Here is a collection of information provided in the class.
Here is a link that explains gimbal lock: http://www.anticz.com/eularqua.htm
“Every system that uses Eular angles will have this problem. The reason for this is that Eular angles evaluate each axis independently in a set order.”
The solution for this is to use Quaternion.
2. Different Kinds of IK Approaches
Classic Approach ( Linear Approximation): Not efficient
– Jacobian Transpose
– Jacobian Pseudoinverse
– Damped Pseudoinverse
– Use Nullspace to achieve objectives ( can be used to solve singularity problem)
– Priority ( e.g. hands moving but want to keep feet on the ground)
Solve 1st priority with damped, 2nd with nullspace.
Priority Chain: keep projecting into nullspace
– Linear Search
– CCD ( much faster)
– FABRIK (forward and backward reaching IK)
Other Interesting papers can be found here: http://graphics.cs.cmu.edu/nsp/course/15464-s17/lectures/Feb6ReferenceList.pdf
After the class, I find not classic ways easy to implement and also, from what I have seen, have more natural result than classic ones. More research will be put into this and I will keep updating this post.
Imagine convert this into 3D world. Center of the capturing area is the origin. Axes shown in the picture. 15 cameras tracking the markers to get tracking information.
The reference cameras are used to see if the mapping of the rig is always right. The reference cameras includes software that can map the ref camera into the 3D space and you can get a clearer view of that.
Before using the system, the cameras should be calibrated.
This is the markers. It has a variety of sizes and can cost about $8. The cameras emits rays and captures the reflected frequencies to identify the markers. There’s something it can not capture: hair.
After putting on the markers, the markers will be mapped in the software, but not necessary to the rig. Information of motion at each point are stored and then processed. You can map the points to rigs, skins, and you can use multiple points to calculate one joint. It is also said that you can get forces.
There are some movements that involves a lot of occlusions. E.g. swimming. They tried to hang the actor to get the motion.
// Here is the beginning of a blog series. This post will be updated after every technical animation class
from Walk Cycle to Motion Capture
– Stop Motion Animation
My classmates in class asked why it still exist. The reason is, there is too many factors that influence the reality, and we have only discovered ways to mimic it. We may achieve it in the future, or may not; but stop motion uses REAL PHYSICS. That is why I love stop motion animation. Actually, Laika is one of my favourite company.
What I love about stop motion animation:
The Randomness in motion. Posing a real model with the artist’s hands feels different from using Maya. The frame-by-frame posing adds more style to the animation than setting key-frames. Think about flash animation verses hand-drawn animation and you will understand that different feelings.
The Details in material and the lighting. Stop motion animation use real materials, and because the physics behind lighting is so complicated, every object respond uniquely. When lighting a scene, the materials and the lights always work together. The stop motion animations made by Laika are like exhibitions of the beauty of the materials, always make me feel intimate and satisfied.
The Style itself. The animation in the movie Kubo and the Two Strings looks very fluent, but there’s still some parts (especially facial animation) that you can tell it is stop motion. Instead of feeling skeptical, I feel strongly into this kind of style. It is lovely and artistic.
And you can not deny, stop motion is the first 3D animation. In a way, computer animation referred to it when it first came out.
– Key Framing Animation and Motion Capture Animation
Different from my classmates’ opinion, I like the key framing animation better. And in animation feature, people seldom use motion capture. Motion capture can preserve the motion, but it has the limitation of reality. In animation, there is a balance between novelty and reality. If it looks too real, it is boring. If it is not real enough, it is not believable. Motion Capture creates realistic result, but too real. The philosophy of exaggerating in animation is lost in Motion Capture, and make it lose the style. Because of the fact that if you use motion capture, you will need to adjust the animation frame by frame, it is not ideal for animation features.
– Different Problems: Calculate Key Frame Animations, Transition Between Animation Clips and Motion Capture Displacement Curves
After the class I have some doubts about the difference between the three I mentioned above, so I did some research.
Keyframe Animation and Animation Curves
Here is a short description of maya’s graph editor.
Keyframe animation is calculated based on interpolation.
Transition between 2 Animation Clips:
The actual problem is Animation Blending, It is used both on smooth transition between 2 animation clips and create new animations from existing ones. Also it can be used to combine multiple clips according to time-varying weights to get better result for motion capture.
The animations are represented with rotations. It is more complex than linear interpolation since it also involves complex problems like synchronization.
Use the example I found on stackoverflow:
Basically, if you have two animations clips A and B:
If A and B are completely different (blending jump and swimming), then chances are you will have strange/funny results;
If A and B are similar (two walking animation clips), then, you must first synchronize them to blend them when both left feet are on the ground for example.
The answer also mentioned a paper used a method called registration curves.
stackoverflow link : http://gamedev.stackexchange.com/questions/22402/animation-blending-basics
paper link: http://graphics.cs.wisc.edu/Papers/2003/KG03/regCurves.pdf
Displacement Curves on Motion Capture Results
This problem is more of a “spacetime constraint problem“. (A Hierarchical Approach to Interactive Motion Editing for Human-like Figures, 1999, http://dl.acm.org/citation.cfm?id=311539)
In this paper, it “combines a hierarchical curve fitting technique with a new inverse kinematics solver”. IK is calculated frame by frame to meet the constraints. The motion displacement of every joint at each constrained frame is interpolated and then the curve is smoothed.
Conclusion: All based on motion graph. Keyframe animations use linear interpolation as default. The two later problems both connect closely to the skeleton. The goal of transition is to get a right and smooth result, and adding displacement curves is more of a constraint problem.