 0,0,0 now all verts are 1.2, 2.5, 5.5 ). 

Now I am getting confused.  So I go back to the poly plane.  I write a simple  function that gets the list of verts, gets the verts point position, the reassigns that value to the vert.  The result of this, the object keeps its form but the scale is larger.

string $selected[] = `ls -sl fl`;
for ( $s in $selected )
{
	float $pp[] = `pointPosition $s`;
	setAttr ( $s + ".pntx" ) $pp[0];
	setAttr ( $s + ".pnty" ) $pp[1];
	setAttr ( $s + ".pntz" ) $pp[2];
}

[joojaa]

Yes Mayas working as intended, just not the way you imagined. This is one of the reasons why many guides on how to script Maya are in fact wrong. The scripting of Maya should build nodes not manipulate the intrinsic database (that is unless you know what your doing).

Explanation. In order to facilitate manipulation of objects with history Maya has a concept called the tweak (named after a node that allows you to redirect the manipulation in history). In normal operation Maya works on nodes. Nodes construct a history. Now if you make a shape in history like this:

generatingNode -> shapeNode

Then generating node would always override the shape node changes. Unfortunately artist couldn't really do anything in this kind of situation. Enter tweaks; What Maya does is it implements a cache that's done on top of this history. Because its a common occurrence its been included into the shape node. This means Maya has 2 point arrays in your shape node, positions and the tweak potions. The tweak array is called pnts, and the other array is called vrts. You can find this info by reading the documents right. Mayas most important document section is the NODE REFERENCE, something that escapes nearly everyone. From the node reference:

pnts (pt) 	float3		array output input connectable storable
 
This attribute is the array of vertex tweaks (relative movements). The 
attribute values are always significant regardless of whether the object
has history or not

and (yes this ones a little bit cryptic, but since you can/should not directly manipulate this array it does not matter)

vrts (vt) 	float3		array output input storable hidden
 
This is an internal attribute representing local space vertex position,
used for file I/O. It should never be directly manipulated by the user.
If it is modified, the results will be unpredictable.

So in essence the pnts is added on top of other movements. Hence ist not a local space representation but offset space representation. So yes putting the local coordinate in pnts would indeed scale your object.

So what to do? Well do not manipulate the pnts array directly. If you use the commands

xform

move

instead then Maya will automatically calculate the needed pnts transforms for you. There is really only about 6-10 reasons to manipulate pnts directly. The most common operation being that you want to erase the tweak. The other is to relocate the tweak form elsewhere when a tweak node is present to redirect move and xform, but this is very uncommon already.  Rest of the operations are really fringe use cases. In fact you should allays manipulate things with the accessors if humanly possible.

PS: to understand try this, make a plane that animates a wave. Kill the inbuilt tweak node that goes ito history and manipulate points of your shape. They still move with teh animation but at a newly defined location.

PPS: no Maya at hand.