Trying to have an object 'track' down another

[mark_wilkins]

Because Maya doesn't see the xform -q or move commands as equivalent to a connection, it doesn't know when it has to execute the expression.

This is a big problem with using MEL commands in the middle of expressions. You can set the expression's "always evaluate" toggle on, which may help (though there are certain circumstances under which even this won't solve the problem completely.)

An alternative is to query the control object's translateX and translateZ by assigning them to a local variable (rather than using the xform command), and assign new position values directly to tx and tz rather than using the move command. This will make your updating work out better.

Here's my version of your expression:

float $distance = 0.1;

vector $ctrl_direction = <<locator1.translateX - tx,
0,
locator1.translateZ - tz>>;

vector $norm_ctrl_direction = $ctrl_direction / mag($ctrl_direction);

vector $move_vec = $norm_ctrl_direction * $distance;

tx += $move_vec.x;
tz += $move_vec.z;


"But," you say, "the expression has to have the node name of the control object available in it!" Yes, it does. The solution to this is to build a small MEL script that you use for setting up the car/control relationship. This MEL script will look at the name of the control object you've selected and use the expression command to make the above expression.

Another alternative which I *MUCH* prefer if you're going to have a lot of these things, because it will be much, much faster and much simpler to set up, is to use particle dynamics. You can set up a particle object whose particles follow per-particle goals around just as you've done with your expression, then use geometry instancing to put a car at each particle. Not only will this be faster, but it will also save memory if you have multiple of the same car in the scene.

-- Mark

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