Inverse Kinematics

Ted Boardman tedb@tbmax.com         http://www.tbmax.com

Spring is here…right, there’s six inches of new snow in my driveway and there’s a cold raw wind blowing off the ocean. There were a few green shoots emerging from between the remnants of last year’s fallen leaves just before this recent snowfall so it seems inevitable that I’ll be sitting on the front porch again, soon.

One of the sure signs of spring is that tankers of asphalt for summer paving projects are replacing the ships coming up the river loaded with road salt. Portsmouth is an active shipping port for goods like petroleum products, road salt, gypsum, scrap metal, fishing, and nuclear submarines, as well as large and small yachts stopping by on their way up the New England coast later in the spring and summer.

I had to put the column aside for a trip and I’m just back from a trip to Palm Springs, CA where it reached 102 deg F, much too hot for me even in mid-summer and, while there is no apparent link between New England weather and that of the southern California desert, this month’s column is going to be all about linking in VIZ and max.

I’ll take a look at some fundamentals of using several tools in Autodesk VIZ and 3ds max that allow you to set up relationships between objects in the scene that can make animations and the movement of objects easier. There are several lessons to be taken from the process that I’ll go through step by step to illustrate a practical use of linking, including:

  • Hierarchical Linking
  • Dummy Helper objects
  • Interactive Inverse Kinematics

Yes, that is a lot of big words, but their use is not so difficult once you get the basic concepts down.

Hierarchical Linking is setting up a Parent-Child relationship between objects in the scene usually to control move and rotate transformations. Transformations of the parent object are passed to the child, while the child may have it’s own transformations. Complex ancestries may be created with a parent having many children and grandchildren, etc, but a child can only have one parent.

Dummy Helper objects are non-renderable objects that can be included in hierarchical linking to act as control objects or secondary pivot points.

Interactive Inverse Kinematics allows you to set transformation limits to objects or helpers and have them interactively update as you transform the parent objects in the scene. Check the online help files for more definitions of Forward and Inverse Kinematics.

In the example in this column I will show you how to create the telescoping pistons found in a tip body dump truck. In reality the pistons would extend, one at a time, with hydraulic pressure to pivot the body of the dump truck to an angle appropriate for the cargo to slide from the body. In VIZ or max, however, you will use Inverse Kinematics to have a control object at the end of the pistons pull the pistons as the truck body is manually rotated.

The scene is made up of the truck body and three nested cylinders. Two dummy objects are in the scene to act as connection points for the cylinders. One will be linked to the truck body to serve as an apparent attachment point for the end cylinder; the other dummy will be a connecting pin at the base of the bottom cylinder.

This scene is created in Autodesk VIZ 4, but may also be opened in 3ds max 4, 5, and 6. However, max 5 and 6 are slightly different in the basic IK operation as will be noted in the text.

Hierarchical Linking

Open the VIZ 4 file called DumpTruckBody_orig.max and save it as DumpTruckBody01.max.

The first thing to do is to analyze the action you want and to setup the parent-child relationships. The driving object in this scenario is the rotation of the truck body itself. The telescoping pistons will follow the dummy linked to the truck body as it rotates and the pistons, which are linked to each other from the top down will remain pinned to the second dummy object, the last parent in that hierarchical chain. Figure 1 shows the scene and the location of the Select and Link tool in Autodesk VIZ 4.


Figure 1: Select and Link tool in Autodesk VIZ 4 and scene setup for telescoping pistons

The Pivot Point locations of objects have a big effect on linking and certainly animation and the objects have been created or edited with that in mind. The Pivot Point locations for each object can be seen in Figure 2. Several objects share common pivot points.

Figure 2: Pivot Point locations for objects

The actual linking process is a drag and release operation. You click the Select and Link button, pick when you see the link cursor and hold on the child object. Hold the left mouse button and drag to the parent object. You will see a dotted “rubber band” line from the child’s pivot to the cursor. Release the left mouse button when you see the new link cursor and you will see the parent object flash white briefly to indicate the link has taken place.

The first link you will establish is the DumpBody object to the Top_Dummy01. Click the Select and Link button and, in the Front viewport, pick Top_Dummy01 and drag to DumpBody. Release when you see the cursor indicating a valid parent object as seen in Figure 3.

Figure 3: Click Select and Link and pick and drag from the child Top_Dummy01 to the parent DumpBody. Changing cursors and the rubber band line are the indicators the link is being performed. The parent flashes white when you release the left mouse button to complete the link.

Zoom in on the cylinders in the Front viewport and use the same Select and Link process to link the smaller Cylinder03 to Cylinder02. Again to link Cylinder02 to Cylinder01, and finally link Cylinder01 to BasePin_Dummy. Click the Select button in the main toolbar to exit the Select and Link tool. Click the Select by Name button on the main toolbar or type H to call the Select by Name dialog. Check Display Subtree at the bottom left to show the hierarchy as children indented below their parents in the list. It should look just like Figure 4.

Note: if the list is not the same and there is a Pick button instead of Select in the dialog, you have not yet exited Select and Link mode. This process can be used to link objects by name rather than picking them in the viewports.

 

Figure 4: Checking Display Subtree in the Select Objects dialog will indent children below the parent allowing you to visually check the hierarchy.

Setting Up IK Joints

You will now learn to set up restrictions to control the amount of sliding and rotation possible in the three cylinders of the piston. Each should be able to slide almost their full length, but not pull out of the larger cylinder in the system. They should not be able to rotate in any axis and the smaller Cylinder03 should appear to be attached to Top_Dummy01 as DumpBody rotates to dump its load.

