I am in the final phase of my project. I want to do animation using model imported from MakeHuman. I remark in the properties of the model there is two mode FK (forward kinematic) (default mode) and IK (inverse Kinematic) to move our model. What is the difference between both modes. I could use easily the FK mode but in the IK mode some lines are added (see figure 2)and when I try to move it in order to move leg or arms of a person, i get nothing!!

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  • The lines you're talking about are not controlers but targets. As you may know, elbow and knee bend only 1 side, the target allows an IK to know wich side by pointing at the target. try to make your character kneel and move the knee target to see what i mean. – Bithur Feb 12 '15 at 9:40
  • IK Will make you be able to bend. FK Will not let you bend the characters knees. – user24239 May 1 '16 at 18:53
up vote 6 down vote accepted

With inverse kinematics (IK) the hands and feet will keep their position when the body is moved. When you switch from FK to IK you need to enable the layers for IK also.

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FK and IK have different control bones (custom shapes) for both modes. With IK you control the pose with the large plate shaped widgets at the hands and feet.

The lines you see are pole targets and allow to control the position of the knee or elbow.

IK example

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With forward kinematics (FK) each bone from shoulder to wrist needs to be adjusted. This is exactly how bones can be moved right after creation.

FK example

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Movement with arms still set to IK but leg to FK

Rigs like Makehuman or MHX allow you to switch from IK to FK separately for the legs and arms. You can even control the influence of the FK/IK controls.

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The snap buttons allow copying from the IK position to FK and vice versa.

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Here Rigify

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  • thanks, Snap is very important because i can swich between modes without lost of motion. I think that IK is very important to move a group of bones by moving the parent and to details some motion, we can use the FK, for example to move just the toe or foot, we can note use the IK but it is obligate to use FK mode. – BetterEnglish Feb 12 '15 at 16:54

If you get nothing that is no motion when you move the arm or leg IK .. Go to user preferences and check "Autorun Scripts". That should work.

Original Article: Overview of Inverse Kinematics

What is Forward Kinematics?

Before we can get into what Inverse Kinematics is, we need to figure out what Forward Kinematics is. The Forward Kinematics function/algorithm takes a pose as the input, and calculates the position of the end effector as the output. Forward Kinematics is the inverse function of Inverse Kinematics. With Forward Kinematics, you need to define the whole pose of an articulated body so as to provide the function/algorithm with the pose input. This means you need to define the articulation of each joint in the articulated body. This might be fine if you have a low number of joints, but with a high number of joints this tends to be tedious.

What is Inverse Kinematics?

Now, imagine if you’d like the end effector of your articulated body to reach a particular target position. This means that you know the end effector position you’d like to target, but you don’t know what the pose of the articulated body needs to be for the end effector to reach this target position. This is where Inverse Kinematics shines!

Inverse Kinematics is the inverse function/algorithm of Forward Kinematics. The Forward Kinematics function/algorithm takes a target position as the input, and calculates the pose required for the end effector to reach the target position — the pose is the output.

With Inverse Kinematics, you do not need to define the whole pose of an articulated body — this gets calculated for your by the IK algorithm. With IK, you only need to define a position as the input.

Inverse Kinematics does all the challenging computational work of calculating what the pose is. In the Before State, there is an articulated body with some known pose. It defines a target position for the end effector to try to reach. Once the IK algorithm is applied to the articulated body, we have reached the After State. The After State shows that a new pose has been computed, such that the end effector is now at the target position.

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