Tag Archive: stretch


An IK Leg and a Bug Hunt

Im not sure I can truely put into words how frustrating creating the IK leg for this rig was. I do know, however, that creating a stretchy IK leg should not take almost 48 hours to get working.

Before I duplicated my bones, I decided I wanted to put some extra bones along the lower leg to enable smoother twisting. To ensure things worked nicely later on I wanted to ensure that the three new bones I inserted were exactly a quarter, half and three-quarters of the way along the calf. To do this, I parent constrained each of the three bones to both the knee and the ankle, and made sure maintain offset was unticked. This placed all three bones exactly half way between the knee and ankle. I then changed the weighting of the parenting to two of them. Femur01 was weighted 0.75 to the knee and 0.25 to the ankle, whilst Femur03 was weighted 0.25 to the knee and 0.75 to the ankle. I then deleted the constraints and used comet tools to orient the joints correctly.

Joint Creation 11

Once this was complete I created two duplicates (one for IK, one for FK). I then created a third duplicate in which I deleted the three femur joints and reparented the ankle straight to the knee. This was so that I had just two bones in the IK system. I called it the IKGuide. I created an IK on the joints and made a simple cube controller for the foot. I parented the IK handle to this control. I then orient constrained the ankle bone to this controller so that it would not rotate as the leg moved. Finally, I created a simple circle controller and parent constrained the hip bone at the top of the IK to the circle.

IKLeg01

With the IK built and working, I started to set up the IK stretch. Like the IK spine, I needed to know how long the leg was at any one point in time. However, I only needed the straight line distance from hip to ankle. I used the distance tool for this. I aligned one locator with the hip and one with the ankle and then parented them to the corresponding controllers. Now, as the leg moved, the distance tool would always give the distance from hip to ankle. At this point I also realised I had not created a knee controller. For this I used an arrow shape. I point constrained the arrow to both the hip and ankle (with maintain offset unticked). This placed the arrow on the plane between the two, directly in the centre. I then used an aim constraint to ensure the arrow was pointing directly at the knee. After deleting both constraints I moved the arrow in front of the knee and set up a pole vector constraint on the IK.

IKLeg02

To wire up the stretchy leg I used another multiply divide node and, like the spine, the distance was wired to the first input. The second input needed to be the length of the thigh bone plus the length of the calf bone. As the joints had been oriented correctly this could be found simply by adding the x transform of the knee and ankle together. I changed the node to divide and wired the output to the “true” output of a condition node. The “false” output was left as 1. The condition node also had the distance in the first input and the length of the two bones as the second. It then compared the two lengths, and if the distance was greater than the bone length, the condition was true. I then wired this condition node to the x scale (length) of the thigh and calf. The two bones of the IK scaled nicely. Unfortunately, the ankle and foot bones were being scaled strangely when the leg stretched, despite not being wired to the condition node. I even checked the scale of both and x, y and z were all still showing as 1. This meant the bones shouldnt have been scaling at all.

FootScaleBug FootScaleBug02

I tried deleting the IK and remaking it, but the problem persisted. I then tried moving the controls with no IK present at all, and the ankle and foot continued stretching strangely. I could only assume there was something strange with the bones, so I deleted the IKGuide joints and re-created them. I set everything back up, re-wired the thigh and calf x-scale to the condition node and tested it again. I had exactly the same problem all over again. I tried re-creating the bones once more that evening but with no success. I finally decided the only option was to go to bed and look at it with a fresh mind in the morning.

As is often the way with re-visiting a problem the next day, I tracked the issue down quite quickly. I had all the joints in the hypershade to make sure my ankle and foot definitely hadnt managed to end up wired to anything and I realised there was no line showing the parenting of ankle to knee. I un-parented the ankle bone and re-parented it to the knee and the problem disappeared. I was delighted, until I found yet another problem. Whilst the ankle was no longer scaling strangely, it still was not doing what I expected when I moved the hip too far away. Despite being orient constrained the foot controller (and as such theoretically unable to rotate by itself) when I moved the hip controller forwards or backwards so that the leg stretched, my ankle would rotate.

FootBugRotate FootBugRotate02

I decided to fix the problem by simply creating a new version of the ankle and foot bone. I simply point constrained the new ankle joint to the one on the IK leg and orient constrained it to the controller again. Success! Problem solved, just not as tidily as I would have liked. It also left me feeling frustrated because I wanted to know why the problem had occured so I could avoid it in the future. Still, at least the problem was gone and I could get on with parent constraining the IK joints to the IKGuide joints. I unticked maintain offset and parent constrained all the joints to their respective guide joints. I parented the three femur joints in the same way I created them; by parenting to both knee and ankle and then editting the weights. However, what I hadn’t thought of was that a parent constraint would cause the joints to rotate out of alignment due to the ankle’s orientation. I pondered the problem for a while and decided I would simply ensure to maintain offsets when constraining the deform joints to the IK joints.