Note: Initial setup of the IK joints is where there are noticeable variations between Autodesk VIZ 4 and 3ds max 5 & 6. In 3ds max 5 & 6 you must first select Cylinder03, then go to the Animation pull-down menu, IK Solvers, and choose HD Solver. Then pick BasePin_Dummy as you see the dotted rubber band line attached to the cursor. This applies a Bones system that allows adjustment of sliding joints.

In both VIZ and max, select Cylinder03 and, in Hierarchy panel, IK, open the Rotational Joints rollout and clear the Active checkbox for each axis. See Figure 5. Repeat this for Cylinder02 and Cylinder01. For BasePin_Dummy clear the X and Z rotational axes, but leave the Y axis Active so the dummy will rotate in that axis freely.



Figure 5: For all three cylinders clear the Active option in the Rotational Joints rollout.

You have to do each cylinder individually and this is a good habit to get into for general practice. Deactivating the rotational axes will prevent wild flipping of hierarchical links while you are trying to set a system up. You can then go back and activate them as needed.

Cylinder01, the large base cylinder will not slide in this system. Activate the Front viewport. Select Cylinder02 and expand the Sliding Joints rollout in the Hierarchy panel. Check the Active and the Limited options in the Z axis area. This allows the joint to slide, but limits it to settings of 0 in either direction (positive or negative Z axis).

Pick and hold on the spinners for the From: numeric field and move the new double arrow cursor until the number is about 100. Cylinder02 will just project slightly from Cylinder03 on the left side as seen in the Front viewport. See Figure 6.



Figure 6: Check Active and Limited then click and hold on the From field spinner until it reads about 100. You will see the Cylinder02 slide interactively as you adjust the spinner.

Next, click and hold on the To: spinner and adjust it to about 190 where Cylinder02 is almost out of Cylinder01. See Figure 7. You have just set limits for that cylinders IK sliding when in Interactive mode.

Figure 7: Use the spinners in the To: numeric field to set the limits for Cylinder02 to extend to.

Select Cylinder03 and, in the Hierarchy panel, check Active and Limited in the Z Axis area and adjust only the To: field spinners to read about 100. The actual numbers are determined by an objects size, pivot point location, and starting position and you are controlling relative sliding limits.

To test the sliding limits and the action of the cylinders, toggle the Inverse Kinematics Mode Toggle (or pick the Interactive IK button at the top of the Hierarchy panel). Select Cylinder03 and, in the Top viewport, move the cylinder to the left. As it pulls out of Cylinder02 it will reach a point where it pulls Cylinder02 from Cylinder01. See Figure 8. Moving Cylinder03 back collapses the pistons into each other again. Right-click to cancel the Move or click Undo to return the cylinders to their original position.

Figure 8: Toggling Interactive Mode on allows you to test the restrictions you set by moving Cylinder03 in the viewports.

The cylinders have the correct action now but there is nothing for them to do. You will Bind Cylinder03 to the Top_Dummy object linked to DumpBody so that it will always try to track the dummy.

With Cylinder03 selected in the Front viewport, go to Hierarchy panel, IK, Object Parameters rollout. In the Bind to Follow Object area, click the Bind button. In the Front viewport drag the cursor to the Top_Dummy01 object and release the mouse when you see the pin cursor. See Figure 9. Click the Bind button to disable it. This tells the Pivot Point of Cylinder03 to try to stay attached to the dummy.

Figure 9: Use the Bind option in the Hierarchy panel, to bind the Pivot Point of the top cylinder to the dummy object.

To test the final connection, zoom out in the Front viewport and select the DumpBody. Now rotate the DumpBody in the Z-axis of the Front viewport a little over 90 degrees so the body goes past vertical and back to horizontal. The piston cylinders always try to stay with the Top_Dummy01 and will slide accordingly. If you rotate way past vertical you will see that the top cylinder is not actually connected.

Making It All Work

So far, you have been able to observe the interactive process of the linking system. When it comes to animating the scene though you do not just turn the Animate button on and set the appropriate keys. You must allow VIZ or max to calculate the keys.

You must turn off the Interactive IK button in Hierarchy panel, or toggle Inverse Kinematics Mode button off. You can then turn the Animate button on and animate the DumpBody rotating over the 100 frame default. Then, in Hierarchy panel, Inverse Kinematics rollout, click the Apply IK button.

Now play the animation back and you will see the cylinders behave, as you would expect for this type of system and animation.

Summary

For IK to work correctly the position of object’s Pivot Points is an important factor. Linking the Parent-Child relationships in the correct order is next. It is extremely helpful to deactivate all Rotation axes in the Hierarchy panel, IK tab. The Sliding joints are deactivated by default.

Setting Limits, either for Sliding or Rotation Joints depending on your needs will help control the linked system and constrain objects in a more realistic manner. In order to see the results of any changes to Sliding or Rotation Joints in the viewports you must have the Interactive IK turned on and the best results are seen by adjusting the spinners of the From and To restrictions manually rather than entering the numbers directly.

For any animation to be correct you must disable Interactive IK and use the Apply IK in the Hierarchy panel to actually create the necessary keys in the Track bar.

There are a multitude of other adjustments that you can use to fine tune the results but this should get you started with IK and Sliding Joints.

You can open the file DumpTruckBody.max to view the results of the steps performed here.

 

Good luck and have fun.

 

Ted

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