Hi, I am currently trying to implement a "simple" analytic IK solver to compute the position of a knee in a simple 2 bone system (Thigh / Calf). I have found many example of such solver in 2D on the Internet but nothing in 3D... For example the analytic case here: http://cose.math.bas.bg/Sci_Visualization/compAnim/animation/rick_parent/Modl.html I would like to stay away from more complex methods and numerical solvers (Jacobian, CCD...). My idea is actually a foot placement system, and I need IK to compute the new knee angle once I knew where the foot should be. Has anyone already seen this kind of thing with an example to understand how to move from a 2D to a 3D problem? Thanks.

Read my response here: http://www.gamedev.net/community/forums/viewreply.asp?ID=3446115

Oh, very sorry, didnt see the part about 3D.

Vector C will just be in 3d. The blue and red angles will be computed in the plane that makes up the 3 segements, which is a 2d problem then.

The purple angle will be result in polar angles, rather than a single angle from atan2. So you'll need a cartesian to polar conversion there.

That is if you want to use euler angles. You could just construct the pink angle as a matrix or quaternion, creating the transformation from the pelvic to the plane in which the blue and red angles lay.

Not very much different.

Here's an example with source you might want to check out:

http://mrl.nyu.edu/~perlin/gdc/ik/
http://mrl.nyu.edu/~perlin/gdc/ik/ik.java.html

Thank you both for your answers. The code from Ken Perlin will surely help. I think I'm going to use the same method, using the x axis as he does and then translate the results back to where my model is.

Thanks ;)