Archive for category maya

a quick note on object snapping

A few weeks ago I got stuck while writing a foot snapping tool — the snapping code I used to use didn’t work on the rigs we inherited because there were a lot of pivot changes. I tried a bunch of more elegant ways to fix this but ended up doing the old standby:

delete `orientConstraint`;
delete `pointConstraint`;

Now, that solution wasn’t a satisfying one to me, so I kept looking for a better one.

Turns out there’s an easy fix. If you use getAttr on a transform node’s .worldMatrix, you get back the proper 4×4 set of float values that represents its final position in world space. Afterwards you can set the matrix of another object using xform. It’s pretty simple in either MEL or Python.

// this snaps one object to another using worldSpace matrix coordinates
string $sel[] = `ls -sl`;
float $m[] = `getAttr ($sel[1] + ".worldMatrix")`;
xform -ws -m $m[0] $m[1] $m[2] $m[3] $m[4] $m[5] $m[6] $m[7] $m[8] $m[9] $m[10] $m[11] $m[12] $m[13] $m[14] $m[15] ($sel[0]);

The MEL code is a bit ugly because of having to specifically reference each item in the array of floats, but it works. Here’s the Python equivalent. I keep it now as a button my shelf.

# uses matrices to snap one object to another
import maya
# yes, I put all my commands into the main namespace
from maya.cmds import *

sel = ls(sl=True)
mat = getAttr(sel[1] + ".worldMatrix")
xform(sel[0], ws=True, m=mat)

Katt's Mysterious 3D Lectures – Vector Application: A Better Rivet

You have the aim constraint under your belt. You can guess how a pole vector’s motion will change rotations with a look. You’re feeling a new sense of power and a desire to accomplish… things.

Now what?

Let’s start with something simple. Remember that old standby, the Rivet script, that I mentioned in my last post? Every good tutorial DVD I’ve bought over the years had it. Every rigger I’ve ever met uses it. As scripts go, it’s probably the most useful 2k on

But did you know it’s also broken?

Let me go back for a moment. A year ago, while I was working under a talented rigger who liked the Farenheit Digital system for creating nurbs ribbon rigs, was saddened by the fact that all licenses of Maya at our company were Maya Complete save two: his, and mine. This meant that the standard way of building ribbons, where you attach a Maya Hair follicle to the surface, wasn’t going to work as Maya Hair only comes with Unlimited. He mentioned something about using an aim constraint and a pointOnSurfaceNode or through the decomposeMatrix node to accomplish the same, although it didn’t work as well. So I was tasked with writing a python node plugin that accomplished the task. It worked well and quickly enough; 40 or so of them were live in the final rigs.

However I prefer to keep the number of non-standard nodes in rigs to a minimum. At my current place of work we realized a need for a follicle-like setup again, so I started researching.

At one point we’d thought we could solve the problem with the Rivet script. Rivet takes two edges, lofts them into a nurbs surface with history, then attaches to the surface a locator using a pointOnSurfaceInfo and an aim constraint. When the lofted surface is relatively square and doesn’t deform much, this works fine. When you try to use just the pointOnSurfaceInfo and aim constraint setup on a longer nurbs surface that deforms and bends, however, we found that the locators do not behave properly. Past a certain amount of twisting, the locators would rotate off in odd directions.

I played with the script and found that the pointOnSurfaceInfo node was feeding the surface normal into the aim constraint as the aim vector, with one of the tangent vectors as Up. Because of this, the aim constraint was causing the locator to flip. The way aim constraints work makes up vectors into suggestions, not rules. It also makes the third axis a product of the other two, as I showed in my last post.

In the end it was a simple fix: instead of using the surface normal (which wasn’t an illogical choice), I fed both surface tangents into the aim constraint and let the third axis, the normal, be the derived one. Since the tangent u and v vectors are always orthogonal regardless of how much you distort the surface, and since they always run in the right directions along the surface, you can be certain that the surface normal — a third orthogonal vector — will still end in the right place. (I bet the surface normal is derived from the cross product of the two tangent vectors anyway, internally.) No need for a custom node or to force the loading of decomposeMatrix; so far I haven’t seen any problems with this setup.

Steps for those who want to try this at home:

1) Create a pointOnSurfaceInfo node attached to your nurbs surface. Set it’s U and V parameters to get the output to the right place on your surface.

2) Use the createNode command to make an aimConstraint node.

3) Plug the pointOnSurfaceInfo’s tangentU into the aimConstraint’s target[0].targetTranslate, and the tangentV into the constraint’s up vector.