IKLeg03

I then set to work creating a control system for the toes so that I could create a simple set of foot roll controllers. Initially I decided to place a circle controller around each joint of the toes, but it quickly became clear that some of them would be hard to select.

Foot01

 

Instead, I moved the curve shapes of the controllers above each toe joint and this made them much clearer and easier to see. Finally, I also made a main controller that would be used to curl all the joints of a toe. I then began creating a simple set of foot roll controls and with some re-parenting of the IK handle my IK leg was complete.

Foot02 Foot03

Unfortunately, I quickly checked things in my orthographic side view and realised that at some point during the creation process I had managed to cause the entire IK system to move out of alignment from its starting position, despite all the controls being at 0, 0, 0.

Foot04

The only option was to yet again build the entire IK leg. I deleted all my bones, mirrored the right hand leg to the left hand side. The good thing was, that at least this time all the controllers were already built so all I needed to do was wire everything up correctly, and make sure my controllers were correctly aligned before constraining/parenting things to them. Fortunately, this time I got it right and my left IK leg was finally complete. Hooray!

The IK Spine

I finally got the mesh back yesterday, and Im happy to say its actually symmetrical this time.

Symmetrical

As such, I’ve been able to get cracking with building the control system for the toony rig. I started with the spine as I feel its such a central part of the control system, and almost everything else is parented to it in some way. My first step was to duplicate the bones that I want to apply the IK spine to. This means if I made any mistakes, moved anything by accident, I wouldn’t ruin the position/orientation etc. of the deform skeleton and I could easily delete the duplicate and start again. I tend to insert IK (or whatever is appropriate) to the name of the joint to differentiate it from the deform skeleton.

Once I’ve got my duplicate I hid the deform skeleton so that I couldn’t affect it or move it whilst working on the controls. I applied an IK spline with two spans to the spine. This means that there are control points for each end, as well as a single control point in the center to affect the curve. I created a cluster for each of the control points. These control the shape of the curve and the shape of the curve drives the position of the bones.

IKSpine01

I then needed to build the actual controllers for the spine. Comet tools provides a quick way to make a bunch of spline shapes, but they are all quite simple, sharp edged splines. They never look particularly nice, and they don’t fit the shape of the body all that well. As such, I like to make a lot of my controllers by hand. To do this, I used the mesh itself to guide the shapes. I turned on snap to vertex and created a selection of curves that flowed around the area of the body that I wanted to control. I generally tweak them slightly afterwards to make sure the ends of the curves meet up and dont leave gaps anywhere. With this complete, I had a bunch of individual curves that could be selected seperately. What I actually want is to be able to click anywhere on any of the curves and to have them all selected. To do this, I had to reparent the individual “curveshapes” to a single curve. This can be easily done by selecting the “curveshape”, then shift selecting the curve I want to parent it to. I then simply use a single line of MEL script: “parent -r -s”. This leaves an empty curve node with no shape that can be deleted.

IKSpine02 IKSpine03

Once all my controllers were built I aligned them with the correct bones and parented the clusters to each controller. I also created two groups for each controller to be parented within. One I suffix with _SDK and one with _0. The _0 is my null group. The 0 point so that I do not need to use freeze transformations. The _SDK group allows me to set up parent constraints for a controller, whilst still giving the animator the ability to animate it. For the spine, I parent constrained the middle controller _SDK to both the top and bottom spine controllers. This means that the middle controller will always remain halfway between the top and bottom of the spine.

Once this was complete I decided to test the spine to check it was working correctly. Unfortunately, it wasn’t. I hadn’t realised I had only given the IK spline four bones to move around. When the curve had extreme bends the bones just averaged out their positions and the shape of the curve was lost. This meant rebuilding the deform spine with more bones so that there were enough joints to follow the spline curve more accurately. Having added them, I made sure to tidy up their orientation with comet tools again.

IKSpine04 IKSpine05

I then repeated the process of applying an IK spline to the duplicate set of bones and creating clusters for the three control points of the curve. I re-positioned the controllers to ensure they were correctly aligned with the new bones and then parented the clusters to the controls. I also set the twist controls for the IK spline to make the hip and chest controllers control

the spine rotation.

 

IKSpine07

Finally, I wanted my spine to be stretchy as this is meant to be a “cartoony” rig. I created a multiplydivide node which I set to divide. I also created an arc length info node for the spline curve. This provided me with the length of the curve at any time. I wired the length into the first input of the divide node and put the length of the curve when all controls were at 0, 0, 0 into the second input of the divide. This means that the output will be the current length of the spine divided by the original length. I then simply wired the output into the scale x (the length)  of all the joints in the spine.

IKSpine06

Success! An easy to use stretchy IK spine.