Inverse kinematics: getting started

By Randy R. Wall.

Well I am pretty new to the Kinematics in Imagine but I'll get You started so that You can experiment. First make three axis making one and then copy it and paste it down twice. Now pick Axis.1 and then set it's Z position to 50 (using the Transformation requester). Now set Axis.2 Z position to 100. Select the bottom Axis and the Axis.1 just above it by using the Shift key as You select it. Group them together and then click anywhere on the screen to unselect these Axis. Select the middle Axis.1 and the top Axis.2 and group them together. Ok this is a basic bone structure or group, if You select the bottom Axis while in Pick Groups mode all of the axis will be selected. So with this I can explain to You some simple kinematics... at least enough to get You started.

Go into Pick Objects mode (Kinematics will only work in this mode). Select the bottom Axis and and then select Freeze: in the Freeze menu set Rotation World X, Y and Z on. Set Constrain and select the top Axis.2 of the group and press M. If You are in the quad view (all four views showing) You can go in the top view and rotate Your pointer around the Axis. You will see that all the axis follow the pointer as You move it but the bottom one stays stationary and does not rotate with the others. If we unset the Rotation World X, Y and Z it would follow along with the other axis. If we wanted to limit this group more we could do the following.

Set ALL axis so Rotation World Y and Z are set, un-set the bottom Axis's Rotations. Select the top axis and try rotating the pointer around the axis in the top view, as You can see the axis now only move along the Y axis or should I say will only rotate on the X axis. As we have limited its movement by only letting it rotate on X. Hit the spacebar, undo this and select the Right View Bar.

Now we can make the axis move in a more controllable means. Select the top axis again, press R and then X and move the pointer to the right slowly till the axis angle is at 45 degrees. Imagine will tell You this at the top right corner of the menu bar. The top axis will rotate on its X axis. Press M and move the pointer to the left and slightly down. As You can see the top axis keeps its alignment of X and all the other bones or axis bend to make it look like a finger bending. This may cause a problem if You move to far but You can either try moving it back and try again or hit the spacebar, select the middle axis and press R then X to rotate the middle axis into the correct position.

If You wanted to stop the bottom axis from causing this problem You could Freeze its World Tranlations and then move or rotate the top axis to the position or alignment You want and, un-set the bottom axis Translations and Rotate its axis on X to rotate the whole group. Or to do a similar effect keep Your top axis free to move and rotate, You could rotate the middle axis into the angle You want and then Lock it's X Joint, select the top axis and move or rotate it's X axis. By doing this the bottom and middle axis stay aligned to each yet still follow the top axis and leaves the top axis free to manipulate...

Oopps! typo on my part it should read: set ALL the axis so Rotation World Y and Z are set.

Last Update: Aug 26, 1995
Back to Ian's HomePage. -- Up to FAQ #8 Index