Update skeleton_kinematics.md

docs/animation/skeleton_kinematics.md

@@ -92,7 +92,7 @@ We'll extend this so we can have multiple targets, which will then use the funct
 
 which is a simple extension actually. Since each term is independent and added together, we can get the gradient of this new cost function just by summing the gradients of each of the constituent cost functions!
 
-You should implement multi-target IK, which will take a `std::map` of `Joint`s and target points for that joint. Each joint can only have 1 target point.
+You should implement multi-target IK, which will take a `vector` of `IK_Handle*`s called `active_handles` which stores the information a target point for a joint. See `scene/skeleton.h` for the definition of `IK_Handle` structure.
 
 In order to implement this, you should update `Joint::compute_gradient` and `Skeleton::step_ik`. `Joint::compute_gradient` should calculate the gradient of <img src=task2_media/0079.png height="14"> in the x,y, and z directions, and add them to `Joint::angle_gradient` for all relevant joints. `Skeleton::step_ik` should actually do the gradient descent calculations and update the `pose` of each joint. In this function, you should probably use a very small timestep, but do several iterations (say, 10s to 100s) of gradient descent in order to speed things up. For even faster and better results, you can also implement a variable timestep instead of just using a fixed one. Note also that the root joint should never be updated.